Class 10 Physics Chapter 2 Notes Kerala Syllabus Lenses Questions and Answers

The comprehensive approach in SCERT Kerala Syllabus 10th Standard Physics Textbook Solutions and Class 10 Physics Chapter 2 Lenses Notes Questions and Answers English Medium ensure conceptual clarity.

SSLC Physics Chapter 2 Notes Questions and Answers Pdf Lenses

SCERT Class 10 Physics Chapter 2 Lenses Notes Pdf

SSLC Physics Chapter 2 Questions and Answers – Let’s Assess

Question 1.
The focal length of a convex lens is 20 cm. An object of height 3 cm is located at a distance of 60 cm from its optic centre on the optic axis.
a) Calculate the height of the image.
b) What are the characteristics of the image obtained?
Answer:
a) f = +20 cm, u = -60 cm, h₀ = +3 cm

∴ = -1.5 cm

b) Diminished, inverted and real

Question 2.
The focal length of a lens is 20 cm.
a) An object is placed 30 cm away from the lens.
Calculate how far the screen should be placed to get a clear image.
b) If the height of the object is 1.2 cm, what will be the height of the image appearing on the screen?
Answer:
a) The lens is Convex as the image is obtained on the screen.
f = +20 cm
u = -30 cm
v = \(\frac{u f}{u+f}\) = \(\frac{-30 \times+20}{-30+(+20)}\)
= \(\frac{-600}{-10}\) = 60 cm
Screen should be placed 60 cm away from the lens.

b) h₀ = 1.2 cm
m = \(\frac{v}{u}\) = \(\frac{60}{-30}\) = -2
m = \(\frac{h_i}{h_0}\)
h_i = m × h₀ = -2 × 1.2
h_i = -2.4 cm

Question 3.
The focal length of a convex lens is 100 mm. An object of height 15 mm is located 60 mm from the optic centre on its optic axis.
a) Draw its ray diagram on a graph paper and find the position and height of the image.
b) Calculate the magnification if the distance to the object is 20 mm.
Answer:
a) f = +100 mm = 10 cm
h₀ = +15 mm = 1.5 cm
u = -60 mm = -6 cm

h_i – m × h₀ – 2.5 × 1.5
h_i = 3.75 cm
u = -20 mm = -2 cm

b) f = 10 mm = 10cm
v = \(\frac{u f}{u+f}\) = \(\frac{-2 \times 10}{-2+10}\) = \(\frac{-20}{8}\) = -2.5 cm
m = \(\frac{v}{u}\) = \(\frac{-2.5}{-2}\) = 1.25

Question 4.
Four statements are given regarding the image formed by a concave lens. Find and choose the correct answer.
i. It will be diminished and inverted
ii. It will be diminished and virtual
iii. It will be magnified and virtual
iv. It will be diminished and erect
a) Only the second statement is true
b) Only the first statement is true
c) Second statement and fourth statements are true
d) Only the third statement is true
Answer:
c) Second statement and fourth statements are true

Question 5.
A concave lens has a focal length of 50 cm. What will be its power?
a) +2 D
b) +0.5 D
c) -2D
d) -0.5 D
Answer:
c) -2 D
f = -50 cm = \(\frac{-50}{100}\)m
P = \(\frac{1}{f}\) = \(\frac{1}{-\frac{50}{100}}\) = \(\frac{-100}{50}\) = -2D

Question 6.
Find the most appropriate statement related to a telescope.
a) The objective lens has a shorter focal length and the eyepiece lens has a longer focal length.
b) The objective lens has a longer focal length and the eyepiece has a shorter focal length.
c) Objective lens and eyepiece lens are concave lenses.
d) Objective lens will be concave lens and eyepiece lens will be convex lens.
Answer:
b) The objective lens has a longer focal length and the eyepiece has a shorter focal length.

Question 7.
When an object is placed in front of a lens, the image formed is inverted.
a) Is it real or virtual?
b) What will you do if you want another image of this obtained image to be real, erect and of the same size?
Answer:
a) Real
b) Arrange such that this image is formed at 2F of another convex lens.

Question 8.
When an object is placed at the principal focus of a lens, an image that is erect and diminished is obtained.
a) What kind of lens is this?
b) Draw the ray diagram of the image formation.
Answer:
a) Convex lens

Question 9.
The image (IM) obtained when an object is placed in front of a lens is depicted.

a) If PQ is a lens in the figure, what type of lens does PQ represent?
b) Complete the ray diagram and find the position of the object.
c) The height of the object is …………………… than the height of the image (greater/lesser).
Answer:
a) Convex lens
b)
c) greater

Question 10.
Match the items in the columns A, B and C appropriately.

