The comprehensive approach in Kerala Syllabus 9th Standard Physics Textbook Solutions Chapter 1 Refraction of Light Notes Questions and Answers English Medium ensures conceptual clarity.
Std 9 Physics Chapter 1 Notes Solutions Refraction of Light
SCERT Class 9 Physics Chapter 1 Notes Solutions Kerala Syllabus Refraction of Light Questions and Answers
Class 9 Physics Chapter 1 Let Us Assess Answers Refraction of Light
Question 1.
Ray diagram showing the path of light through mediums A and B is given.
a) In which medium will the speed of light be less A or B ?
b) Which will be the optically denser medium? Justify your answer.
Answer:
a) Speed of light will be less in medium B.
b) Bis the optically denser medium. Here, when light enters from A to B, the light ray deviates towards the normal. It happens when light enters from an optically rarer medium to a denser medium. So B will be the optically denser medium.
Question 2.
Complete the given diagram. Mark the angle of incidence and angle of refraction.
Answer:
Question 3.
Light passes from medium X to Y. Here, the angle of refraction is greater than the angle of incidence.
a) In which medium is the speed of light more?
b) Which is the medium of larger refractive index?
c) Draw the path of light.
Answer:
a) Y
b) X
c) 
Question 4.
Refractive index of different mediums are given in the table.
| Medium | Refractive index |
| Crown glass | 1.52 |
| Glycerine | 1.47 |
| Sunflower oil | 1.47 |
| Water | 1.33 |
| Flint glass | 1.62 |
a) In which medium does light travel with maximum speed?
b) Will a ray of light entering obliquely from glycerine to sunflower oil deviate? Explain the reason.
c) Light is transmitted from glass to each medium listed in table above If light is incident at an angle of 30°, which medium will have the largest angle of refraction? Why?
Answer:
a) In water
b) No, As speed of light is same in both these mediums, (as refractive index of both mediums are same) light does not undergo refraction.
c) In water. Among these, water has least refractive index.
Question 5.
Observe the figure. Ray of light incident on two mediums is depicted.
a) Which is the medium of higher optical density? Why?
b) Which is the medium of greater refractive index?
Answer:
a) Medium 1 is the medium of higher optical density.
The angle of refraction in medium 1 is less than that in medium 2. That is when light enters from air to medium 1, the refracted ray bends more towards the normal.
b) Medium 1 has greater refractive index. Observe the figure.
Question 6.
Observe the figure.
a) Which figure indicates total internal reflection?
b) Which figures indicate refraction?
Answer:
a) Fig (i)
b) Fig(ii) and Fig (iii)
Question 7.
The critical angle of glass is 42o. Choose the angle of incidence for which total internal reflection takes place.
a) 40°
b) 49°
c) 38°
d) 42°
Answer:
49°
Question 8.
We can see many small prisms in the reflectors used in motorcycles. Describe the benefits of using them.
Answer:
Many small prisms are used in reflectors used in motor cycles. Light falling on the prisms undergoes total internal reflection and reflects back without loosing the intensity of light. The image can be seen with greater visual clarity.
Question 9.
Observe the table.
| Medium | Refractive index |
| Air | 1.0003 |
| Kerosene | 1.33 |
| Turpentine oil | 1.44 |
| Crown glass | 1.47 |
| Diamond | 1.52 |
a) Choose the medium in which light has the least speed.
b) The speed of light in air is 3 × 108 m/s. What is the speed of light in kerosene?
c) When a ray of light enters obliquely from air to diamond, will the refracted ray deviate towards the normal, or away from the normal? Justify your answer.
Answer:
a) Diamond
c) The refracted ray deviates towards the normal. Diamond has greatest optical density. As light enters obliquely from air to diamond, it deviates towards the normal.
Question 10.
The path of light through mediums A, B, C and D are given. Choose the correct figures. (The optical density of the mediums are in the order A<B<C<D.)
Answer:
Fig(b), (d), (e)
Question 11.
Speed of light in ethanol is lesser than that in methanol. Which medium has lower refractive index? Why?
Answer:
Methanol has lower refractive index. Speed of light in methanol is higher than that in ethanol.
