Class 10 Physics (English Medium) Chapter 05 Refraction of Light - Questions and Answers
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Class 10 Physics Questions and Answers
Chapter 05 Refraction of Light 
Refraction of Light: Textual Questions And Answers

1. Fill three fourth of a transparent vessel with water as shown in the figure. Add one or two drops of milk into it. Fill the portion of the vessel above water with smoke. Close the vessel using an OHP glass sheet. Allow the light from a laser torch to pass through water as shown in the figure. 
i. Observe the path of light, Do you observe anything out of the ordinary?, Can you depict it in the science diary? 
ii. Which are the media involved here? 
iii. What happens to the path of the light?
iv. Where does the deviation of the ray take place?
v. Why does the ray of light undergo a deviation here?
vi. Does light pass through all the media at the same speed?
Answer:
i. The light rays below the water undergo deviation at the surface of the water.
ii. Air & Water
iii. Path of light undergoes deviation
iv. At the point on the surface where the media get separated.
(The ray of light entering water undergoes a deviation at the point on the
the surface where the media get separated.)
v. The difference in speed of light rays in different media.
vi. No 

2. Does light pass through all the media at the same speed? Analyse the table given below.
- The speed of light through various media differs. 
- As the optical density of a medium increases, the speed of light through it decreases and vis-versa.

3. Can the media are given in the table be arranged in the increasing
order of their optical densities?
Air < Water < Glass < Diamond

4. Refraction of Light
- It is the difference in the optical densities that causes the deviation.
When a ray of light entering obliquely from one transparent medium to another, its path undergoes a deviation at the surface of separation. This is refraction. 

5. The figure below shows the refraction through the glass slab.
i. Which is the incident ray on the surface of separation CD?
ii. The angle between the incident ray and the normal is called the angle of incidence. If so, can you explain what is the angle of refraction?
iii. Using a protractor measure the angle of incidence and the angle of refraction.
iv. Is the angle of refraction greater or lower than the angle of incidence when it goes from air to glass?
v. What about from glass to air?
vi. Which is of greater optical density — air or glass?
vii. While going from air to glass, the refracted ray deviates towards the normal/ deviates away from the normal.
viii. What happens while it goes from glass to air?
ix. Are the angle of incidence, angle of refraction and the normal at the point of incidence on the same plane?
Answer: 
i. QR
ii. Between the refracted ray and the normal
iii. Angle of incidence i = 45⁰ , Angle of refraction r = 28⁰
iv. Lower
v. grater
vi. Glass
vii. Deviates towards the normal
viii. Deviates away from the normal.
ix. Yes. The angle of incidence, angle of refraction and the normal at the point of incidence are in the same plane.

6. Does refraction take place for a ray while entering a glass slab normal to it?
Answer: No

7. Examine using a ray of light from a laser torch. Ray diagrams of a light ray passing through different media are depicted. Find out the appropriate figures by observing these figures and also based on the concepts you have developed from the textbook.
Answer:

8. Figure shows the path of laser light that passes obliquely through a glass prism. 
a) The ratio of the sine of the angle of incidence to the sine of the angle of
refraction is a constant. Name the constant. Name this law.
b) Identify the letter that represents this constant
(i, n, r, v, e)
c) As shown in the figure when a light ray passes obliquely from air to gas, find out the value of the refractive index on the basis of Snell’s law. (sin 13°= 0.34, sin 20° = 0.22)
d) When a ray of light passes obliquely from one medium to another medium “The angle of incidence of the angle of refraction and the normal at the point of incidence on the surface of separation of the two media will always be in a different plane. Correct the statement, if it is wrong.
e) When a ray of light passes from one medium to another medium, what is the relationship between angle incidence and angles of refraction
Answer:
a) Refractive index, Snell's law
b) n
c) 1.5
d) same plane
e) directly propositional

9. The experimental results of a ray of light entering from air to glass slab
and from glass slab to air are tabulated. 
Can you analyse the table and find out the change?
What other conclusions can you arrive at the table?
Answer: When light passes through different pairs of media, the angle of refraction increases with the angle of incidence. The ratio of the sine of the angle of incidence to the sine of the angle of refraction (sin i / sin r) will be a constant. This constant is known as the refractive index.

