Class 10 Physics Chapter 3 Notes Kerala Syllabus The World of Colours and Vision Questions and Answers

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SSLC Physics Chapter 3 Notes Questions and Answers Pdf The World of Colours and Vision

SCERT Class 10 Physics Chapter 3 The World of Colours and Vision Notes Pdf

SSLC Physics Chapter 3 Questions and Answers – Let’s Assess

Question 1.
Find the most appropriate answer.
Name the optical phenomena taking place when light rays pass through water droplets to form a rainbow.
a) internal reflection
b) refraction
c) refraction and internal reflection
d) none of these
Answer:
c) refraction and internal reflection

Question 2.
Which of the following pairs of colours can produce white light?
a) magenta, blue
b) yellow, green
c) red, green
d) magenta, green
Answer:
d) magenta, green

Question 3.
When a ray of white light enters obliquely and passes through a prism
i) does not undergo refraction
ii) undergoes dispersion and deviation
iii) undergoes dispersion
iv) not subjected to any of these Choose the correct option.
a) ii <& iii
b) iv
c) i & iv
d) none of these
Answer:
a) ii & iii

Question 4.
Fill in the blanks appropriately:
a) Cyan colour + ………………. → white light
b) Blue colour + ……………… → white light
c) Magenta colour + green colour …………………..
d) Magenta colour + cyan colour + yellow colour → ………………….
Answer:
a) Red
b) Yellow light
c) White light
d) White light

Question 5.
Give scientific explanations based on scattering for the following:
a) Red light is used for emergency lamps.
b) The sky of the moon appears dark even during the day.
c) The deep sea appears blue.
Answer:
a) Red light having more wavelength undergoes less scattering. So it can travel longer distances and can be seen from far away. Hence red light is used for emergency lamps.

b) Since there is no atmosphere on the moon, scattering of light does not takes place. So the sky on the moon appears dark even during the day because of the absence of scattered light.

c) When sunlight enters the ocean, water molecules absorb and scatter the different colours of light. Blue light, with its shorter wavelength, is scattered more than other colours like red and orange. Since blue light is scattered more, it reaches our eyes, making the deep sea appear blue.

Question 6.
Complete the path of a light ray falling on the glass prism (Fig. 3.31).

Answer:

Question 7.
What are the radiations that are seen on either side of visible light in the electromagnetic spectrum. Write one use of the radiation with a shorter wavelength than visible light.
Answer:
Radiation seen in the electromagnetic spectrum on both sides of visible light are UV radiation and infrared radiation
Ultraviolet radiation has a shorter wavelength than visible light.
Use: UV radiation produces vitamin D.

Question 8.
The near point of a person with hypermetropia is 40 cm.
a) Can this person read the letters in a book held 25 cm away?
b) Can this person see an object at infinity?
c) How can this defect of the eye be rectified?
Answer:
a) No
b) Yes, he can see an object at infinity
c) Long sightedness can be rectified using a convex lens with suitable power.

Question 9.
Water is colourless, but it appears white in waterfalls. Why?
Answer:
Water appears white in waterfalls due to scattering. When water falls, it forms tiny droplets and bubbles that scatter light in different directions, making it look white or foamy, similar to clouds.

Question 10.
Based on the colour of illumination in the room, how can the coloured objects given in the table be seen? Complete the table.

Answer:

Physics Class 10 Chapter 3 Notes Kerala Syllabus The World of Colours and Vision

Activity
Pass a beam of light from a laser torch through a prism as shown in figure 3.1.

Question 1.
What is the reason for deviation in the path of light?
Answer:
Refraction of light

Question 2.
Identify the faces on which the light ray undergoes deviation.
Answer:
AB, AC

Question 3.
Depict the path of light ray in your science diary.
Answer:

Question 4.
Towards which part of the prism does the light ray deviate when it enters into the prism from air?
Answer:
The deviation occurs towards the base of the prism.

Question 5.
What about when the light ray passes from the prism into air?
Answer:
The deviation occurs again towards the base of the prism.
When light ray enters and leaves a prism, it deviates towards the base of the prism due to refraction.

Question 6.
Write the colours seen on the wall in the science diary in the decreasing order of variation.
Answer:
The component colours seen on the wall in the decreasing order of variation are violet, indigo, blue, green, yellow, orange, red (VIBGYOR).
The orderly arrangement of the component colours in white light ¡s called the spectrum.

Observe the splitting of sunlight into its component colours when it passes through a prism as shown in figure 3.4

Question 7.
What could be the reason for the deviation of the light ray?
Answer:
When light ray passes through a prism, it deviates at the two faces due to refraction.