A	B	C
Magnification	1/f	hi negative
Power of lens	Inverted image	v/u
Real image	hi/ho	hi positive
	Erect image	Dioptre

Answer:

A	B	C
Magnification	hi/ho	v/u
Power of lens	1/f	Dioptre
Real image	Inverted image	hi negative

Physics Class 10 Chapter 2 Notes Kerala Syllabus Lenses

Observe figure 2.1. Have you noticed older people using reading lens to make letters appear larger?

Question 1.
Where else are lenses used? Write them down.
Answer:

• Toys
• Spectacles
• Door lens (lens fixed on the door to view the outside)
• Telescope
• Microscope
• Eye piece used to repair watches
• Binoculars
• Camera

What makes these lenses different from a sheet of glass?
Let’s examine.

Reading lens and sheet of glass
Question 2.
Allow sunlight to fall on a paper through a thin sheet of glass. What is observed?

Answer:
The part of the paper on which light falls get illuminated.

Question 3.
Vary the distance between the paper and the glass sheet. What do you observe?
Answer:
It is seen that the size of the illuminated area does not change whether the glass sheet is near or far from the paper.

Question 4.
Do the same activity using reading lens.What is your observation?

Answer:
When the lens is held at a specific distance from the paper, the size of the illuminated area is greatly reduced and the intensity of light at that area increases.

When the lens is held at that point for a longer time, we can see the paper smouldering and catching fire.

Question 5.
What feature does the lens have that the glass sheet doesn’t?
Answer:
Reading lenses can converge light rays.

Convex lens and Concave lens
Question 6.
Observe the lens used in the previous activity and note down its characteristics.

Answer:

• Thicker in the middle
• Shows the objects magnified
• Edges are thinner

The lens you are now familiar with is called convex lens. It is understood that such lenses can converge light rays

Question 7.
Observe another type of lens (Fig 2.4). What are its features?

Answer:

• Thinner in the middle
• Edges are thicker
• Shows the objects as diminished

Question 8.
Try to bum a piece of paper with such lenses. Is it possible?
Answer:
Concave lenses cannot converge light rays. It is not possible to bum a piece of paper using these lenses as they diverge light rays.

It is understood that this type of lenses cannot converge light rays. Such lenses are called concave lenses.

Question 9.
List the characteristics of concave lenses and convex lenses in the table.
Answer:

Convex lens	Concave lens
Thicker in the middle	Thinner in the middle
Thinner at the edges	Thicker at the edges
Shows the objects magnified	Shows the object as diminished

Question 10.
Observe the letters through each lens and move the lens to one side. What is the observation?
Answer:

• When a convex lens is used, the letters appear to move in the opposite direction.
• When a concave lens is used,the letters appear to move in the same direction.
  This activity can be used as a method to distinguish between convex and concave lenses.

Question 11.
Observe figures 2.5 (a) and 2.5 (b).

How many surfaces does a lens have? How many refracting surfaces does it have?
Answer:
Each lens has two surfaces.
When light passes through them, refraction occurs. That means a lens has two refracting surfaces.

Question 12.
Refracting surfaces of a lens are parts of (spheres / circles)
Answer:
Spheres
A lens is a transparent medium in which each refracting surface is part of the spheres.

Terms related to lenses

• Optic centre: The midpoint of a lens is the optic centre (O).
• Centres of curvature: Each refracting surface of lens is part of a sphere. The centres of such spheres are the centres of curvature.
• Optic axis: The optic axis is the imaginary line passing through the centres of curvature and the optic centre of a lens.
• Aperture: The area of the lens through which light passes is called aperture. In optical instruments such as cameras and microscopes, the aperture can be varied by using the stop.

Observe figures 2.6 (a) and 2.6 (b).
Question 13.
Which figure represents convex lens? And which one represents concave lens?
Answer:
Figure 2.6(a) represents convex lens. Figure 2.6(b) represents concave lens

Question 14.
What do C, and C, indicate?
Answer:
C₁, C₂ indicates centres of curvatures

Question 15.
Which refers to the optic centre? (C₁, O, C₂)
Answer:
O

Question 16.
Which represents the optic axis?
Answer:
C₁ OC₂

Question 17.
Depict the path of light.

Answer:
There is a change in the direction of light rays parallel to the optic axis of the convex lens as they pass through the lens. (It is to be noted that all light rays coming from a source or reflected from an object may not be parallel. But here we are considering only parallel rays). Light rays converge to a point on the other side of the lens after refraction. This point is the principal focus of the convex lens.

