Wed. Jul 6th, 2022

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

Question 1:
What happens when a ray of light falls normally (or perpendicularly) on the surface of a plane mirror?
Solution :
Light ray will be reflected back along the same path.

Q2: Write Snell’s law.

Ans: Snell’s law is defined as: “The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for the light of a given colour and for the given pair of media”.

Q3: What is the Refractive Index?

Ans: The ratio of the speed of light in vacuum to its speed in a particular medium is known as the refractive index of the medium.

It is represented n.

n=c/v

where

n is refractive index

c is speed of light in vacuum.

v is the speed of light in the medium.

Question 4:
A ray of light is incident normally on a plane mirror. What will be the :
(a) angle of incidence ?
(b) angle of reflection ?
Solution :
(a) 00
(b) 00

Question 5:
What type of image is formed :
(a) in a plane mirror ?
(b) on a cinema screen ?
Solution :
(a) Virtual image
(b) Real image

Question 6:
What kind of mirror is required for obtaining a virtual image of the same size as the object?
Solution :
Plane mirror.

Question 7:
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?
Solution :
Lateral inversion.

Question 9:
If an object is placed at a distance of 10 cm in from of a plane mirror, how far would it be from its image?
Solution :
Distance between object and mirror=10cm.
Distance between mirror and image=10cm.
Therefore, the distance between object and image=10+10=20cm

Question 11:
The image seen in a plane mirror cannot be formed on a screen. What name is given to this type of image?
Solution :
Virtual image.

Question 12:
Fill in the following blank with a suitable word :
When light is reflected, the angles of incidence and reflection are…………
Solution :
Equal.

Question 13:
State whether the following statement is true or false :
A student says that we can see an object because light from our eyes is reflected back by the object.
Solution :
False.

Question 14:
Where is the image when you look at something in a mirror ?
Solution :
At the back side of mirror.

Question 16:
Write the differences between a real image and a virtual image?
Solution :

Question 18:
What is a lateral inversion?
Solution :
Lateral inversion is the reversal of mirror image where the right side of the object appears on the left side behind the mirror.

Question 26:
What is meant by ‘reflection of light’?

Define the following terms used in the study of reflection of light by drawing a labelled ray-diagram :
(a) Incident ray
(b) Point of incidence
(c) Normal
(d) Reflected ray
(e) Angle of incidence
(f) Angle of reflection
Solution :
The process of sending back the light rays which fall on the surface of an object is called the reflection of light. (a) Incident ray: The ray of light that falls on the mirror surface is called the incident ray.
(b) Point of incidence: The point at which the incident ray falls on the mirror is called the point of incidence.
(c) Normal: The normal is a line at right angle to the mirror surface at the point of incidence.
(d) Reflected ray: The ray of light which is sent back by the mirror is called the reflected rays.
(e) Angle of incidence: The angle of incidence is the angle made by the incident ray with the normal at the point of incidence.
(f) Angle of reflection: The angle of reflection is the angle made by the reflected ray with the normal at the point of incidence.

Question 27:
State and explain the laws of reflection of light at a plane surface (like a plane mirror), with the help of a labeled ray diagram. Mark the angles of ‘incidence’ and ‘reflection’ clearly on the diagram.

If the angle of reflection is 47.5°, what will be the angle of incidence ?
Solution : Laws of reflection of light:

I) The incidence ray, the reflected ray and the normal (at the point of incidence), all lie in the same plane.

ii) The angle of reflection is always equal to the angle of incidence.

If the angle of reflection is 47.5o, the angle of incidence will also be 47.5o in accordance with the second law of reflection.

Question 28:
State the characteristics of the image formed in a plane mirror.
Solution :
Characteristics of the image formed in a plane mirror:
(i) The image formed in a plane mirror is virtual.
(ii) The image formed in a plane mirror is erect.
(iii) The image in a plane mirror is of the same size as the object.
(iv) The image formed by a plane mirror is at the same distance behind the mirror as the object is in front of the mirror.
(v) The image formed by a plane mirror is laterally inverted.

Question 2:
Out of convex mirror and concave mirror, whose focus is situated behind the mirror?
Solution :
Convex mirror

Question 6:
What is a spherical mirror?

Distinguish between a concave mirror and a convex mirror.

Solution :
A spherical mirror is a mirror whose reflecting surface is part of a hollow sphere of glass.