Question 8.
Is this deviation the same for all the colours?
Answer;
No

Let’s see how this is related to the wavelength of different colours of light.
Compare the wavelengths of the colours given in table 3.1

Question 9.
Which colour of light has the shortest wavelength?
Answer:
Violet

Question 10.
Which has the longest?
Answer:
Red

Question 11.
Which colour deviates the most as it passes through a prism?
Answer:
Violet

Question 12.
Which has the least deviation?
Answer:
Red

Question 13.
What is the reason for the changes observed in the deviation of colours? Compare the deviation of colours with their wavelengths.
Answer:
The wavelength will be different for different colours.

Question 14.
How does the deviation of colours change with the increase in the wavelength as it passes through a prism?
Answer:
As the wavelength increases, the deviation of colours decreases.

Question 15.
What are the factors on which the deviation of a ray of light depend?
Answer:
Refractive index of the medium, Wavelength of the colour of light.

When light passes through a glass prism, it undergoes refraction at the two refracting faces of the prism. The extent of deviation depends on the wavelength of light. Red deviates the least because of its longer wavelength. Violet, which has a shorter wavelength deviates the most. The wavelength of other colours lie in between red and violet. Hence their deviation occurs proportionally and is arranged between red and violet.

Observe figure 3.8

Question 16.
When passing through water droplets where do light rays undergo refraction?
Answer:
When entering the water, coming out of the water.

Question 17.
What happens to the refracted light rays inside the water droplets?
Answer:
The refracted light rays undergo internal reflection.

A ray of sunlight passing through a water droplet undergoes refraction twice and internal reflection once. This is a natural phenomenon. A rainbow is formed as a result of the combined effect of refraction, dispersion, and internal reflection.

Question 18.
What kind of light is obtained on the wall? (coloured light / white light)
Answer:
White light

Question 19.
What could be the reason for this?
Answer:
The rays of different colours from the first prism undergo a deviation in the opposite direction by the second prism. This results in the recombination of colours to produce white light on the wall.

Question 20.
Are there any other components in sunlight besides visible light?
Answer:
Yes

Question 21.
Excessive exposure to sunlight is harmful? What could be the reason for this?
Answer:
Sunlight contains infrared and ultraviolet radiations in addition to visible light. The infrared radiation in the sunlight is the main reason for the heat in the Sun’s rays.

In the activity shown in figure 2.2 (b) (burning paper using a lens), the paper burns because of the convergence of infrared radiation on the paper. Ultraviolet radiation helps to produce vitamin D in our body.

Solar radiations reach the earth’s atmosphere after travelling an average distance of 150 million kilometre through air and vacuum. The distance it travels through the air is negligibly small compared to that in vacuum.

Solar radiations contain visible light, infrared radiation, ultraviolet radiation, etc. They do not require a medium to travel. They travel through vacuum at a speed of 300,000 kilometre per second (3 × 10⁸ m/s). Such radiations are electromagnetic radiations.

The orderly distribution of electromagnetic radiations is known as the electromagnetic spectrum.

Question 22.
Observe figure 3.10. Name the radiations that constitute the electromagnetic spectrum. List them in the ascending order of wavelength.

Answer:
The electromagnetic spectrum is composed of several types of radiation, including radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.

Thus, the increasing order of the wavelengths of the given electromagnetic radiations is:
Gamma rays < X-rays < Ultraviolet rays

Question 23.
Based on your observations, complete table 3.2.

Answer:

Overlapping colours	Resulting colours on overlapping
Red + Green	Yellow
Red + Blue	Magenta
Blue + Green	Cyan
Red + Green + Blue	White light

Not only white light, but all other coloured lights can also be created using red, green, and blue lights. Therefore, these colours are called the primary colours of light. The coloured light formed by combining any two primary colours is a secondary colour of light

Question 24.
Find the secondary colours from figure 3.12 and write them down.
Answer:
Yellow, magenta, cyan.

Question 25.
What are the primary colours in yellow light?
Answer:
Red and Green

Question 26.
Which primary colour is not present in yellow light?
Answer:
Blue

Question 27.
Which colour will be obtained when yellow light is combined with the primary colour that is not present in yellow?
Answer:
When yellow light is combined with the primary color blue (that is not present in yellow), the resulting color is white.

The yellow light from a sodium vapour lamp is not a composite light. Hence, red and green objects appear dark in this light.