Light rays near and parallel to the optic axis incident on a convex lens, after refraction converge at a point on the optic axis on the other side of the lens. This point is the principal focus (F) of a convex lens.

Question 18.
Repeat the experiment shown in figure 2.7 by passing the light through the opposite hole. Didn’t the light rays converge in this case too?
Answer:
Yes. The light rays converges in this case too.

Question 19.
How many principal foci does a convex lens have ?
Answer:
Convex lenses have two principal foci, one on each side of the lens. These foci are equidistant from the optic centre.

Question 20.
The principal focus of a convex lens is considered real. Why ?
Answer:
The principal focus of a convex lens is considered real because light rays passing parallel to the optic axis of a convex lens pass through the principal focus, after refraction.

Question 21.
Write an activity to find the principal focus of concave lens.
Answer:
Smoke box experiment

Procedure:
Fix the concave lens on the stand inside the box. Fill the box with smoke. Allow the light beam from laser to fall on the lens through the hole.

Observation
The light rays passing near and parallel to the optic axis of a concave lens, appears to diverge from a point on the optic axis on the same side, after refraction. This is the principal focus of concave lens. Concave lens diverges the light rays.

Question 22.
Draw the path of the ray of light.
Answer:

There is a change in direction of the light rays passing parallel to the optic axis through the concave lens. Here the refracted rays appear to diverge from a point [Fig. 2.10 (b)]. This point is the principal focus of the concave lens. A concave lens diverges the rays of light.

Light rays, near and parallel to the optic axis incident on a concave lens, after refraction appear to diverge from a point on the optic axis on the same side of the lens. This point is the principal focus of a concave lens (F).
Repeat the experiment by passing light through the other side of the concave lens.

Question 23.
What is your observation?
Answer:
The light rays diverges

Question 24.
Haven’t you understood that the concave lens also has two principal foci?
Answer:
Concave lens has two principal foci on both of its sides.These foci are equidistant from the optic centre.

Question 25.
Do refracted rays pass through the principal focus of a concave lens?
Answer:
No

Question 26.
If so, is the principal focus of a concave lens considered virtual or real?
Answer:
Light rays, near and parallel to the optic axis incident on a concave lens, after refraction appear to diverge from a point on the optic axis on the same side of the lens. Refracted rays do not pass through the principal focus of a concave lens. Therefore the principal focus of a concave lens is considered virtual.

Image Formation by Lenses
Question 27.
Project the image of a window onto a screen using a convex lens as shown in figure 2.11 .Write down the results of the observation in the science diary.

Answer:
Image can be formed on the screen.
Characteristics of image formed diminished, real and inverted.

Question 28.
Try to form an image using a concave lens. Is it possible?
Answer:
It is not possible to form an image on the screen using a concave lens.

Question 29.
Which lens was used to form an image on the screen?
Answer:
Convex lens
Images that can be projected on a screen are real images.

Question 30.
Write down examples for real images.
Answer:

• Image that is captured on a camera
• Image that is formed on a cinema screen
• Image formed on retina of the eyes.

Question 31.
Record the positions and properties of the image in the table by placing the object at various positions in table 2.2.

Answer:

Position of the object	Position of the image	Characteristics of the image
Beyond 2F	Between F and 2F	Diminished, inverted, real
At 2F	At 2F	Same size, inverted, real
Between F and 2F	Beyond 2F	Magnified, inverted, real
At F	At infinity (Far away)	Magnified, inverted, real
Between F and lens	Between F and 2F at the same side of the object	Magnified, virtual, erect

Rag Diagram of the Image Formation by a Convex Lens
Question 32.
Observe figure 2.13 (a) (b) and (c). Write down in table 2.3 the details of the path of light rays passing through the convex lens through different paths from point A.


Answer:

A ray of light from a point which is parallel to the optic axis incident on a convex lens	Passes through the principal focus on the other side.
A ray of light passing through the optic centre	Passes undeviated along the straight line without undergoing refraction
A ray of light passing through the principal focus on same side of the object incident on a convex lens	Passes parallel to the optic axis after refraction

A ray of light coming from a point on an object and passing through any point of the lens follows the laws of refraction.

Question 33.
What are your observations from figure 2.14?
Answer:
We can see that the light rays which have taken different paths pass through a single point Hence the image of A is formed at D. Similarly, the image of any point on the object is formed at the point of convergence of refracted light rays originating from the corresponding points.

If a screen is placed at the point of convergence of the refracted rays, the image is formed there.

