FRANK Solutions for Class 9 Physics Chapter 6 - Light

Chapter 6 - Light Exercise 261

Question 1

Name the mirror having a wider field of view.

Solution 1

Convex mirror has a wider field of view.

Question 2

Name the type of mirror having a focal length of -15 cm.

Solution 2

The mirror having -15 cm as its focal length is a concave mirror because focal length is taken negative only in case of concave mirror.

Question 3

What do you mean by principle axis?

Solution 3

Principal axis is the straight line passing through the pole and the centre of curvature.

Question 4

Define linear magnification of a mirror.

Solution 4

Linear magnification is defined as the ratio of the height of the image to the height of the object. It is taken to be positive for an image to be virtual and erect and negative when image is real and inverted.
Magnification = height of image / height of object.

Question 5

Define pole of a mirror.

Solution 5

Pole is the centre of the reflecting surface, in this case spherical mirror.

Question 6

What do you mean by centre of curvature of a mirror?

Solution 6

Centre of curvature is the centre of the imaginary sphere to which the mirror belongs.

Question 7

State three characteristics of light.

Solution 7

Three characteristics of light are:-
(a). Light waves can travel through vacuum.
(b). Light waves are transverse waves.
(c). The velocity of light in vacuum is 3 x 10⁸ m/s.

Question 8

Give three differences between sound and light.

Solution 8

Three distinctions between light and sound waves are
(i) Light waves can travel through vacuum while sound waves cannot.
(ii) Light waves are transverse waves while sound waves are longitudinal waves.
(iii) The velocity of light in air is 3 x 10⁸ m/s while the speed of light in air is just about 330 m/s.

Question 9

State three characteristics of the image formed by a concave mirror.

Solution 9

(i) When position of object is at infinity, concave mirror forms a point and Real image at Focus point.
(ii) When position of object is beyond C, concave mirror forms a Diminished, Real and inverted image between F and C.
(iii)When position of object is at C, concave mirror forms a Magnified, Real and inverted image at C.

Question 10

State three characteristics of the image formed by a convex mirror.

Solution 10

Image formed by a convex mirror is always Diminished, Virtual and Erect.

Question 11

Give three uses of concave mirrors.

Solution 11

Concave mirrors are used in reflecting microscope, in shaving and make up glasses and in ophthalmoscope.

Question 12

Name the mirror which always produces an erect image of the objects.

Solution 12

Convex mirror always produces an erect image of the object.

Question 13

Write the sign convection used in optics.

Solution 13

(i) The distance from the pole in the direction of incident ray is taken positive.
(ii) The distance from the pole in the direction opposite to the incident ray is taken negative.

Question 14

What is a mirror formula? Define linear magnification produced by a mirror?

Solution 14

Mirror formula is the relation between the focal length f of the mirror, the distance u of the object from the pole of the mirror, and the distance v of the image from the pole.
Mirror formula is
1/v +1/u = 1/f.
Linear magnification is defined as the ratio of the height of the image to the height of the object. It is taken to be positive for an image to be virtual and erect and negative when image is real and inverted.
Magnification = height of image / height of object.

Question 15

Define linear magnification. Does it have any units? Write down a formula for the magnification produced by a concave mirror in terms of image distance and object distance.

Solution 15

Linear magnification is defined as the ratio of the height of the image to the height of the object. It is taken to be positive for an image to be virtual and erect and negative when image is real and inverted.
Magnification produced by concave mirror is:
Magnification = height of image / height of object.
It is a pure ratio and does not have any units.

Question 16

Distinguish between real and virtual images.

Solution 16

Question 17

Distinguish between regular and irregular reflection.

Solution 17

A smooth and polished surface causes regular reflection while a rough and unpolished surface causes irregular reflection.

Question 18

What do you mean by reflection? State the laws of reflection.

Solution 18

When rays of light fall on a surface, they are turned back into the same medium in accordance with some definite laws. This process is known as reflection.
Reflection obeys following two laws
(a). The incident ray, the reflected ray, and the normal at the point of incidence, all lie in the same plane.
(b). The angle of incidence and the angle of reflection are always equal.

Question 19

How will you distinguish between a plane mirror, a concave mirror and a convex mirror without touching them?

Solution 19

You can distinguish between plane mirror, a concave mirror, and a convex mirror without touching them. When you look into these mirrors by bringing your face close to each mirror, they will produce an image of your face of different types.
A plane mirror will produce an image of the same size as your face.
A concave mirror will produce a magnified image of your face.
A convex mirror will produce Diminished image of your face.

Question 20

How can you distinguish between a convex mirror and concave mirror? Explain.

Solution 20

You can distinguish between a concave mirror and a convex mirror without touching them. When you look into these mirrors by bringing your face close to each mirror, they will produce an image of your face of different types.
A concave mirror will produce a magnified image of your face.
A convex mirror will produce Diminished image of your face.

Question 21

What kind of mirror is used in vehicles to see the traffic on the rear side?

Solution 21

Convex mirror is used in vehicles to see the traffic on rear side.

Question 22

If you want to see an enlarged image of your face, state whether you will use a convex mirror or a concave mirror?

Solution 22

We will use convex mirror to see an enlarged image of our face.

Question 23

An object is placed at a long distance in front of a convex mirror of radius of curvature 20 cm. State the position of the image.

Solution 23

Image of object placed at a long distance in front of a convex mirror is formed at principal focus. Radius of curvature of convex mirror is 20 cm.
Focal length of convex mirror = radius of curvature/2.
Focal length of convex mirror = 20/2 = 10 cm.
So image will form at principal focus 10 cm away from pole.

Question 24

Name the spherical mirror which can produce a real and diminished image of an object.

Solution 24

Concave mirror can produce real and diminished image of the object.