Class 9 Physics Chapter 1 Extended Activities Answers Refraction of Light
Question 1.
Make periscopes using prisms instead of mirrors. Exhibit them.
Answer:
Hint:
Periscopes are devices used to see over or around objects by using a series of mirrors or prisms. Usually, periscopes use mirrors angled at 45 degrees, but prisms can also be used.
Materials required
- 2 right-angled prisms
- 2 PVC pipes or cardboard tubes (30-40 cm each)
- Strong glue
- Cutter
- Sandpaper
Steps:
1. Prepare Tubes
- Cut two PVC pipes or cardboard tubes to desired length.
- Smooth edges with sandpaper.
2. Fix Prisms
- Attach a prism inside each tube end with the hypotenuse facing the tube opening.
- Secure with hot glue or adhesive.
3. Join Tubes
- Align tubes at a right angle.
- Securely join them.
- Optionally, paint inside black to reduce reflections.
- Ensure all parts are securely fixed.
- For demonstration,position the periscope to look over obstacles and explain how prisms helps in providing a clear image in periscopes.
Question 2.
Find out the critical angle in different mediums like glycerine, water, coconut oil, glass etc., and compare them. Prepare a report including different stages like objective, materials, method of study, results etc.
Answer:
Short Report on Critical Angle in Various
Media
Objective
To determine the critical angle of various substances, such as glass, coconut oil, glycerine and water.
| Material | Refractive index |
| Glass | 1.52 |
| Coconut oil | 1.45 |
| Glycerine | 1.47 |
| Water | 1.33 |
Method of study
Formula to find critical angle
Snell’s law helps us to find the critical angle (θc). The critical angle is the angle of incidence above which total internal reflection occurs for light travelling from a denser to a rarer medium.
Snell’s law states that n1sin (θ1) = n2sin(θ2)
When θ2 = 90° (at critical angle), sin(θ2) = 1
So we have sin (θc) = \(\frac{n_2}{n_1}\)
θc = sin1\(\left(\frac{n_2}{n_1}\right)\)
where n1 is the refractive index of the denser medium and n2 is the refractive index of the rarer medium (air). Refractive index of air is taken as n = 1
Calculating critical angle
Critical angle for glass
| Substances | Critical angle |
| Glass | 41.14° |
| Coconut oil | 43.60° |
| Glycerine | 42.86° |
| Water | 48.75° |
It can be observed that water has the largest critical angle, indicating that it has the lowest refractive index among the mediums studied and glass has the smallest critical angle, indicating it has the highest refractive index. Because of their same refractive indices, glycerine and coconut oil have relatively close critical angles.
Conclusion
The critical angle is one important property in optics that varies depending on the refractive index of the medium. The critical angles for glycerine, water, coconut oil, and glass were computed and compared in this study. Applications requiring light transmission and total internal reflection, such as fibre optics and different optical devices, require an understanding of these angles.
Question 3.
Can the angle of refraction be 90° when light enters from an optically rarer medium to a denser medium? Do an activity and write it down in the science diary.
Answer:
Objective
To find out if angle of refraction can be 90° when it moves from air (less dense) to water or glass (denser).
Assumption
Angle of refraction cannot be 90° when it goes from a less dense to a denser medium.
Materials required
- Laser pointer or flashlight
- Protractor
- Clear container (like a glass tank)
- Water
- Piece of glass or plastic
- Pen or pencil
Procedure
- Fill the clear container with water and place it on a table.
- Put the piece of glass or plastic next to the container.
- Shine the laser or flashlight at the water surface at different angles (0°, 30°, 45°, 60°, etc.)
- Measure the angle of incidence and the angle of refraction.
- Record Observations
Results
- When light goes from air to water, the angle of refraction is always less than the angle of incidence.
- The same happens with the glass or plastic.
Snell’s law states that n1sin (θ1) = n2sin(θ2) Here n, and n, are the refractive indices of the rarer and denser media, respectively, and θ1, and are the θ2, angles of incidence and refraction.
When light passes from a rarer to a denser medium, the refractive index of the denser medium (n2) is greater than that of the rarer medium (n1). Thus, the sine of the angle of refraction sin(θ2) (cannot be greater than 1, limiting θ2 to less than 90°.