Question 28.
If we add a primary colour that is not a constituent of the secondary colour, won’t we get white light?
Answer:
If a secondary colour is added to a primary colour that does not contain it, white light is obtained.
When a secondary colour is combined with a primary colour, we get white light. Such pairs of colours are called complementary colours.

Question 29.
Complete table 3.3 with regard to complementary colours.

Answer:

Secondary Colour	Component Colours	Component Colours
Yellow	Red + Green	Red + Green
Magenta	Red + Blue	Red + Blue
Cyan	Green + Blue	Green + Blue

Question 30.
Take a circular disc. Colour half of it with light yellow colour and the other half with light blue colour using crayons. Rotate this circular disc very fast. What do you observe? Why? Why does the circular disc appear white?
Answer:
When complementary colour pairs are combined we get white light. The circular disc appears white due to persistence of vision.

Question 31.
Find more examples of persistence of vision.
Answer:

• The continuous line of light you can see when a sparkler is waved around on bonfire night.
• When looking at rotating blades on a fan, they appear to be continuous.
• A torch rotated rapidly appears as an illuminated circle.

Question 32.
Explain the following based on the persistence of vision:
When a torch or a burning stick is rotated very fast, a ring of fire is seen.
Answer:
When a torch or a burning stick is rotated very fast, the light from other parts of the object suddenly reaches our eyes before the visual experience of any part of the object disappears from the eyes. Due to persistence of vision, all the parts appear to be together and a ring of fire is seen.

Question 33.
Record the observations of each case in table 3.4.

Answer:

Light falling on the filter	Colour of the filter	Colour of light passing through the filter
White light	Red	Transmits red colour in the white light
	Green	Transmits areen colour in the white light
	Blue	Transmits blue colour in the white light

Question 34.
Now let us pass each of the primary colours and white light through a yellow filter. What do you observe?
Answer:
The yellow filter passes the yellow colour and its component colours, red and green.

Question 35.
Identify the components of white light transmitted through each filter. What happens to the other colours that fall.on the filter?
Answer:
The filters transmit only the colour of the filter and its component colours from the white light, and block the other colours.

Question 36.
Complete table 3.5 related to secondary colours.

Answer:

Filter	Light falling on the filter	Transmits / Does not transmit
Magenta	Red	Transmits red
	Green	Does not transmitgreen
	Blue	Transmits blue
	Yellow	Transmits red
	White	Transmits red and blue

A filter of secondary colour transmits light of its own colour and its component colours.

Question 37.
Which colours will be reflected when sunlight (white light) falls on the objects given below? Complete table 3.6.

Answer:

Object	Reflected light
Blue car	Blue
Green mango	Green
White vessel	White
Red apple	Red

When sunlight falls on an opaque object, it reflects the colour of the object as well as the colours associated with adjacent wavelengths. It should be remembered that the colour of an object is not of a single wavelength. Similarly, there may be slight differences in colour perception depending on the light-sensitive cells (rods and cones) in the eyes of each individual. What happens to all other colours? The object absorbs all other colours.

Question 38.
Write down the results obtained by observing the given objects in green, blue, and yellow lights and complete the table.

Answer:

What are the inferences obtained on analysing the table?

Question 39.
What colours do a green leaf reflect?
Answer:
Green

Question 40.
And what about a red flower?
Answer:
Red

Question 41.
Can a yellow flower reflect only the yellow colour?
Answer:
Yellow flower can reflect the colour yellow and also its component colours, red and green.
An opaque object of a secondary colour can reflect light of its colour and its component colours.

Question 42.
In which colour will a surface appear if it reflects all colours of light falling on it?
Answer:
A surface that reflects all colours will appear white in white light.

Question 43.
And what about a surface that absorbs all colours?
Answer:
A surface that absorbs all colours appears dark.
A surface that reflects all colours will appear white in white light. A surface that absorbs all colours appears dark.

Sunlight is a composite light. It contains different colours. When sunlight falls on objects, each object reflects different colours according to its colour. Accordingly, objects are seen in different colours.

Question 44.
Which type of scattering does light undergo when it falls on microscopic particles? (regular / irregular)
Answer:
Irregular

Question 45.
Does this type of scattering cause sunlight to spread everywhere? Discuss.
Answer:
Irregular scattering causes the sun’s light to spread everywhere. The phenomenon of spreading of light in this manner is scattering.
Scattering is the irregular and partial directional deviation of light when it encounters particles in a medium.

Take about three quarters of water in a rectangular glass j ar. Allow light rays from a torch to pass through the water in the jar onto the screen as shown in the figure. Dissolve sodium thiosulphate in the water at a concentration of two gram per litre. Add one or two drops of hydrochloric acid to it. Observe the gradual change of light in the solution and on the screen.