10. Laws of Refraction
Answer: The angle of incidence, the angle refraction and the normal at the point of incidence on the surface of separation of the two media will always be in the same plane.
The ratio of the sine of the angle of incidence to the sine of the angle of refraction will always be a constant. This is known as Snell’s Law
The constant from Snell’s Law is known as Refractive Index. This is indicated by the letter n. 

11. Speed of light in media and refractive index
a) How is the refractive index of a medium related to the speed of light? 
b) Does it have any relation bearing on the ratio of the speed of light in air to the speed of light in glass? 
Answer: 
a) The lower the refractive index in any medium, the higher the speed of light. That is, the refractive index is inversely proportional to the speed of light.
b) The refractive index can also be calculated as the ratio between the speed of light in air and the speed of light in the glass.

12. We consider the speed of light in air to be equal to the speed of light in a vacuum, so what is it?
Answer: 3X10⁸m/s

13. Speed of light in air is 3X10⁸m/s, speed of light in glass is  2X10⁸m/s. Find the refractive index of glass?


14. Imagine that the speed of light in a medium is v₁ and that in a medium is v₂.
The refractive index of medium 1 with respect to medium 2 is represented
as n₁₂ and of medium 2 with respect to a medium is represented as n₂₁. If so
find the refractive index.

15. Define the following.
a) Relative Refractive index
The refractive index of a medium with respect to another is called the relative refractive index
b) Absolute refractive index
The refractive index of a medium with respect to vacuum is called the absolute refractive index.
 
16. If the speed of light in the air (in a vacuum) is considered as 'c' and that in a medium is considered as ' v ', then the absolute refractive index of the medium =
Answer: 
17. The refractive index of glass and water are given in Table.
If the speed of light in water is 2.25 x 10⁸ m/s
(a) Calculate the speed of light in vacuum
(b) Calculate the speed of light in the glass

18. Observe the following figure and answer the following questions.
a) Don't you see that the position of the portion of the pencil underwater
has changed? What may be the reason? 
b) Does the ray of light coming after reflection from the pencil undergo a deviation? What is the reason?
c) Is there any change likely to occur if kerosine is used instead of water?
Answer: 
a) Yes, Refraction 
b) Yes when light travels from water to air it deviates.
c) Pencil appears to be bent more

19. Explain why you got the frog when you shot the fish in the picture below.
Answer: The exact position of the fish could not be seen because the light reflected from the fish was refracted at the splitting level.

20. Take an opaque vessel. Place a coin at its bottom. Ask a child to walk backwards looking at the coin. Ask her to stop at the place where the coin disappears. Now ask another child to add water into the vessel without moving the coin. What is observed here?
Answer: 
The coin is seen again. This is due to the refraction of light itself.

21. Draw a thick line on a paper using a pen. Place a glass over it and observe as suggested below.
a) Look from one side as shown in Fig.5.8(a) (placing a glass slab perpendicular to the line)
b) Look from one side as shown in Fig. 5.8(b) (glass slab oblique to the line)
c) Look from one side as shown in Fig. 5.8(c) (from vertically above the glass)

a) The line seems to be slightly higher than the glass slab.
b) The line can be seen curved on the surface of the glass slab.
c) The line can be seen in its true position.

22. Try to take out a coin from the bottom of a trough filled with water by viewing it from one side. Can you easily pick up the coin? What is the reason for the failure?
Answer: 
Can't. Because the coin does not look real, it looks a little high.