Question 34.
Complete the ray diagram ofthe formation of image when the object is placed at different positions. Find the position and characteristics of the images.
Answer:
(i) Object beyond 2F

• Position of the image : Between F and 2 F on the other side
• Characteristics of the image : Inverted, diminished, real

(ii) Object at 2F

• Position of the image :At 2F on the other side
• Characteristics of the image :Same size, inverted, real

(iii) Object Between F and 2F

• Position of the image: Beyond 2F on the other side
• Characteristics of the image: Magnified, inverted, real

(iv) Object at F
a) Draw the ray diagram of the image formation.
Answer:

b) Do refracted rays converge?
Answer:
Refracted rays do not converge

c) What would be the characteristics of the image?
Answer:
Magnified, inverted, real
A convex lens does not form real images always.

(v) Object between F and the Lens
Observe the ray diagram of the image formation when the object is placed between the focus (F) and the optic centre (O).

• Position of the image: On the same side of the object
• Characteristics of the image : Erect, Virtual, Magnified.

Question 35.
Here, do the light rays coming from the object and passing through the lens pass through a common point?
Answer:
Light rays coming from the object and passing through the lens do not pass through a common point.

The light rays coming from the object is diverging. When we observe this object from the other side of the lens, we can see the magnified image of the object.

Thus the image that cannot be obtained on the screen, but can only be seen, is virtual.

Images that cannot be captured on a screen, but can only be seen are virtual images.

Ray diagram of image formation by concave lens

Question 36.
Complete table 2.4 by observing the change in the path of light as it passes through a concave lens.

Answer:

Rays of light parallel to the optic axis incident on a concave lens (after refraction)	The rays appear to emerge from the principal focus on the same side of the light source and appear to move away from the optic axis
Light rays passing through the lens directed towards the principal focus on the other side (after refraction)	Passes parallel to the optic axis
Rays of light passing through the optic centre	Passes undeviated along the straight line without undergoing refraction

Characteristics of image formed at different positions of a concave lens

Question 37.
Draw the ray diagram of the image formation for
a) Object between F and 2F
b) Object between F and Lens
Answer:
a)

• Position of the image : Same side of the object between F and lens
• Characteristics of the image : Virtual, erect, diminished

b)

• Position of the image : Same side of the object between F and lens
• Characteristics of the image: Virtual, erect, diminished

Question 38.
Complete the given table based on the image formation by a concave lens.

Answer:

Question 39.
On analysing table 2.5, it is understood that the image formed by a concave lens is virtual. What could be the reason for this?
Answer:
As concave lens diverges light rays, the image it forms is always virtual. The position of the image is always between F and the lens on the same side of the object.

We can calculate how far the image from the lens will be, if an object is placed at a given distance.For this we make- use of the lens equation.

LENS EQUATION
With regard to the image formation by a lens, we consider the focal length, the distance from the optic centre to the object and to the image. Observe these distances marked in the figure.

Question 40.
Which letter indicates the distance to the object (OB) in the figure?
Answer:
u indicates distance to the object

Question 41.
Which distance does the letter v represent in the figure?
Answer:
v indicates distance to the image

Question 42.
Which distance does the letter f stand for?
Answer:
f indicates focal length

As the lenses and positions of the object change, appropriate signs for the measurements related to the lens should be considered.

Cartesian sign convention
While solving mathematical problems related to lens, appropriate signs should be given for the measurements. These rules can be used in lenses in general.

• All distances should be measured from the optic centre of the lens.
• Distances measured in the same direction as the incident ray should be considered positive and those in the opposite direction should be considered negative.
• Distances measured above the optic axis should be considered positive and those below should be considered negative.
• Cartesian sign convention can be used to solve mathematical problems using general equations in different contexts. There is no need to consider whether the object is on the left or right side of the lens.

Question 43.
Observe figures 2.22 (a) and (b) and complete table 2.6 based on Cartesian sign convention.


Answer:

Lens equation
The position of the image formed by a lens is determined by the position of the object and the focal length of the lens. The relation between them is made clear by the lens equation.
\(\frac{1}{f}\) = \(\frac{1}{v}\) – \(\frac{1}{u}\)
f = focal length; u = distance to the object;
v – distance to the image
This equation can also be written as
f = \(\frac{u v}{u-v}\)
We can also use the formula for u and v
u = \(\frac{f v}{f-v}\)
v = \(\frac{f u}{u+f}\)

Question 44.
Observe the distance to the object and the distance to the image depicted in the figure.

a) Write down the measurements using the sign conventions.
b) Calculate the focal length of the lens.
Answer:
a) Distance to the object, u = – 90 cm (measurement in a direction opposite to that of the incident ray)
Distance to the image, v = +30 (measurement in the same direction as that of the incident ray)

b) f = \(\frac{u v}{u-v}\)
f = \(\frac{-90 \mathrm{~cm} \times+30 \mathrm{~cm}}{-90 \mathrm{~cm}-30 \mathrm{~cm}}\)
= \(\frac{-2700 \mathrm{~cm}^2}{-120 \mathrm{~cm}}\) = +22.5 cm
Since the focal length is positive, it can be understood that the principal focus is real and the lens used here is convex.