Question 7:
Name the two types of spherical mirrors. What type of mirror is represented by the:
(a) back side of a shining steel spoon ?
(b) front side of a shining steel spoon ?
Solution :
Type of mirror represented by the:
(a) back side of a shining steel spoon – convex mirror
(b) front side of a shining steel spoon – concave mirror

Question 11:
Define

(a) centre of curvature

(c) pole

(d) principal axis, and

(e) aperture, of a spherical mirror
Solution :
(a) Centre of curvature: Centre of curvature of a spherical mirror is the centre of the hollow sphere of a glass of which the mirror is a part.

(b) Radius of curvature: The radius of curvature of a spherical mirror is the radius of the hollow sphere of a glass of which the mirror is a part.

(c) Pole: The center of a spherical mirror is called its pole.

d) Principal axis: The straight line passing through the center of curvature and pole of a spherical mirror is called its principal axis.

(e) Aperture: The portion of a mirror from which the reflection of light actually takes place is called the aperture of the mirror.

Question 20:
When a spherical mirror is held towards the sun and its sharp image is formed on a piece of carbon paper for some time, a hole is burnt in the carbon paper.
(a) What is the nature of spherical mirror ?
(b) Why is a hole burnt in the carbon paper ?
(c) At which point of the spherical mirror the carbon paper is placed ?
(d) What name is given to the distance between spherical mirror and carbon paper ?
(e) What is the advantage of using a carbon paper rather than a white paper ?
Solution :
(a) Concave mirror
(b) A lot of sun’s heat rays are concentrated at the point of sun’s image which burn the hole in carbon paper.
(c) At the focus.
(d) Focal length.
(e) A black carbon paper absorbs more heat rays and hence burns a hole more easily (than a white paper).

Question 23:
Which type of mirror is used in a solar furnace? Support your answer with reason.
Solution :
Concave mirror is used in a solar furnace.
A concave mirror is used in the design of solar furnaces because they converge the parallel sun rays at a point. This helps to increase the temperature of the furnace.

Question 24:
Name the type of mirror used by dentists. How does it help ?
Solution :
Concave mirror is used by dentists.
The dentist holds a small concave mirror in such a way that the tooth lies within its focus. A magnified image of the tooth is then seen by the dentist in the concave mirror. Since the tooth looks much bigger, it becomes easy to examine the defect in the tooth.

Question 25:
Explain why concave mirrors are used as shaving mirrors.
Solution :
Concave mirrors are used as shaving mirrors. This is because when the face is placed close to a concave mirror (so that the face is within its focus) the concave mirror produces a magnified and erect image of the face. Since a large image of the face is seen in the concave mirror, it becomes easier to make a smooth shave.

Question 26:
Give two uses of concave mirrors.
Solution :
Uses of a concave mirror
(i) Concave mirrors are used as shaving mirrors.
(ii) Concave mirrors are used by dentists to see the large images of the teeth of patients.

Question 3:
What is the nature of a mirror having a focal length of, +10 cm ?
Solution :
Convex mirror (since focal length is positive).

Question 4:
What kind of mirror can have a focal length of, – 20 cm ?
Solution :
Concave mirror (since focal length is negative).

Question 21:
Give two uses of a convex mirror. Explain why you would choose convex mirror for these uses.
Solution :
Two uses of convex mirror:
(i) A convex mirror is used as a rear-view mirror in vehicles because it forms erect and dimished images of the objects and has a wider field of view.
(ii) Big convex mirrors are used as security mirror in shops so that a large number of goods displayed in the shop can be seen in the convex mirror.

Question 23:
Which of the following are concave mirrors and which convex mirrors ?
Shaving mirrors, Car headlight mirror, Searchlight mirror, Driving mirror, Dentist’s inspection mirror, Torch mirror, Staircase mirror in a double-decker bus, Make-up mirror, Solar furnace mirror, Satellite TV dish, Shop security mirror.
Solution :
Shaving mirror – concave mirror.
Car headlight mirror – concave mirror.
Searchlight mirror – concave mirror.
Driving mirror – convex mirror.
Dentist’s inspection mirror – concave mirror.
Torch mirror – concave mirror.
Staircase mirror in a double-decker bus – convex mirror.
Make-up mirror – concave mirror.
Solar furnace mirror – concave mirror.
Satellite TV dish – concave mirror.
Shop security mirror – convex mirror.

Q. What is refraction of light?

Ans: Refraction of light is the phenomenon of change in the path of light in going from one medium to another.