Conclusion
Light cannot bend at a 90° angle when it goes from a less dense medium (like air) to a denser medium (like water or glass). This is because the refraction law shows it is impossible.
Refraction of Light Class 9 Notes Questions and Answers Kerala Syllabus
Question 1.
Have you ever observed the straw in a glass of lemonade? Does it appear bent? What could be the reason?
Answer:
The straw in a glass of lemonade appears bent because of the phenomenon of light called refraction.
Question 2.
Don’t you think the light rays reflected from the straw dipped in lemonade travel through different mediums before they reach our eyes?
Answer:
Yes
Activity
Fill three fourth of a trough with water. Add two or three drops of milk into it. Cover the trough with a transparent sheet of paper. Fill the remaining space in the trough with a smoke. Then flash the light from a laser torch such that light should fall obliquely on the surface of water as shown in figure below.
Observe the path of light and record the observations in the science diary. Ray diagram showing the path of light travelling from air to water
Question 3.
Which are the mediums through which light travels within the trough?
Answer:
Air, water
Light is travelling from air to water
Question 4.
How will the path of light ray be, when it passes through only one medium?
Answer:
The path of light will be in a straight line.
Question 5.
Is there any deviation in the direction of the ray of light when it enters obliquely from one medium to another?
Answer:
Yes, there is deviation.
Question 6.
Where does the ray of light undergo a change in the direction?
Answer:
At the surface of water.
Activity
Allow light from a laser torch to fall perpendicular to the surface of water taken in a trough.
Question 7.
Is there a deviation in the path of the ray of light? Record your observation.
Answer:
No, there is no deviation in the path of the ray of light because light is incident normally.
| When light travels through a single medium the path of light will be a straight line. A ray of light undergoes a deviation at the surface of separation when it enters obliquely from one medium to another. There will be no deviation in the path of the ray of light incident normally. |
Activity
Observe the speed and a direction of a toy car moving from a smooth surface to a rough surface.
Question 8.
Is there a change in the direction of motion of the car?
Answer:
Yes, the direction of motion of the car changes.
Question 9.
Where does this change occur?
Answer:
This change occurs at the surface of separation of the two media.
Question 10.
Did the car move with the same speed on both surfaces?
Answer:
No, the car moves with different speed on both surfaces.
Note:The change in direction is caused by the change in the speed of the car as it moves from one surface to the other.
Reason for the change in the direction of light when it passes from one medium to another
The speed of light is different in different mediums. The change in the speed of light causes the change in the direction of the ray of light, when it passes from one medium to another. Examine the table showing the speed of light through various mediums.
| Medium | Speed of Light (approximate) |
| Air | 3 × 108 m/s |
| Water | 2. 25 × 108 m/s |
| Glass | 2 × 108 m/s |
| Diamond | 1.25 × 108 m/s |
By examining the table we can understand that the speed of light differs in various mediums due to the difference in their optical densities.
| The ability of a medium to influence the speed of light through the medium is its optical density. |
| The speed of light will be lower in a medium of higher optical density (optically denser medium). The speed of light will be higher in a medium of lower optical density (optically rarer medium). Note that optical density has no relation with material density. |
Question 11.
Arrange the mediums in the table shown below in the increasing order of their optical densities.
| Medium | Speed of Light (approximate) |
| Air | 3 × 108 m/s |
| Water | 2. 25 × 108 m/s |
| Glass | 2 × 108 m/s |
| Diamond | 1.25 × 108 m/s |
Answer:
Air <Water<Glass<Diamond
Activity to understand about refraction
A straw is kept obliquely in a glass as shown in figure below. Then fill the glass with water.
Question 12.
What difference do you observe?
Answer:
The straw appears bent when water is poured.
Question 13.
Why does the straw appear bent?
Answer:
It is due to refraction of light that the straw appears bent.
Activity
A straw is kept in a trough. Observe the path of the rays of light from the straw falling on the eyes before and after filling the trough with water.
Question 14.
Is there a deviation for the ray of light falling on the eye from point B of the straw before pouring water?