23. Now you have become familiar with many activities regarding refraction. Find out more examples of refraction from daily life.
a) Pool of water appears to be less deep than what it actually is
b) It looks like there is water on Metelled Road on hot days.
c) A stick held partly immersed in water appears to be bend at the water surface.
d) Twinkling of stars in a clear sky.
e) The sun rises in the sky before sunrise and after sunset Seeing for a while‌.

24. Take a glass flask and fill half of it with water. Add one spoon of milk to it. Allow light from a laser torch to fall on the water in the flask. Observe the path of the refracted ray. Gradually increase the angle of incidence. Note the deviation of the refracted ray.
a) What will be the angle of refraction when the refracted ray passes
along the surface of the water?
b) What is a critical angle?
c) When light enters the air from water at an angle of inclination greater than the critical angle, does that refraction occur? What is it called?
Answer: 
a) 48.6⁰
b) When a ray of light passes from a medium of greater optical density to that of lower optical density, the angle of incidence at which the angle of refraction becomes 90° is the critical angle.
c) No, this is called a total internal reflection.

25. What is total internal reflection?
Answer: When a ray of light passes from a medium of higher optical density to a medium of lower optical density at an angle of incidence greater than the critical angle, the ray is reflected back to the same medium without undergoing refraction. This phenomenon is known as total internal reflection.

26. A person who looks at an aquarium as shown in Fig. 5.10 can see the base on the surface of the water. What is the reason?
Answer: Total internal reflection. That is, the light from the bottom hits the surface of the water and falls on the observer's eye so that the bottom can be seen on the surface of the water.

27. The path of light in different media is shown in the figures. Analyse them and answer the following questions.
i. Which are the figures that show total internal reflection?
ii. What is the critical angle of glass?
iii. Will total internal reflection takes place when light passing through water is incident on the surface of separation with air at an angle of incidence of 45°? Why?
Answer:
I. Fig.(a), Fig.(e)
ii. 42⁰
iii.  No, total internal reflection occurs only when the angle of incidence is greater than the critical angle.

28. Find out the practical applications of total internal reflection in our day to day life.
i. Endoscope - Medical field — 
ii. Optical fibre cables - In the field of telecommunications 
iii. Reflector used in vehicles and highways
iv. Diamond cutting in a special shape: total internal reflection Utilizing it to make it shinier‌.
v. In prism binoculars, spectroscopy, and periscope
vi. The phenomenon of mirage

29. Observe the picture below.
a) Which object is shown in the picture above? 
b) Write the structure of the device shown above? What is the difference for this from other metal conductors?
Answer: 
a) Optical fibre
b) Optical fibres are made of light fibres of quartz or glass. On the other hand, there is a cover with a low light density medium and a cover with a plastic cover.
1. Less energy loss
2. transmit signals very quickly in the form of light
3. To send several signals of different frequencies at the same time

30. When viewed through the water drop that fell on a magazine, a child noticed that there was a change in the size of letters on the page. Why did the letters appear to be bigger in size?
Answer: From this, we can understand that the spherical transparent media acts like a lens

31. Write a definition for the lens? What kind of lens do we mainly use? 
Answer: A lens is a transparent medium having spherical surfaces. Convex and concave lenses are the lenses that we mainly use.

32. Write terms related to convex and concave lenses?
Answer: 
1. Optic centre
2. Centre of curvature
3. Principal axis
4. Principal focus
5. Focal length

33. What are the terms and characteristics associated with convex and concave lenses.
Answer: 
1. Optic centre: Optic centre is the midpoint of a lens.
2. Centre of curvature: Centre of curvature is the centre of the imaginary spheres of which the sides of the lens are parts.
3. Principal axis: Principal axis is the imaginary line that passes through the optic centre joining the two centres of curvature.
4. Principal focus: Light rays incident parallel and close to the principal axis after refraction converges to a point on the principal axis of a convex lens. This point is the principal focus of a convex lens.
Light rays incident parallel and close to the principal axis diverge from one another after refraction. These rays appear to originate from a point on the same side. This point is the principal focus of a concave lens.
5. Focal length: Focal length is the distance from the optic centre to the principal focus. This is denoted by the letter f.