It is possible to find how many times the size of the image is to the size of the object by studying magnification.

Magnification
Magnification refers to how many times the height of the object is to the height of the image.

Magnification is the ratio of the height of the image to the height of the object. It has no unit.

Magnification = \(\frac{\text { Height of the image }}{\text { Height of the object }}\) = \(\frac{\mathrm{h}_i}{\mathrm{~h}_0}\)
OR
Magnification = \(\frac{\text { Distance to the image }}{\text { Distance to the object }}\) = \(\frac{v}{u}\)
m = \(\frac{\mathrm{h}_i}{\mathrm{~h}_0}\) = \(\frac{v}{u}\)

According to Cartesian sign convention, it can be understood that the image is erect if magnification is positive and the image is inverted if magnification is negative.

Question 45.
Calculate the magnification using the measurements in the figure 2.23 and write down the characteristics.
Answer:
m = \(\frac{\mathrm{h}_i}{\mathrm{~h}_0}\) or m = \(\frac{v}{u}\)
u = -90 cm ,h_O = +1.8 cm ,v = +30 cm, h_i = -0.6 cm
m = \(\frac{\mathrm{h}_i}{\mathrm{~h}_0}\) = \(\frac{-0.6}{+1.8}\) = – \(\frac{1}{3}\)
m = \(\frac{v}{u}\) = \(\frac{+30}{-90}\) = –\(\frac{1}{3}\)

Since magnification is less than one, it can be understood that the image is smaller than the object.
The negative sign of the magnification indicates that the image is inverted and real.

Question 46.
Complete the table 2.7 by considering the relation of magnification with the nature of the image.
Answer:

Nature of the image	Sign of magnification (positive / negative)
Erect	Positive
Inverted	Negative
Real	Negative
Virtual	positive

Question 47.
An object of height 2 cm is placed on the optic axis at a distance of 12 cm from the optic centre of a convex lens. Focal length of the convex lens is 6 cm.
a) Draw a ray diagram based on the given measurements and write down the characteristics of the image.
b) Calculate the magnification by measuring the height of the image.
Answer:

Position of image: At 2F on the other side of the lens
Characteristics of image: same size, inverted, real,

b) Height of object = 2 cm
Height of image = -2 cm (Since image formed is of same size as that of object)
Magnification \(\frac{\mathrm{h}_i}{\mathrm{~h}_0}\) = \(\frac{-2}{2}\) = –\(\frac{1}{3}\) = -1

Question 48.
A concave lens has a focal length of 20 cm . An object of height 2 cm is placed at a distance 30 cm from the lens on the optic axis.
a) Calculate the distance from the lens to the image.
b) How much will the magnification be? What are the characteristics of the image?
Answer:
a) f = -20 cm
h₀ = 2 cm
u = -30 cm

b) Magnification m = \(\frac{v}{u}\) = –\(\frac{12}{-30}\) = 0.4
m = 0.4
The image formed is diminished,virtual and erect

Question 49.
Which are the instruments where lenses are used?
Answer:
Some of the instruments that make use of these are : Spectacles, simple microscope, compound microscope and telescope.

Question 50.
Have you noticed the prescriptions given by doctors to buy spectacles? Can you identify what is written on it? What does +2.00 refer to?

Answer:
Yes.
It refers to the power of the lens in the spectacles.

Question 51.
What is the power of a concave lens of focal length 25 cm?
Answer:
Focal length of concave lens = – 25 cm
Since the focal length is considered in metre while calculating the power of the lens in the SI unit,
f = \(\frac{-25}{100}\)m = -0.25 m
Power P = \(\frac{1}{f}\)
P = \(\frac{1}{-0.25 \mathrm{~m}}\) = -4D
If the power is negative, it is identified as concave lens.

Question 52.
If it is a convex lens ,what will be the sign of the power?
Answer:
For convex lens the sign of power is positive

Question 53.
What type of lens is in the doctor’s prescription (Fig 2.24)?
Answer:
Convex lens
Let’s take a look at some of the devices that use lenses.

Compound microscope

Question 54.
What is the use of a compound microscope?
Answer:
They magnify objects
The two main parts of a compound microscope are objective and eyepiece.