Answer:
After the water is poured, the ray of light from the immersed part of the straw entering air from water is deviated at the surface of separation.
Question 15.
Doesn’t the ray of light appear to come from C, though it is actually coming from B?
Answer:
The straw appears bent, as the actual position B seems to have risen to the position C.
Refraction
| When a ray of light enters obliquely from one medium to another of different optical densities, it undergoes a deviation at the surface of separation of the mediums. This phenomenon is refraction. The ability of light to undergo refraction in a medium depends on the optical density of the medium. The role of refractive index is very important in designing various optical devices. |
| Refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in the medium. |
| Refractive index (n) = \(\frac{\text { The speed of light in vacuum (c) }}{\text { The speed of light in the medium (v)}}\) n = \(\frac{c}{v}\) The speed of light in vacuum (c) = 3 × 108 m/s |
Question 16.
The speed of light in different mediums is given in the table below. Find the refractive index of each medium and complete the table.
| Medium | Speed of light (v) | Refractive index (n) |
| Air | 3 × 108 m/s | n = \(\frac{c}{v}\) = \(\frac{3 \times 10^8 \mathrm{~m} / \mathrm{s}}{3 \times 10^8 \mathrm{~m} / \mathrm{s}}\) = 1 |
| Glass | 2 × 108 m/s | |
| Water | 2.25 × 108 m/s |
Answer:
| Medium | Speed of light (v) | Refractive index (n) |
| Air | 3 × 108 m/s | n = \(\frac{c}{v}\) = \(\frac{3 \times 10^8 \mathrm{~m} / \mathrm{s}}{3 \times 10^8 \mathrm{~m} / \mathrm{s}}\) = 1 |
| Glass | 2 × 108 m/s | n = \(\frac{c}{v}\) = \(\frac{3 \times 10^8 \mathrm{~m} / \mathrm{s}}{2 \times 10^8 \mathrm{~m} / \mathrm{s}}\) = 1.5 |
| Water | 2.25 × 108 m/s | n = \(\frac{c}{v}\) = \(\frac{3 \times 10^8 \mathrm{~m} / \mathrm{s}}{2.25 \times 10^8 \mathrm{~m} / \mathrm{s}}\) = 1.33 |
Question 17.
If the refractive index of diamond is 2.4, what is the speed of light that passes through it?
Answer:
Refractive index of diamond, n = \(\frac{c}{v}\) = 2.4
V = \(\frac{c}{v}\) = \(\frac{3 \times 10^8}{2.4}\)
v = 1.25 × 108 m/s
Question 18.
How is the speed of light related to refractive index?
Answer:
As refractive index increases, speed of light decreases. The speed of light is less in a medium of higher refractive index. The speed of light is higher in a medium of less refractive index.
Question 19.
Write down how optical density is related to refractive index?
Answer:
Optical density will be higher in a medium having higher refractive index.
Note: A ray of light entering obliquely from one medium to another with different optical densities undergoes deviation at the surface of separation of the two mediums.
Question 20.
Is the deviation of light the same when it enters from air to water and from water to air when the angle of incidence is the same?
Answer:
No, the deviation of light is not the same when it enters from air to water and from water to air when the angle of incidence is the same.
Ray diagrams showing refraction
Observe the figures.
Incident ray:The ray falling on the surface of separation of the two mediums is the incident ray. Refracted ray:The ray which undergoes refraction is the refracted ray.
Angle of incidence: The angle between incident ray and the normal(NN’) .
Angle of refraction: The angle between refracted ray and the normal is the angle of refraction.
Question 21.
Find the incident ray, refracted ray, angle of incidence and angle of refraction from each figure and complete the table.
Answer:
| Fig.(1) | Fig (2) | |
| Incident ray | AB | PQ |
| Refracted ray | BC | QR |
| Angle of incidence (i) | ∠ABN | ∠PQN |
| Angle of refraction (r) | ∠CBN | ∠NQR |
| The angle of refraction is greater than/ less than the angle of incidence | The angle of refraction is less than the angle of incidence | The angle of refraction is greater than the angle of incidence |
Question 22.