Question 46.
Which colour spreads first in the solution?
Answer:
Blue

Question 47.
Write down in order the colour changes seen on the screen.
Answer:
Yellow, Orange, Red

Question 48.
Which is the last colour to appear on the screen?
Answer:
Red

When sodium thiosulphate and hydrochloric acid react, colloidal sulphur is precipitated. The size of the particles gradually increases.

Violet, indigo and blue colours in sunlight, which have shorter wavelengths, undergo more scattering when they encounter particles in the atmosphere. The scattering of red, having a relatively longer wavelength, is very low. Hence red can travel a longer distance through the atmosphere.

The extent of scattering and the size of the particles are related to each other. As the size of the particles increases, so does the scattering. If the size of the particles is greater than the wavelength of light, the scattering will be the same for all colours.

BLUE COLOUR OF THE SKY
Observe figure 3.23.

You know that light undergoes scattering when it passes through the atmosphere.
Question 49.
Which colour undergoes maximum scattering?
Answer:
Violet, Indigo, Blue

Question 50.
Then, which colour of light spreads in the atmosphere?
Answer:
Blue

When sunlight travels through the atmosphere to reach the Earth, some of its components undergo scattering as it passes through the air. Scattering occurs most for colours with shorter wavelengths such as violet, indigo and blue. This scattered light spreads in the sky. The resultant scattered light that reaches the observer’s eye gives the effect of the blue colour. So the sky appears blue.

Question 51.
The distance that the sun’s rays travel through the atmosphere to reach the Earth during sunrise and
sunset compared to other times is ………………….. (more / less)
Answer:
More

Question 52.
Which colours undergo the least scattering at these times?
Answer:
Red, Yellow, Orange

Question 53.
Which colour will be prominent in the light reaching the Earth?
Answer:
Red
If so, explain why the sun appears red, yellow, or orange during sunrise and sunset.

The sun appears red, yellow, or orange during sunrise and sunset because its light travels through more of the Earth’s atmosphere. As it does, the shorter blue wavelengths scatter more, leaving the longer wavelengths (red, orange, and yellow) for us to see.

Question 54.
Now try changing the position of the object to a distance 30 cm from the lens.
Answer:
No

Question 55.
Replace the lens of focal length 10 cm with a lens of focal length 12 cm. What do you observe?
Answer:
A clear image of the object is formed on the screen.

Question 56.
What could be the reason for getting a clear image now?
Answer:
A clear image was obtained at the same position when a lens with a suitable focal length was used.

When the object is placed at different positions, to get a clear image at the same position, the focal length of the lens must be adjusted accordingly.

Question 57.
How are images of objects at different distances formed on the retina?
Answer:
This is made possible by changing the curvature of the lens in the eye with the help of the ciliary muscles by changing the focal length. When the ciliary muscles contract, the curvature of the lens increases and the focal length decreases.

Question 58.
What change will occur in the curvature of the lens while looking at distant objects?
Answer:
While looking at distant objects, curvature of the lens decreases.

Question 59.
What about the focal length when the curvature decreases?
Answer:
When the curvature decreases, focal length increases.
The ability of the eye to change the curvature of the lens and adjust the focal length so that the image of the object always falls on the retina, regardless of the position of the object, is the power of accommodation of the eye.

Question 60.
Will clear images be formed on the retina when objects are kept very close to the eye?
Answer:
No
Activity
Try reading a book by holding it close to your nose.

Question 61.
Can you see the letters clearly?
Answer:
No

Question 62.
What if you move the book away?
Answer:
The letters are seen clearly.

Question 63.
At what distance from the eye can you see the letters clearly?
Answer:
25 cm. This distance is the least distance of distinct vision.
The nearest point at which an object can be seen clearly is the near point. For healthy eyes, the minimum distance for clear vision is 25 cm.

Question 64.
What is the maximum distance at which an object can be seen clearly?
Answer:
Infinity
The farthest point at which an object can be seen clearly is the far point. This distance is considered to be infinity.

Question 65.
Will the near point and far point be alike for everyone?
Answer:
No

Question 66.
When an object is placed at a distant point P as shown in the figure, where will the image be formed?
Answer:
Infront of retina.

Question 67.
Can the object be seen clearly?
Answer:
The object cannot be seen clearly.

Question 68.
What if the object is at Q?
Answer:
Yes, the object can be seen clearly.