34. Light rays incident parallel and close to the principal axis after refraction converges to a point on the principal axis of a convex lens. What is this point called?
Answer: the principal focus of a convex lens.

35. How many principal focus does a convex lens have? ” Why?
Answer: Two, because the convex lens or the convex lens has two transparent surfaces.

36. Identify the following two lenses and clarify the principal focus of both?
Answer:
I. Fig. 5.19 - convex lens
ii. Fig. 5. 20 - concave lens
i. convex lens:
Light rays incident parallel and close to the principal axis after refraction converges to a point on the principal axis of a convex lens. This point is the principal focus of a convex lens.
ii. concave lens
Light rays incident parallel and close to the principal axis diverge from one another after refraction. These rays appear to originate from a point on the same side. This point is the principal focus of a concave lens.

37.  Why is it said that the principal focus of a concave lens is virtual?
Answer: It is impossible to produce real convergence of light using a concave lens. Therefore the principal focus of a concave lens is virtual.

38. How to find the Focal length of a convex lens?
Answer: Cast the image of a distant object on the screen using a convex lens. Measure the distance between the lens and the screen. Repeat the experiment with different distant objects. Find the average of the distances measured. This gives the focal length of the convex lens.

39. Adjust the screen as shown in the figure, keeping the convex lens at different positions on the principal axis in front of a lighted candle. Understand image formation and fill the table.

40. What are the points to be taken care of while drawing ray diagrams.
Answer: 

41. Light rays coming from a distant object are considered as parallel
rays.
a) Where will the rays coming parallel to the principal axis converge?
b) Where is the image formed?
Answer: 
a) On the other side: in the Principal focus.
b) Principal focus

42. Consider two rays from an object placed beyond 2F as shown in
the figure.
a) Describe how the image formation occurred here, using a diagram?
b) Write the features of the image
Answer: 
a) The image is formed by two rays.
i. One ray incident parallel to the principal axis and passes through the principal focus.
ii. The second ray passing through the optic centre passes without any deviation. 
The image occurs at the point where the two rays meet.
b) Position of the image - Between F and 2F
Nature of the image - real, inverted
Size of the image - diminished

43. Complete the diagrams below and write the features of the image?
Answer: 

44. Draw a diagram of the image formation that occurs when placing an object between the F and the lens of a convex lens and write the characteristics of the image.
Position of the image - On the same side of the object
Nature of the image - erect, virtual
Size of the image - magnified

45.  The following is an illustration of an image produced by a concave lens. What is the nature of the image?
Answer: Wherever the object is, the image always occurs between the F and the lens. The small, virtual, and erect image is obtained on the same side of the object.

46. What is New Cartesian Sign Conventions?
Answer: The symbol must be added to measure the distance between the lens and the mirror, the rules related to this are known as New Cartesian Sign Conventions.
1. Length is measured by considering the focal point of the lens as the origin.
2. All measurements must be made from the point of origin.
3. Light is considered to travel from left to right.
4. Those measured in the same direction as the light beam will be positive and those measured in the opposite direction will be negative.
5. The distance above the X-axis is positive and the distance below is negative.

47. Record the measurement shown in the figure as per the Cartesian System.
Distance of the object from the lens (u) = -25 cm
Distance of the image from the lens (v) = +100 cm
Height of object (OB) = +1 cm
Height of image (IM)  =  -4 cm

48. Suggest an experiment to see how the distance characters u and v are related to the focus distance of a lens.
- Take a convex lens of known focal length. Keep a lighted candle at a certain distance from the lens and adjust the lens to get a clear image on the screen. Then measure the values of u and v and tabulate the values on the basis of the New Cartesian Sign Conventions. Repeat the experiment by changing the position of the object.
Average f =
The focus distance obtained in the previous experiment is now the same as the value obtained from the table.