How does the direction of light change when a ray of light enters obliquely from air to water? (deviates towards the normal/deviates away from the normal)
Answer:
deviates towards the normal
Question 23.
How does the direction of light change when a ray of light enters obliquely from water to air? (deviates towards the normal / deviates away from the normal)
Answer:
deviates away from the normal
Question 24.
Observe the figures that show light passing through different pairs of mediums.
Find answers to the following questions based on the optical densities of mediums.
a) Which are the figures in which the ray of light enters obliquely from an optically denser medium to a rarer medium?
Answer:
Fig.(4), Fig.(6)
b) In this case, to which direction does the refracted ray deviate? (towards the normal / away from the normal)
Answer:
away from the normal
c) Choose the figures in which the refracted ray deviates towards the normal.
Answer:
Fig.(3), Fig.(8)
d) In which situation will a ray of light deviate towards the normal as it passes from one medium to another?(from an optically denser medium to a rarer medium/from an optically rarer medium to a denser medium)
Answer:
from an optically rarer medium to a denser medium.
e) There is no refraction of light in Fig.(5) and Fig.(7). What may be the reason?
Answer:
When a ray incident normally at the surface of separation of mediums, it does not undergo refraction.
Note:
|
Allow light from a laser torch to fall on a glass slab.
Observe the path of light.
Question 25.
At which points do refraction occur for the ray of light?
Answer:
Refraction occurs at the points Q and R.
Question 26.
Draw the ray diagram showing the refraction of a ray of light in a glass slab
Answer:
Note: When light passes from air to glass it moves towards the normal and when it moves from glass to air it bends away from the normal. In a glass slab, the incident ray (PQ) and emergent ray (RS) are parallel to each other. The emergent ray is just shifted to the side, but it remains parallel to the original path.
Question 27.
Allow light to incident normally on the glass slab. Does refraction occur?
Answer:
No, the ray incident normally on the glass slab does not undergo refraction.
Put a coin in a vessel. Walk backwards looking at the coin. When the coin disappears, ask one of your classmates to pour water into the vessel.
Question 28.
What do you observe? Analyse the figure given above and find out the reason.
Answer:
On pouring water, the ray of light coming from the coin undergoes refraction and reaches the eye. Hence, the coin becomes visible again.
Question 29.
Place a glass slab on the letters in a textbook. The letters appear raised. What may be the reason? Find out.
Answer:
The ray of light coming from the letters undergoes refraction through the glass slab and reaches the eye. So the letters appear raised.
Question 30.
Observe the figure given below.
You can see a coin lying under the water in a trough. Try to take the coin out by looking through any one side of the trough.
Can you do it with ease? What may be the reason?
Answer:
We cannot do it with ease. When light coming from the coin enters from an optically denser medium to a rarer medium, it deviates away from the normal. So by looking through any one side of the trough we cannot locate the actual position
of the coin.
Question 31.
The bottom of a pond appears elevated when viewed from a distance than from a nearer point. Why?
Answer:
When light coming from the bottom of the pond enters from an optically denser medium to a rarer medium, it deviates away from the normal. As the distance from the pond and the observer increases, the angle of refraction made by the incident ray increases. So the bottom of a pond appears elevated.
Question 32.
People who engage in bow fishing aim at a point slightly below the perceived position of the fish. Why?
Answer:
When the reflected light coming from the fish enters from an optically denser medium to a rarer medium, it is deviated away from the normal at the surface of separation. This refracted light appears to come from a position just above the actual position. So people who engage in bow fishing aim at a point slightly below the perceived position of the fish.
Observe the figures.
Question 33.
Does the ray of light from the star reach our eyes by travelling in a straight line?
Answer:
No, the ray of light from the star does not reach our eyes by travelling in a straight line.
Question 34.
An illustration of the path of a ray of light from a distant star through the Earth’s atmosphere is given in Fig.(9) .Doesn’t the ray of light undergo irregular deviations? What may be the reason?
Answer:
Yes. The ray undergoes irregular deviations.