Question 69.
Why can’t such people see distant objects clearly?
Answer:
For people with this defect, the far point will not be at infinity. It will be at a certain distance from the eye.

Question 70.
What could be the reason for short sightedness? Can you explain the reason based on the size of the eyeball and the power of the lens in the eye?
Answer:
The size of the eyeball is larger.
The power of the lens is more.

Question 71.
Observe figure 3.28 to find out how short¬sightedness is rectified.

Answer:
Short sightedness can be rectified using a concave lens with suitable power.

Question 72.
Find the reason for this defect based on the size of the eyeball and the power of the lens in the eye.
Answer:
The size of the eyeball is smaller.
The power of the lens is less.

Question 73.
Flow can long sightedness be rectified?
Find out from figure 3.30.

Answer:
Long sightedness can be rectified using a convex lens with suitable power.

Question 74.
What is the distance to the near point for a healthy eye?
Answer:
For older people, the distance to the near point may be more than 25 cm. This is because the efficiency of the ciliary muscles decreases. Such people have less power of accommodation. This is presbyopia. It can be rectified using a convex lens with suitable power.

Light Pollution
Although light is essential for the survival of life on Earth, artificial light harms the natural habitat of the biosphere. Light pollution refers to the creation of artificial light in excessive amounts and intensity. Artificial light adversely affects the reproduction and predation of many nocturnal animals.
Excess of artificial light adversely affects the natural activities, mental and physical health of humans.

Question 75.
What are the other consequences of light pollution? Prepare a note on this to present in class.
Answer:

• Causes difficulty during night drive.
• Makes astronomical observations difficult by obstructing the night sky.
• The light from multi-storeyed flats misleads migratory birds.
• It adversely affects the breeding and predation of many nocturnal species.

Std 10 Physics Chapter 3 Notes – Extended Activities

Question 1.
Prepare a note on the differences between colours of light and dyes.
Answer:
Red, green and blue (RGB) are primary colours while considering colours of light. But in the case of dyes, cyan, magenta and yellow (CMY) are taken as primary colours. The combination of primary colours of the same intensity and primary dyes are given in the table.

Colours of Light	Colour obtained
Blue + Green + Red Blue + Green Green + Red Blue + Red	White Cyan Yellow Magenta

Dyes	Dye obtained
Cyan + Yellow + Magenta Cyan + Yellow Cyan + Magenta Yellow + Magenta	Dark Green Blue Red

Primary dyes are used in painting and printing. In printing, black dye is also used.

Question 2.
Construct Newton’s colour disc and operate.
Answer:
Hints to make Newton’s colour disc

• Draw a circle on cardboard.
• Divide the circle into 7 equal segments.
• Color each segment with VIBGYOR colors (Violet, Indigo, Blue, Green, Yellow, Orange, Red).
• Make a hole in the center of the disc.
• Insert a pencil or dowel into the hole.
• Spin the disc quickly.
• Observe the blended colors to see white light.

The World of Colours and Vision Class 10 Notes

The World of Colours and Vision Notes Pdf

• When light ray enters and leaves a prism, it deviates towards the base of the prism due to refraction.
• Dispersion of light is the phenomenon of splitting up of a composite light into its component colours.
  • When light passes through a glass prism, it undergoes refraction at the two refracting faces of the prism.
  • The extent of deviation depends on the wavelength of light.
  • Red deviates the least because of its longer wavelength.
  • Violet, which has a shorter wavelength deviates the most.
  • The wavelength of other colours lie in between red and violet. Hence their deviation occurs proportionally and is arranged between red and violet.
• The orderly arrangement of the component colours in white light is called the spectrum.
• Light composed of different colours is a composite light.
• A ray of sunlight passing through a water droplet undergoes refraction twice and internal reflection once. A rainbow is formed as a result of the combined effect of refraction, dispersion, and internal reflection.
• When white light is passed through a prism, it undergoes refraction and the constituent colours are formed. In the path of the dispersed light arrange an identical prism in an inverted position adjacent to the first. It undergoes refraction in the opposite direction and results in the recombination of colours to produce white light
• Visible light, infrared radiation, ultraviolet radiation do not require a medium to travel. They travel through vacuum at a speed of 300,000 kilometre per second (3 × 10⁸ m/s). Such radiations are electromagnetic radiations.
• The orderly distribution of electromagnetic radiations is known as the electromagnetic spectrum.
  • The electromagnetic spectrum is composed of several types of radiation, including radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.
  • The increasing order of the wavelengths of the given electromagnetic radiations is:
    Gamma rays < X-rays < Ultraviolet rays < Visible light < Infrared rays < Microwaves < Radio waves >
• With red, green and blue lights, we can create not only white light, but also all other colours of light. So, these colours are called primary colours.
• The coloured light formed by combining any two primary colours is a secondary colour of light.
  Red + Green = Yellow
  Red + Blue = Magenta
  Blue + Green = Cyan
  Red + Green + Blue = White light
• When a secondary colour is combined with a primary colour, we get white light. Such pairs of colours are called complementary colours.