Objective:
An objective is a lens placed close to the object to be observed.

Eyepiece:
Eyepiece is the lens through which the image formed by the objective lens is observed. The focal length of the eyepiece is greater than that of the objective.

Question 55.
Observe figure 2.25 (b) and complete table 2.8 identifying the characteristics of lenses used in compound microscope.

Answer:

Question 56.
Where should the object to be observed be kept with reference to the objective? (beyond 2F₀ / between F₀ and 2F₀)
Answer:
between F₀ and 2F₀

Question 57.
What is the position of the image formed by the objective?
Answer:
Forms beyond 2F₀ of objective. The position of this image is between the optic center and F_E of the eyepiece.

Question 58.
What are the characteristics of this image?
Answer:
Magnified, inverted and real

Question 59.
What would be the characteristics of the image formed by the eye piece?
Answer:
Magnified, erect and virtual

The object should be placed between F₀ and 2F₀ of the objective. A large, real, inverted image of the object is formed beyond 2F₀ of the objective. This acts as the object for the eyepiece. Its position is between the optic centre and F_E of the eyepiece. A large and virtual image of this can be seen through the eyepiece.

Question 60.
Increasing the focal length of the objective lens will not be beneficial in the compound microscope. What is the reason?
Answer:
If the focal length of the objective lens is longer, the size of image will be smaller. That means the magnification will be lesser. So the objective lens should have a shorter focal length.

A microscope is usually provided with a range of objective lenses to obtain suitable magnification while observing micro objects.

Refracting Telescope
Telescopes are instruments to see distant objects clearly. The invention of the telescope brought about a great change in the study of the universe. There are different types of telescopes that make use of reflection and refraction of light. Let us see the functioning of a telescope whose working is based on the refraction of light.

Question 61.
Observe figures 2.26 (a) and 2.26 (b).
The main parts of the telescope are objective and eyepiece. Identify their characteristics and complete the table given below.

Answer:

Let’s see how the image is formed in the telescope.
Question 62.
Where, is the position of the object?
(faraway / nearby)
Answer:
faraway

Question 63.
Focal length of the objective is (lesser / greater)
Answer:
greater

Question 64.
What are the characteristics of the image formed by the objective? (small and real / large and virtual)
Answer:
small and real

Question 65.
Which of the lenses use this image as its object? (objective / eyepiece)
Answer:
eyepiece

Question 66.
Through which lens is the image viewed? (objective / eyepiece)
Answer:
eyepiece

Question 67.
The image we see through the eyepiece is (real / virtual)
Answer:
virtual

In a telescope, the objective forms a small, real and inverted image of a distant object.

It is the image formed by the objective that we observe through the eyepiece. Since the position of this image is between the focus of the eyepiece and the optic centre, we can see the virtual image formed by the eyepiece.

It is clear that while making a telescope, the length of the telescope tube should be taken by considering the focal length of the objective lens and the focal length of the eyepiece lens.

Question 68.
Why is it said’not to look at the sun through a telescope? Search and find out.
Answer:
When we look at the sun through a telescope, the sun rays get concentrated at the retina of the eye. It cause bums.To avoid this, it is said not to look at the sun with a telescope.

Std 10 Physics Chapter 2 Notes – Extended Activities

Question 1.
You may know people who use spectacles for various purposes. Collect, tabulate and analyse information regarding the type of lens used in different types of spectacles, the power of lens, age of the users and the problems faced by them.
Answer:
Steps

• Find the people who use spectacles
• Collect information from them, regarding the eye defect they are suffering from,their age,the lens being used in their spectacles,power of the lens
• Tabulate the collected information as shown in the example given below.

Age	Lens Type	Power	Reason	Other Problems
15	Single Vision (concave lens)	-2.00	Nearsightedness	Blurry distant vision
34	Bifocal	-1.50, +2.00	Reading & Nearsightedness	Trouble reading close
65	Single Vision(Convex lens)	+1.50	Farsightedness	Eye strain
42	Single Vision(concave lens)	-1.00	Nearsightedness	No problems

Analysis

• Most-of the people of young age use single vision lens. They are mostly suffering from nearsightedness.
• More People who are aged need bifocal or progressive glasses. They are mostly suffering from presbyopia and farsightedness. People with bifocals or progressives might have trouble adjusting.

Conclusion:
Older people often need special glasses for near and far vision. People with high power glasses feel more discomfort and most common problems due to the use of glasses is blurry vision and eye strain.

