# Chapter 10 Light Reflection and Refraction Class 10 Science Notes

Students should read Chapter 10 Light Reflection and Refraction Class 10 Science Notes provided below. These notes have been prepared based on the latest syllabus and books issued by NCERT, CBSE and KVS. These important revision notes will be really useful for students to understand the important topics given in the chapter Chemical Reactions and Equations in Class 10 Science. We have provided class 10 science notes for all chapters.

## Revision Notes Chapter 10 Light Reflection and Refraction Class 10 Science

Chapter 10 Light Reflection and Refraction is an important chapter in Class 10 Science. The following notes will help you to understand and easily learn all important points to help you score more marks.

Very Short Anwer Type Questions

Question. What is the name of the phenomenon in which the right side of an object appears to be the left side of the image in a plane mirror?
Answer : Lateral inversion is the phenomenon in which the right side of an object appears to be the left side in its image in a plane mirror.

Question. Write two different uses of concave mirrors.
Answer : Uses of concave mirrors:
(1) It is used in torches, headlights of vehicles and search-lights to get powerful parallel beams of light.
(2) It is used as a shaving mirror to see a larger image of the face when held close to the face.
(3) It is used by dentists and ENT specialists to see larger images of teeth and inner parts of ear and throat.
(4) Large concave mirrors or parabolic mirrors are used in solar furnaces to concentrate sunlight.

Question. Under what condition in an arrangement of two plane mirrors, incident ray and reflected ray will always be parallel to each other, whatever may be angle of incidence?
Answer : When two plane mirrors are placed at right angle with each other, then the incident ray and reflected ray will always be parallel to each other, irrespective of the angle of incidence.

Question. An object O is placed in front of a plane mirror as shown below.

At which point is the image of the object ‘O’ formed?
Answer : At point R, the image of object ‘O’ is formed.

Question. Absolute refractive indices of medium ‘A’ and medium ‘B’ are ‘na’ and ‘nb’, What is the refractive index of medium ‘B’ with respect to medium ‘A’?
Answer : The absolute refractive index (n) of a medium is the ratio of speed of light in vacuum (c) to the speed of light in that medium (v).

Question. Which type of mirror should a student use if he wants to project the image of a candle flame on the walls of the school laboratory?
Answer : To project the image of a candle flame on the walls of the school laboratory, he should use a concave mirror as only a concave mirror forms a real image (which can be obtained on a screen) for all positions of object except when the object is placed between the pole and focus of the mirror. A convex mirror and plane mirror,on the other hand, always form virtual images.

Question. What is the power of a converging lens having a focal length of 20 cm.
Answer : As convex lenses are converging lenses and it is given that Focal length = + 20 cm = + 0.2 m, as per the new sign convention.
As Power = 1/f (in m)
Therefore, P = +1/0.2D = +5D

Question. Observe the given figure. What is the angle of reflection?

Answer : The angle of reflection is the angle between the normal and the reflected ray.
Therefore: 90 – 70 = 200

Question. A man stands 10 m away in front of a large plane mirror. How far must he walk before he is 5 m away from his image?
Answer : As the distance between the man and the mirror is 10 m, the distance between man and his image is 10 + 10 or 20 m, as image distance and object distance are equal. Distance between the man and his image is 5 m when the man is 2.5 m away from the mirror. Therefore, the man has to walk 10 m – 2.5m = 7.5 m towards the mirror.

Question. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Radius of curvature = 2 x Focal length (f)
f = R/2 = 32/2 = 16 cm
Therefore, convex mirror’s focal length is 16 cm.

Question. If a magnification of -1 is to be obtained using a concave mirror of focal length 6 cm, then what should be the position of the object?
Answer : Magnification of -1 means that the image is real and size of image is equal to the size of the object. This is possible when the object is placed at C of a concave mirror. Therefore, position of object = C = 2f = 12 cm in front of the concave mirror.

Short Anwer Type Questions

Question. A concave mirror is used for image formation for different positions of an object. What inferences can be drawn about the following when an object is placed at a distance of 10 cm from the pole of a concave mirror of focal length 15 cm?
(A) Position of the image
(B) Size of the image
(C) Nature of the image
Draw a labelled ray diagram to Justify your inferences.
Answer : Object distance u = –10 cm
focal length of concave mirror f = –15 cm
Position of image v = ?
using mirror formula

(A) Position of image 30 cm behind the mirror
(B) size of image: 3 times the size of object
(C) virtual and erect

Question. A pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water. Will the pencil appear to be bent to the same extent if instead of water we use liquids like kerosene or turpentine? Support your answer with reason.
Answer : A pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water due to the phenomenon of refraction of light. No, the pencil will not appear to be bent to the same extent if instead of water we use liquids like kerosene or turpentine. This is because bending will be different in different liquids, since velocity of light at the interface separating two media depends on the relative refractive index of the medium.

Question. Under what condition in an arrangement of two plane mirrors, will the incident ray and the reflected ray always be parallel to each other, whatever may be the angle of incidence? Show the same with the help of diagram.
Answer : When two plane mirrors are placed at right angle i.e., at 90 0 to each other, then the incident ray and the reflected ray will always be parallel to each other, irrespective of the angle of incidence.

Question. Draw a ray diagram showing the path of the rays of light when it enters with oblique incidence (1) from air into water and (2) from water into air.
Answer : (1) Ray diagram showing the path of rays of light when it enters with oblique incidence from air into water.

(2) Ray diagram showing the path of rays of light when it enters with oblique incidence from water into air.

Question. An object 5 cm high is placed at a distance of 2f from a convex lens. What is the height of the image formed?
Answer : We know that when an object is placed at a distance of 2f in front of a convex lens, then the image formed is of the same size as the object.
Hence, the height of the image formed will be 5 cm.

Question. A convex mirror used on a bus has a focal length of 200 cm. If a scooter is located at 100 cm. from this mirror, find the position, nature and magnification of the image formed in the mirror.
focal length ‘f’ = 200 cm
object distance = –100 cm
Since the object (scooter) is placed to the left of the mirror, it is always taken with a negative sign.

Thus, the position of the image is 66.67 cm behind the convex mirror. Since the image is formed behind the convex mirror, its nature will be virtual and erect.

Question. Draw ray diagram in each of the following cases to show what happens after reflection to the incident ray when:
(A) it is parallel to the principal axis and falling on a convex mirror.
(B) it is falling on a concave mirror while passing through its principal focus.
(C) it is coming oblique to the principal axis and falling on the pole of a convex mirror.
Answer : (A) An incident ray parallel to the principal axis, after reflection appear to diverge from the principal focus is case of convex mirror.

Question. Why does a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself? Explain using a diagram.
Answer : When a ray of light is incident on a rectangular glass slab immersed in a medium, the emergent ray comes out parallel to the incident ray.
This is because the extent of bending of the ray of light at the opposite parallel faces (airglass interface) and (glass-air interface) at the rectangular glass slab is equal and opposite.
The path of the light ray is as shown:

In the given figure, EO is the incident ray, OO’ is the refracted ray and O’H is the emergent ray. The light ray at point O has entered from a rarer medium to a denser medium from air to glass and so the light ray has bent towards the normal. At O’, the light ray has entered from the glass to the air from a denser medium to a rarer medium. The light here has bent away from the normal. The extent of bending of the ray of light at the opposite parallel faces AB (air-glass interface) and CD (glass-air interface) of the rectangular glass slab is equal and opposite. This is why the ray emerges parallel to the incident ray. Hence, a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself.

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