Secondary Colour	Component Colours	Complementary Colour
Yellow	Red + Green	Blue
Magenta	Red + Blue	Green
Cyan	Green + Blue	Red

• When we quickly remove an object from our field of vision the visual experience of that object persists for about 1/16 of a second. This phenomenon is the persistence of vision.
  Eg: When a burning incense stick is whirled very fast, a ring of fire can be seen.
• When complementary colour pairs are combined we get white light. The circular disc appears white due to persistence of vision.
• A filter of secondary colour transmits light of its own colour and its component colours.
• An opaque object of a secondary colour can reflect light of its colour and its component colours.
• A surface that reflects all colours will appear white in white light. We know that a surface that absorbs all colours appears dark.
• Scattering is the irregular and partial directional deviation of light when it encounters particles in a medium.
  • Violet, indigo and blue colours in sunlight, which have shorter wavelengths, undergo more scattering when they encounter particles in the atmosphere. The scattering of red, having a relatively longer wavelength, is very low. Hence red can travel a longer distance through the atmosphere.
  • The extent of scattering and the size of the particles are related to each other. As the size of the particles increases, so does the scattering. If the size of the particles is greater than the wavelength of light, the scattering will be the same for all colours.
• When light rays pass through a colloidal liquid or suspension, they get scattered, causing tiny particles to become illuminated, making the path of light visible. This phenomenon is the Tyndall effect.
• When sunlight travels through the atmosphere to reach the Earth, some of its components undergo scatter-ing as it passes through the air. Scattering occurs most for colours with shorter wavelengths such as violet, indigo and blue. This scattered light spreads in the sky. The resultant scattered light that reaches the observer’s eye gives the effect of the blue colour. So the sky appears blue.
• The sun appears red, yellow, or orange during sunrise and sunset because its light travels through more of the Earth’s atmosphere. As it does, the shorter blue wavelengths scatter more, leaving the longer wave-lengths (red, orange, and yellow) for us to see.
• The ability of the eye to change the curv ature of the lens and adjust the focal length so that the image of the object always falls on the retina, regardless of the position of the object, is the power of accommodation of the eye.
• Some people can see nearby objects clearly but cannot see distant objects. This defect of the eye is short sightedness. It can be rectified using a concave lens with suitable power.
• Some people can see distant objects clearly but cannot see nearby objects clearly. This defect of the eye is long-sightedness. It can be rectified using a convex lens with suitable power.
• For older people, the distance to the near point may be more than 25 cm. This is because the efficiency of the ciliary muscles decreases. Such people have less power of accommodation. This eye defect is pres-byopia. It can be rectified using a convex lens with suitable power.
• Light pollution refers to the creation of artificial light in excessive amounts and intensity.

INTRODUCTION

In this chapter we discuss about the wonders of colours around us. The concepts of refraction, dispersion, scattering of light, recombination of colours, rainbow, electromagnetic spectrum, primary colours, secondary colours, complementary colours, persistence of vision, Newton’s colour disc, colour of transparent objects, colour of opaque objects, blue colour of sky, red colour of sun at sunrise and sunset, tyndall effect are clearly depicted. The topics related to eye and vision including myopia, hypermetropia, presbyopia and light pollution are also explained in detail.

Refraction through a Glass Prism
• When light ray enters and leaves a prism, it deviates towards the base of the prism due to re-fraction.

Dispersion of light

• Dispersion of light is the phenomenon of splitting up of a composite light into its component colours.
• When light passes through a glass prism, it undergoes refraction at the two refracting faces of the prism.
• The extent of deviation depends on the wave-length of light.
• Red deviates the least because of its longer wavelength.
• Violet, which has a shorter wavelength deviates the most.
• The wavelength of other colours lie in between red and violet. Hence their deviation occurs proportionally and is arranged between red and violet.

• The orderly arrangement of the component colours in white light is called the spectrum.
• Light composed of different colours is a composite light.
• A ray of sunlight passing through a water droplet undergoes refraction twice and internal reflection once. This is a natural phenomenon. A rainbow is formed as a result of the combined effect of refraction, dispersion, and internal reflection.