Question 2.
Collect a transparent polythene bag. Fill it with water and tie to get it almost in the shape of a sphere. Use it as a convex lens to form various sized images of a burning candle.
Answer:
An example of an activity to make use of water filled polythene bag as a lens and fonnation of various sized images is given below.
Steps
Water-filled Polythene Bag as a Lens

• Fill a clear plastic bag with water and tie it so that it takes the shape of a ball or sphere.
• This bag acts like a lens.

Image formation

• When you hold it in front of a candle, the light passing through the bag bends in such a way that it forms an image of the candle.
• Place a white paper which acts like a screen to capture the image
• Place the candle at different positions to obtain image of varied size and nature.
• Record the position and nature of the image obtained.

Lenses Class 10 Notes

Lenses Notes Pdf

• A lens is a transparent medium in which each refracting surface is part of the spheres.
• The lens which is thicker in the middle and thinner at edges is the convex lens. It Shows the objects magnified. These lenses converge light rays.
• The lens which is thinner in the middle and thicker at edges is the concave lens.lt Shows the objects as diminished. These lenses diverge light rays.
• Optic centre: The midpoint of a lens is the optic centre (O).
• Centres of curvature: Each refracting surface of a lens is part of a sphere. The centres of such spheres are the centres of curvature.
• Optic axis: The optic axis is the imaginary line passing through the centres of curvature and the optic centre of a lens.
• Aperture: The area of the lens through which light passes is called aperture. In optical instruments such as cameras and microscopes, the aperture can be varied by using the stop.
• Light rays near and parallel to the optic axis incident on a convex lens, after refraction converge at a point on the optic axis on the other side of the lens. This point is the principal focus (F) of a convex lens.
• The focal length (f) is the distance from the optic centre of the lens to the principal focus.
• Light rays, near and parallel to the optic axis incident on a concave lens, after refraction appear to diverge from a point on the optic axis on the same side of the lens. This point is the principal focus of a concave lens (F).
• Images that can be projected on a screen are real images.
• Images that cannot be captured on a screen, but can only be seen are virtual images.
• A convex lens can form diminished ,same sized and magnified images based on the position of the object.
• A concave lens always forms virtual, erect and diminished image

INTRODUCTION

Lenses are transparent optical devices that refract light to converge or diverge it, allowing for the formation of images. They are crucial components in various optical systems, such as cameras, eyeglasses, microscopes, and telescopes. Lenses come in different types, including convex and concave, each with distinct properties affecting the way light is focused. Understanding lens characteristics and their applications is fundamental in optics and imaging technology. This chapter deals with the topics like Convex and concave lens, Image formation in concave and convex lens, Lens equation, Power of lens, Compound microscope and refracting telescope.

Lens

• Each lens has two surfaces. When light passes through them, refraction occurs. That means a lens has two refracting surfaces.
• A lens is a transparent medium in which each refracting surface is part of the spheres.

Convex lens
• The lens which is thicker in the middle and thinner at edges is the convex lens .It Shows the objects magnified. These lenses converge light rays.

Concave lens
• The lens which is thinner in the middle and thicker at edges is the concave lens .It Shows the objects as diminished. These lenses diverge light rays.

Terms related to lens

• Optic centre: The midpoint of a lens is the optic centre (O).
• Centres of curvature: Each refracting surface of lens is part of a sphere. The centres of such spheres are the centres of curvature.
• Optic axis: The optic axis is the imaginary line passing through the centres of curvature and the optic centre of a lens.
• Aperture: The area of the lens through which light passes is called aperture. In optical instruments such as cameras and microscopes, the aperture can be varied by using the stop.
• Light rays near and parallel to the optic axis incident on a convex lens, after refraction converge at a point on the optic axis on the other side of the lens. This point is the principal focus (F) of a convex lens.
• The focal length (f) is the distance from the optic centre of the lens to the principal focus.
• Light rays, near and parallel to the optic axis incident on a concave lens, after refraction appear to diverge from a point on the optic axis on the same side of the lens. This point is the principal focus of a concave lens (F).

Image Formation by Lenses

• Images that can be projected on a screen are real images.
• Images that cannot be captured on a screen, but can only be seen are virtual images.
• Image formation by a convex lens

Position of the object	Position of the image	Characteristics of the image
Beyond 2F	Between F and 2F	Diminished, inverted, real
At2F	At 2F	Same size, inverted, real
Between F and 2F	Beyond 2F	Magnified, inverted, real
At F	At infinity (Far away)	Magnified, inverted, real
Between F and lens	Between F and 2F at the same side of the object	Magnified, virtual, erect

Image formation by a concave lens

Lens equation
The position of the image formed by a lens is determined by the position of the object and the focal length of the lens. The relation between them is made clear by the lens equation.
\(\frac{1}{f}\) = \(\frac{1}{v}\) = \(\frac{1}{u}\)
f = focal length; u = distance to the object; v = distance to the image
This equation can also be written as
f = \(\frac{u v}{u-v}\)

Cartesian sign convention
While solving mathematical problems related to lens, appropriate signs should be given for the measurements.
These rules can be used in lenses in general.