Stars appear as point sources of light because they are at a greater distance from the Earth compared to the planets. The light coming from the stars reaches our eyes by traversing through the atmosphere. The optical density of the medium through which the light travels goes on changing as the physical conditions (pressure, temperature etc.) of the layers of atmosphere change continuously. Hence, the light undergoes an irregular refraction. Therefore, when the light rays from the stars reach the eyes after refracted several times, the star cannot be seen continuously at the same position. This is the reason for the twinkling of stars.
Question 35.
Even after the Sun has passed the western horizon, the Sun is visible for some more time. Similarly, the Sun can be seen a few seconds before it reaches the eastern horizon in the morning. What is the reason? Analyse the figure given below and record the explanation in the science diary.
Answer:
This is due to atmospheric refraction. The temperature difference causes density difference in the layers of atmosphere. Due to this, optical density of the medium through which the light travels goes on changing. Light enters to the atmosphere undergoes continuous refraction. We can see the apparent position of the Sun just before and just after the actual Sunrise and actual Sunset.
Question 36.
Will refraction occur whenever light enters obliquely from one medium to another?
Answer:
No
Question 37.
When light passes through the fibres of the decorative lamps as shown in figure, it emerges only through the fibre tips. What may be the reason?
Answer:
It is due to total internal reflection of light.
Activities to understand total internal reflection
Activity 1
Allow light from a laser torch to fall at different angles on the surface of water taken in a trough.
Question 38.
Which are the mediums through which light passes inside the trough?
Answer:
water, air
Question 39.
Does the light enter from an optically denser medium to a rarer medium or from an optically rarer medium to a denser medium?
Answer:
Here, light enters from an optically denser medium to a rarer medium (That is from water to air).
Question 40.
Increase the angle of incidence gradually. What do you observe?
Answer:
Here the light enters from an optically denser medium to rarer medium. The angle of refraction goes on increasing with the increase in the angle of incidence. The incident ray reflects back to the water completely when the angle of incidence is above a specific value.
Question 41.
When does the light reflect completely to the same medium, without refraction?
Answer:
The incident ray reflects completely to the same medium when the angle of incidence is above a specific value.
Activity 2
Draw a circle on a chart paper. Mark angles on the paper as if two protractors are placed together. Keeping this on the surface of a table, place a thick semicircular glass slab on the circle drawn on the chart paper as shown in figure given above. Allow the light from a laser torch to fall on this slab at different angles.
Question 42.
Observe the angle of incidence and angle of refraction in each case. Draw the ray diagram.
Answer:
Question 43.
From which medium to which does the ray of light enter?
(from an optically denser medium to a rarer medium/from an optically rarer medium to a denser medium)
Answer:
From an optically denser medium to a rarer medium.
Question 44.
What is the change in the angle of refraction when the angle of incidence increases?
Answer:
The angle of refraction increases, when the angle of incidence increases.
Question 45.
What is the angle of incidence when the angle of refraction is 90o ?
Answer:
42°.
Question 46.
What peculiarity do you notice when the light is incident at an angle greater than this angle of incidence?
Answer:
The incident ray is reflected back completely to the same medium without undergoing refraction.
| Critical Angle When a ray of light enters from an optically denser medium to a rarer medium, the angle of incidence at which the angle of refraction becomes 90° is the critical angle. In the glass-air pair, the critical angle is 42°.Total Internal Reflection When a ray of light enters from an optically denser medium to a rarer medium, at an angle of incidence greater than the critical angle, the ray is reflected back completely to the same medium without undergoing refraction. This phenomenon is total internal reflection. |
Question 47.
The path of light through different mediums is given. Analyse the figures and answer the following questions.
Air
(a) Which of the above figures represent total internal reflection?
Answer:
Fig. (a), (e)
b) What is the critical angle of glass in this case?
Answer:
42°
c) Will total internal reflection occur for a ray of light entering from water to air at an angle of 500? Why? What is the critical angle for the water-air pair?
Answer:
Total internal reflection will occur for a ray of light entering from water to air at an angle of 50°. Because the angle of incidence is greater than the critical angle of water. The critical angle for the water-air pair is 48.6°
d) What are the two conditions required for total internal reflection to occur?
Answer:
The conditions required for total internal reflection to occur are
- The ray should enter from an optically denser medium to a rarermedium.