• When white light is passed through a prism, it undergoes refraction and the constituent colours are formed. In the path of the dispersed light arrange an identical prism in an inverted position adjacent to the first. It undergoes refraction in the opposite direction and results in the recombination of colours to produce white light

• Visible light, infrared radiation, ultraviolet radiation do not require a medium to travel. They travel through vacuum at a speed of 300,000 kilometre per second (3 × 10⁸ m/s). Such radiations are electromagnetic radiations.
• The orderly distribution of electromagnetic radiations is known as the electromagnetic spectrum.
• The electromagnetic spectrum is composed of several types of radiation, including radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.
• The increasing order of the wavelengths of the given electromagnetic radiations is:
  Gamma rays < X-rays < Ultraviolet rays < Visible light < Infrared rays < Microwaves < Radio waves
• With red, green and blue lights, we can create not only white light, but also all other colours of light. So, these colours are called primary colours.
• The coloured light formed by combining any two primary colours is a secondary colour of light. Red + Green = Yellow
  Red + Blue = Magenta
  Blue + Green = Cyan
  Red + Green + Blue = White light
• When a secondary colour is combined with a primary colour, we get white light. Such pairs of colours are called complementary colours.

Secondary Colour	Component Colours	Complementary Colour
Yellow	Red + Green	Blue
Magenta	Red + Blue	Green
Cyan	Green + Blue	Red

• When we quickly remove an object from our field of vision the visual experience of that object persists for about 1/16 of a second. This phenomenon is the persistence of vision.
  Eg: When a burning incense stick is whirled very fast, a ring of fire can be seen.
• When complementary colour pairs are combined we get white light. The circular disc appears white due to persistence of vision.
• A filter of secondary colour transmits light of its own colour and its component colours.
• An opaque object of a secondary colour can reflect light of its colour and its component colours.
• A surface that reflects all colours will appear white in white light. We know that a surface that absorbs all colours appears dark.

Scattering of Light

• Scattering is the irregular and partial directional deviation of light when it encounters particles in a medium.
• Violet, indigo and blue colours in sunlight, which have shorter wavelengths, undergo more scattering when they encounter particles in the atmosphere. The scattering of red, having a relatively longer wavelength, is very low. Hence red can travel a longer distance through the atmosphere.
• The extent of scattering and the size of the particles are related to each other. As the size of the particles increases, so does the scattering. If the size of the particles is greater than the wave-length of light, the scattering will be the same for all colours.
• When light rays pass through a colloidal liquid or suspension, they get scattered, causing tiny particles to become illuminated, making the path of light visible. This phenomenon is the Tyndall effect.
• When sunlight travels through the atmosphere to reach the Earth, some of its components undergo scattering as it passes through the air. Scattering occurs most for colours with shorter wavelengths such as violet, indigo and blue. This scattered light spreads in the sky. The resultant scattered light that reaches the observer’s eye gives the effect of the blue colour. So the sky appears blue.
• The sun appears red, yellow, or orange during sunrise and sunset because its light travels through more of the Earth’s atmosphere. As it does, the shorter blue wavelengths scatter more, leaving the longer wavelengths (red, orange, and yellow) for us to see.

Eye and Vision

• The ability of the eye to change the curvature of the lens and adjust the focal length so that the image of the object always falls on the retina, regardless of the position of the object, is the power of accommodation of the eye.
• Some people can see nearby objects clearly but cannot see distant objects. This defect of the eye is short sightedness. It can be rectified using a concave lens with suitable power.
• Some people can see distant objects clearly but cannot see nearby objects clearly. This defect of the eye is long-sightedness. It can be rectified using a convex lens with suitable power.
• For older people, the distance to the near point may be more than 25 cm. This is because the efficiency of the ciliary muscles decreases. Such people have less power of accommodation. This eye defect is presbyopia. It can be rectified using a convex lens with suitable power.
• Light pollution refers to the creation of artificial light in excessive amounts and intensity.

DISPERSION OF LIGHT
Activity

Let’s pass sunlight through a prism instead of laser light.
Using a plane mirror, reflect sunlight onto a white wall. Place a narrow slit in the path of the sunlight so that only a thin beam of light passes through it.
Now you can see a white patch of light on the wall.
Arrange a prism in the path of this beam so that it falls obliquely on one of its sides (Fig. 3.2).
Instead of white light, you can see different colours (as in a rainbow) on the wall. You can see that sunlight splits into different colours.
Activity

Place a small plane mirror slightly inclined in a tray of water as shown in the figure 3.6
Adjust its position so as to reflect sunlight onto a screen.
Instead of white light various colours appear on the screen like that formed in a rainbow. As a result of dispersion of light, the sunlight separates into component colours.
Light composed of different colours is a composite light. Dispersion of light is the phenomenon of splitting up of a composite light into its component colours.