• All distances should be measured from the optic centre of the lens.
• Distances measured in the same direction as the incident ray should be considered positive and those in the opposite direction should be considered negative.
• Distances measured above the optic axis should be considered positive and those below should be considered negative.

Magnification

• It is the ratio of the height of the image to the height of the object. It has no unit.
• Magnification = \(\frac{\text { Height of the image }}{\text { Height of the object }}\) = \(\frac{\mathrm{h}_i}{\mathrm{~h}_\theta}\)
• Magnification = \(\frac{\text { Distance to the image }}{\text { Distance to the object }}\) = \(\frac{v}{u}\)

Power of tens

• Power is the reciprocal of focal length. The lower the focal length, the higher the power of the lens.
  Power P = \(\frac{1}{f}\)
• The SI unit of power is dioptre. It is denoted by the letter D.
• The power of a lens with a focal length of one metre is one dioptre (1 D).

Compound microscope

• Used to magnify objects.
• Its main parts are objective and eye piece

Refracting telescope

• To see distant objects clearly.
• Its main parts are objective and eye piece.

Activity to find Principal foci of lenses

Materials required
A box about 50 cm length, 30 cm width and 20 cm height, transparent on one side (a small hole should be made on the two opposite sides of the box and the holes should be sealed with a transparent sheet), laser torch (high beam type), incense stick, match box, convex lens, concave lens and lens stand.

Procedure
After fixing the convex lens on the stand inside the box, fill the box with smoke. Allow light rays from the laser torch to pass through the hole and allow it to fall on the lens as shown in the figure.

Observation
The ray of light passing near and parallel to the principal axis of a convex lens, converges at a point on the optic axis on the other side of the lens ,after refraction.

A method to find the approximate focal length of a convex lens

The distant object method can be used. Project the image of a distant tree or a building onto a screen using a convex lens. Measure the distance between the lens and the image using a scale. This distance is the approximate focal length of that lens.

Focal length
The focal length (f) is the distance from the optic centre of the lens to the principal focus.

Image Formation by a Convex Lens
Activity
Materials required : Light source, the convex lens with pre-determined focal length, the lens stand, screen

Procedure
Arrange a light source, the convex lens with pre-determined focal length, the lens stand and the screen as shown in the figure. Measure and mark F and 2F on the experiment table on both sides of the lens. Here we have to get the image of the object on the screen. Hence the light source is used as the object in this experiment.

First place the object (light source) beyond 2F and adjust the position of the screen to get a clear image.Then place the object at 2F .between 2F and F, at F and between F and lens. Observe the features of the image for varied positions and record them

Ray Diagram of the Image Formation by a Convex Lens
Let’s draw the path of light rays from an object placed in front of a convex lens as they pass through the lens.

Image Formation by a Concave Lens
We have seen how to form an image using a convex lens. Similarly try to form an image using a concave lens.

POWER OF LENS
Power is a term related to the focal length of the lens. The power of a lens is its ability to converge or diverge light rays incident on it.
Power is the reciprocal of focal length. The lower the focal length, the higher the power of the lens.
Power P = \(\frac{1}{f}\)
The SI unit of power is dioptre. It is denoted by the letter D.
The power of a lens with a focal length of one metre is one dioptre (1 D).

Making a Telescope
Materials required
Approximately 1 m long PVC pipe (having a diameter suitable enough to fix the lens), convex lens (of diameter and focal length ), plastic bottle, eyepiece used for watch repair.

Things to be considered while choosing lenses :

• The focal length and aperture of the objective lens should be greater.
• The focal length and aperture of the eyepiece lens should be lesser.
• Use high quality lenses.

Method of Construction
Fix a convex lens of approximately 10 cm diameter and focal length 100 cm at one end of a PVC pipe having approximately 10 cm diameter. Cut the bottom of a plastic bottle of two litre and insert it into the other end of the pipe. Insert and fix the eyepiece (used for watch repair) at the mouth of the plastic bottle. Distant objects can be observed by adjusting the distance between the eye piece and the objective by pushing or pulling the plastic bottle.

Special Attention
Do not look at the sun through a telescope. It is preferable to fix a telescope on a stand while observing other celestial bodies.