Question 25

What do you mean by focal length of a mirror?

Solution 25

The distance of the principal focus from the pole of the mirror is called the focal length of the mirror.

Question 26

Name the type of mirror having a focal length of +20 cm.

Solution 26

The mirror having +20 cm as its focal length is a convex mirror because focal length is taken positive only in case of convex mirror.

Question 27

What is the focal length of a plane mirror?

Solution 27

The focal length of plane mirror is infinity.

Chapter - Exercise

Solution 1

Chapter 6 - Light Exercise 262

Question 1

Solution 1

Question 2

Give two uses of convex mirror.

Solution 2

Uses of concave mirror:
(i) Concave mirrors are used in reflecting microscope
(ii) Concave mirrors are used in shaving and make up glasses.
Uses of convex mirror: Convex mirrors are used as a rear view mirror in automobiles as it provides a wider view of following traffic.

Question 3

You can see the reflection of your clearly on a polished table-top is not polished. Explain.

Solution 3

We can see the reflection of our face on a polished table top because a regular reflection occurs in case of a polished surface while on a unpolished table top irregular reflection occurs which make image of our face unclear.

Question 4

State whether the following statements are true or false. If false, correct them.
(i) The angle of incidence is the angle made by the incident ray with the plane mirror.
(ii) If a ray of light incident on a plane mirror is such that it makes an angle of 30⁰ with the mirror, then the angle of reflection is 60⁰.
(iii) If the incident ray makes an angle of X⁰ with the normal, then the reflected ray is 2X⁰.
(iv) The image in a plane mirror is situated in the mirror.
(v) The image formed in a plane mirror is real, erect and same size as that of the object.

Solution 4

(i)The angle of incidence is the angle made by the incident ray with the plane mirror. {FALSE}
Correct statement is the angle of incidence is the angle made by the incident ray with the normal to the surface of plane mirror.
(ii) If a ray of light incident on a plane mirror is such that it makes an angle of 30^o with the normal, then the angle of reflection is 60^o.{FALSE}
Correct statement is if a ray of light incident on a plane mirror is such that it makes an angle of 30^o with the normal, then the angle of reflection is 30^o.
(iii)If the incident ray makes an angle of Xo with the normal, then the angle between the incident ray and reflected ray is 2X^o. {TRUE}
(iv)The image formed in a plane mirror is real, erect and same size as that of the object. {FALSE}
Correct statement is the image formed in a plane mirror is virtual, erect and same size as that of the object.

Question 5

Solution 5

Question 6

You are provided with two plane mirrors. Draw a ray diagram to show how the ray of light can be turned through an angle of 180⁰.

Solution 6

Question 7

What are the characteristics of the image formed by a plane mirror?

Solution 7

The image formed by a plane mirror is erect and virtual. It is a laterally inverted image. The image formed is of the same size as that of the object. Also, the image and the object are equidistant from the mirror.

Question 8

Solution 8

Question 9

A boy is standing in front of a plane mirror at a distance of 3 m from it.
(i) What is the distance between the boy and his image?
(ii) If the boy moves 1 m backward, find the distance between the image and the boy.

Solution 9

Given, distance of boy from the mirror = 3 m
(a) Distance of image from mirror = distance of boy from the mirror = 3 m
Distance between boy and his image = distance of boy from the mirror + distance of image from mirror = 3+3 = 6 m
(b) Now, distance of boy from the mirror = 4 m
Distance of image from mirror = 4 m
Distance between boy and his image = distance of boy from the mirror + distance of image from mirror = 4+4 = 8m.

Question 10

Draw a neat and labeled ray diagram of a simple periscope. Give two uses of a periscope.

Solution 10

Periscope is used to see over the top of an obstacle. It is also used in submarines for observing for movement of ships. It can be used from the trenches for observing the movement on the surface of earth.

Chapter 6 - Light Exercise 263

Question 1

Solution 1

Question 2

Define the following terms:
(i) Pole
(ii) Centre of curvature
(iii) Principle focus
(iv) Principle axis
(v) Focus of a concave mirror
(vi) Normal.

Solution 2

(a) Pole is the centre of the reflecting surface, in this case spherical mirror.
(b) Centre of curvature is the centre of the imaginary sphere to which the mirror belongs
(c) Principal focus of a spherical mirror is a point on the principal axis of the mirror, where all the rays travelling parallel to the principal axis and close to it after reflection from the mirror, converge to or appear to diverge from.
(d) Principal axis is the straight line passing through the pole and the centre of curvature.
(e) Focus of a concave mirror is a point on the principal axis of the mirror, where all the rays travelling parallel to the principal axis and close to it after reflection from the mirror converge to that point.
(f) Normal to the surface of a mirror at any point is the straight line at right angle to the tangent drawn at that point.

Question 3

Solution 3

Question 4

Solution 4

Question 5

Solution 5

Question 6

An object is placed in front of a convex mirror of focal length 25 cm. By scale drawing show the formation of image and state the nature of image.
Given that v = 10 cm.

Solution 6

Question 7

(a) Draw a ray diagram to show that a convex mirror has a wider field of view.
(b) A concave mirror can be used to produce a parallel beam of light. Draw a ray to illustrate this.

Solution 7

Question 8

An object 10 cm high is placed at a distance of 20 cm in front of a spherical mirror of focal length 25 cm. By scale drawing find the nature, position and magnification of image in each of the following cases:
(a) Concave mirror,
(b) Convex mirror.

Solution 8

Chapter 6 - Light Exercise 264

Question 1

Solution 1

Question 2

An object 5 cm in height is placed at a distance of 30 m in front of a concave mirror of focal length 40 cm. By scale drawing find the nature, size, position and magnification of the image.

Solution 2

Question 3

Solution 3

Question 4

Solution 4