RAINBOW
A rainbow is always formed in a direction opposite to the sun. The Sun is at the west when a rainbow is seen in the east. The Sun is at the east when a rainbow is seen in the west. When you spray fine droplets of water into the air with the sun shining behind you, you will observe a rainbow which is created artificially. A rainbow is not formed when you spray fine droplets of water towards the sun.

RECOMBINATION OF COLOURS OF LIGHT
We get dispersed light from the prism in the experiment as shown in figure 3.2. In the path of the dispersed light arrange an identical prism as shown in figure 3.9.

ELECTROMAGNETIC SPECTRUM
We feel hot when sunlight falls on our body. Sunlight is beneficial to our body.

PRIMARY COLOURS AND SECONDARY COLOURS
Arrange LEDs emitting red, green, and blue light at an angle of 120° on a circular disc. Pass the light from the LEDs through a PVC pipe and project it onto a screen. Set the position of the PVC pipe such that the red, green, and blue colours overlap.

In the region where red, green, and blue colours of the same intensity are combined, we see white light. The region where red and green combine appears as yellow, the part where green and blue combine appears as cyan, and the part where red and blue combine appears as magenta.

PERSISTENCE OF VISION
When a burning incense stick is whirled very fast, a ring of fire can be seen. This is due to the peculiarity of eye called the persistence of vision.

When we quickly remove an object from our field of vision the visual experience of that object persists for about 1/16 of a second. This phenomenon is the persistence of vision.

NEWTON’S COLOUR DISC
The experiment (Fig. 3.9) clearly shows that white light is obtained when the seven colours of sunlight are combined. Newton’s colour disc is a circular disc painted with the colours of sunlight in the same order and proportion.

When Newton’s colour disc is rotated very fast, before the visual experience of any one colour vanishes from the eye, the rays from the succeeding colours reach the eyes in quick succession. Due to the phenomenon of persistence of vision, the combined effect of all the colours persists in our eyes and appears almost white.

COLOUR OF TRANSPARENT OBJECTS
Pass white light through the filters (through transparent objects) given in figure 3.15 and project it onto a white screen.

COLOUR OF OPAQUE OBJECTS
We see an object in the colour of the light that is reflected from the object to our eyes.

SCATTERING OF LIGHT
The schematic diagram shows the scattering of light rays due to their collision with the microscopic particles in the atmosphere (Fig.3.19).

TYNDALL EFFECT
Activity
Take water mixed with chalk powder in a beaker, as shown in figure 3.21. Pass light from a torch through it.

The path of light can be clearly seen due to the scattering of light when it passes through a suspension.
Eg: 1. An example of suspension is water mixed with chalk powder. When light passes through a suspension, the light path can be clearly seen due to scattering.
2. Similarly, in winter, paths of light through the gaps of the branches of trees can be seen clearly due to scattering.

When light rays pass through a colloidal liquid or suspension, they get scattered, causing tiny particles to become illuminated, making the path of light visible. This phenomenon is the Tyndall effect.

The intensity of scattering depends on the size of the particles in the colloid. As the size of the particles increases, the intensity of scattering increases.
Let’s consider some other situations related to scattering.

COLOUR OF SETTING AND RISING SUN

Why does the sun appear red or yellow or orange during sunrise and sunset9 Find out by analysing figure 3.24.

EYE AND VISION
We can see the beautiful sights in nature with the help of our eyes.

How do eyes enable vision?
Take a convex lens of focal length 10 cm. Place a burning candle at a distance of 20 cm away from the lens. Adjust the position of the screen to get a clear image of the flame.
Observe and understand the arrangements used here to form the image.
In the same way, an image of an object is formed in our eyes.

SHORT SIGHTEDNESS / MYOPIA
Some people can see nearby objects clearly but cannot see distant objects. This defect of the eye is short sightedness. The schematic diagram shows the vision of a person with this defect.

LONG SIGHTEDNESS / HYPERMETROPIA
Some people can see distant objects clearly but cannot see nearby objects clearly. This defect of the eye is long-sightedness.
The figures showing the image formation in the eye of a person with long sightedness are given [Fig.3.29 (a), (b)].

A clear image is not formed on the retina of a person with long sightedness.
The near point of a person with long sightedness will be more than 25 cm.