- The angle of incidence should be greater than the critical angle.
Note: If only one medium is mentioned while stating the critical angle, the second medium will be either air or vacuum.
Question 48.
Observe the figure.
The bottom of the aquarium is seen above the surface of water. What may be the reason?
Answer: The light coming from the bottom of the aquarium undergoes internal reflection at the surface of water. Hence, the bottom is seen above the surface of water. The distance from the surface of water to the bottom of the aquarium is the same as the distance from the surface to the image caused by total internal reflection.
Fibres in decorative lamp
One end of each optical fibre in the decorative lamp is connected to a suitable source of light. Light rays from this source travel through the fibre. While travelling through the fibre, it makes an angle of incidence greater than the critical angle with the walls of the fibre. Hence The ray should enter from an optically denser the light undergoes successive total internal reflection and emerges through the other end.
Optical illusions caused by total internal reflection in our surroundings.
Question 49.
During summer season there seems to be water logging on roads when viewed from a distance. What may be the reason?
Answer: During summer season the layers of air closer to the road have a low optical density as it is warmer than the upper layers. The optical density of the air increases gradually as we go higher. When light rays coming from the surrounding objects pass through different layers of air with different optical densities, they undergo refraction and then total internal reflection. Such deviated light rays fall on our eyes. Hence their image appears to have formed on roads. This phenomenon is known as mirage. Such familiar images are usually seen on the surface of water. That is why water seems to be logged on roads when viewed from a distance
Reflector
A large number of small prisms are fixed inside the reflectors in the tail lamps of vehicles. The rays of light incident on a prism get reflected back due to total internal reflection. It can be seen even in dim light.
Question 50.
How does a ray of light incident on a prism get reflected? Explain on the basis of the critical angle of glass.
Answer:
The light ray is incident normally on the side PQ. Hence, there is no refraction. The critical angle of glass is 42o. The angle of incidence at A and B is 45°. Hence the light falling on A undergoes total internal reflection and reaches B. There it undergoes total internal reflection again and comes out of the reflector. The same process happen in other prisms in the reflector as well.
Two equilateral prisms are used in periscope. Due to total internal reflection, the image can be seen with greater visual clarity.
The outer coating of optical fibre with higher refractive index is known as cladding and inner coating is known as the core. Core has a lower refractive index. Light rays passing along the core at an angle greater than the critical angle undergoes total internal reflection. Thousands of optical signals can be sent simultaneously through single cable without the loss of intensity. Such signals can be sent to distant places with the speed of light. This is the reason for using optical fibre cables in communication.
Uses of optical fibre cables
Optical fibres are used in the medical field and communication field.
Question 51.
The use of optical fibres in medical field is increasing day by day. Collect information increasing day by day. Collect information regarding this and present in your class.
Answer:
Hint:
These are some of the applications in medical field which make use of optical fibres
Endoscopy uses optical fibres to obtain the image of the inside of the body.
An optical fibre bundle can be inserted into the body by a physician. Some transport light into the body, while others return light that has been reflected off interior body surfaces. This enables the physician to look clearly into the body to diagnose conditions like cancer
Picture showing endoscope used to view inside of stomach.
Optical fibres helps to observe what is happening during keyhole surgery.
Laser Surgery
Optical fibres help deliver laser light for precise surgical procedures. The fibres guide laser light to the target area for cutting or repairing tissues.
Uses: Common in eye surgeries (like LASIK), skin treatments, and removing tumors.
Comparison of total internal reflection and reflection from a plane mirror
Question 52.
Statements regarding total internal reflection and reflection from plane mirror are given. Tabulate them suitably.
- Occurs only when a ray of light enters from an optically denser medium to a rarer medium at an angle greater than the critical angle.
- The ray of light is not completely reflected.
- Reflection occurs on a surface at any angle of incidence.
- The ray of light is completely reflected.
Answer:
| Reflection from a plane mirror | Total internal reflection |
| The ray of light is not completely reflected | The ray of light is completely reflected. |
| Reflection occurs on a surface at any angle of incidence. | Occurs only when a ray of light enters from an optically denser medium to rarer medium at an angle greater than the critical angle. |