49. Write the lens‌ equation?

50. Magnification
a)  Is there any relation between the height of an object and the height of its image?
b) Can it be related to the ratio between the distance to the object and that to the image?
c) What is Magnification?
Answer:
a) yes
b) yes, both are same
c) Magnification is the ratio of the height of the image to the height of the object. It shows how many times the image is larger than the object.

51. Write the mathematical form of magnification?
Answer:


52. Find the magnification of the image formed by the convex lens?
Answer:

53. Find out the uses of lenses in our day-to-day life.
* In telescope
* In spectacles
* In camera
* In the watch repair shop
* In magnifying glasses
* In projectors
* The power of the lens
* In microscopes

54. Each lens has a different ability to focus light rays parallel to the principal axis.
a) When a person suffering from defective vision met a doctor, in the prescription it was written as +2D for buying spectacles. What has the doctor indicated in the prescription?
b) What is mean by the power of the lens?
c) What is the difference between the symbol of a convex and a concave lens?
Answer:
a) The power of the lens of the spectacles to be used by him.
b) Power of a lens is the reciprocal of focal length expressed in metres. 
Power, p=1/f
If the focal length is given in centimetres, power will be p = 100 / f
c) The power of a convex lens is positive and that of a concave lens is negative. 

55. Answer the following.
a) Calculate the power of a lens of focal length + 25 cm.
b) You can guess what the +2D in the prescription stands for. What is its
focal length? What kind of lens is it?

56. What change happens in the optical density of air on heating it?
Answer: optical density decreases

57. What happens to the light when it travels through media of different optical densities?
Answer: Light is refracted continuously.

58. Why do the stars seem to twinkle at night?
Answer: Light coming from distant stars passes through different layers of air. Each layer differs from the other in their optical densities. Hence light undergoes successive refraction. Since stars, at a greater distance, they appear like a point source. The rays of light appear to come from different points on reaching the eye after refraction. This is the reason for the twinkling of stars.

58. Do the stars really shine? Why?
Answer: The stars do not really shine. It seems so because of the refraction of the atmosphere.

59. Do planets produce energy on their own like stars?
Answer: No, The light from the sun gets reflected by the planets.

Let us assess
1. Refractive indices of different materials are given. Find out the medium through which light passes with maximum speed.
Answer: Water, Water has the lowest refractive index here.

2) The nature of images formed by two lenses are given.
(i) An erect and magnified virtual image
(ii) An erect and diminished virtual image
(a) What type of lens is used in each case?
(b) By using which type of lens will we get an image having the same size as the object? What is the position of the object?
Answer: a) 
i. Convex lens
ii. Concave lens
b) Convex lens

3. 
(a) MN represents a lens. What type of lens is this?
(b) What are the characteristics of the image?
(c) Copy the ray diagrams in the science diary and complete it.
Answer: 
a) Convex lens
b) The image is Magnified, real and inverted.
c) 

4. What do you mean by the power of a lens? What is the SI unit of the power of a lens? Calculate the power of a concave lens of focal length 25 cm.
Answer: Power of a lens is the reciprocal of focal length 
The SI unit of power is diopter
If the focal length is given in centimetres, power, p = 100 / -25 = -4D

5. Observe the figure. Light falling on two different media are shown.
(a) Which medium has greater optical density? Why?
(b) Which medium has greater refractive index?
Answer:
a) Medium 1, Here the angle of incidence is same in both figures but the angle of refraction is different. While entering from a medium of lower optical density to that of a greater one the refracted ray deviates towards the normal.
b) Medium 1

6) An object of height 3 cm is placed in front of a convex lens of focal length 20 cm at a distance of 30 cm.
(a) What is the distance to the image formed?
(b) What is the nature of the image?
(c) What is the height of the image?
Answer:

7) In the table the absolute refractive indices of certain transparent media
are given. 
(a) Find out from the table the medium of highest and lowest optical densities.
(b) If the speed of light in air is 3X10⁸m/ട, what will be the speed of light through kerosene?
(c) Will a ray of light deviate towards the normal or away from the normal when it enters from air to diamond obliquely?
(d) The refractive index of diamond is 2.42. What do you mean by this? Calculate the speed of light through the diamond. 
Answer:
a) highest - Dimond, lowest - air
c) deviates towards normal
d) Light travels through the air at 2.42 times the speed of light through a diamond.
Evaluation Questions
1. The critical angle of glass is 42˚.
a) What is meant by Critical Angle?
b) When the angle of incidence in the glass is 42˚ what will be the angle of refraction?
c) Name the Phenomenon occurs when the angle of incidence is 40˚, Define this phenomenon.
d) Name the Phenomenon occurs when the angle of incidence is 45˚, Define this phenomenon.
Answer:
a) When a ray of light passes from a medium of greater optical density to that of lower optical density, the angle of incidence at which the angle of refraction becomes 90⁰ is the critical angle.
b) 90⁰
c) refraction. When a ray of light entering obliquely from one transparent medium to another, its path undergoes a deviation at the surface of separation. This is refraction.
d) Total internal reflection.
When a ray of light passes from a medium of higher optical density to a  medium of lower optical density at an angle of incidence greater than the critical angle, the ray is reflected back to the same medium without undergoing refraction. This phenomenon is known as total internal reflection.

2. Pick out the wrong statements from the following and rewrite them after correction.
a) Refraction is due to the difference in the optical density of different media.
b) Velocity of light is greater in the media with higher optical density.
c) Optical density of glass is less than that of water.
d) Velocity of light in a vacuum is 3X10⁸m/ട.
Answer: wrong statements b,c

3. Find the magnification of the image formed when the object is placed at 2F of a convex lens.
(Greater than 1, 1, Less than 1, 0
Answer: Magnification=1 (one)

4. Statements related to the images formed by lenses are given below. Tabulate them into those related to real images and those related to virtual images.
a) Inverted
b) cannot be captured on screen
c) can be captured on a screen
d) image formed when the actual intersection of light rays occur
e) erect
f.magnification will be negative
Answer:
Real images - a, c, d,f
virtual images - b,e

5. The terms given are related to the lens. Using these fill up the following statements.
(Focal length, Principal axis, Optic centre, Centre of curvature, Radius of curvature.)
a) ...............is the centre of the lens.
b) The distance between the optic centre and the principal focus is............
c) The centre of a sphere of which lens is a part is known as..............................
d) The imaginary line joining the two centres of curvature of the lens and passing through the optic centre is ......................................................
Answer:
a) Pole                        
b) Focal length            
c) Centre of curvature  
d) Principal axis      

6. The light lays falling parallel to the principal axis of a convex lens is shown in the diagram. Complete the diagram & label the principal focus of the lens. 
 F is the focus

7. Lightray entering glass from the air at an angle is shown in the diagram. Which among the following is the correct one.
Answer: Figure iii, It bent towards the normal

8. Observe the figure. XY is the surface of separation of the media A and B. Lightray enters B from A
a) When the light ray enters B from A
(no deviation, move towards the normal, moves away from the normal)
b) Among media A& B. Which has more optical density?
c) In which medium does the light travel with more velocity?
Answer:
a) It bent towards the normal      
b) Medium B                               
c) Medium B   

9. The diagram shows the image formation by a convex lens.
a) Find out the focal length using the new Cartesian method.
b) Find its magnification using the ray diagram,?
Answer:

10. Method of measuring distances according to New Cartesian Sign Convention is given below. Choose the correct statements from these.
a) All the distances are measured form F.
b) The distances measured in the direction of the incident ray are positive.
c) It is assumed that the incident rays travel from right to left.
d) X-axis is considered as the principal axis.
Answer:
b) All distances measured along the direction of incident light is positive.
d) X-axis is considered as the principal axis.          
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