# Maharashtra Board Class 9 Science Chapter 11 Reflection of Light Solution

## Maharashtra Board Class 9 Science Solution Chapter 11 – Reflection of Light

Balbharati Maharashtra Board Class 9 Science Solution Chapter 11: Reflection of Light. Marathi or English Medium Students of Class 9 get here Reflection of Light full Exercise Solution.

 Std Maharashtra Class 9 Subject Science Solution Chapter Reflection of Light

a) Explain the difference between plane mirror, a concave mirror and a convex mirror with respect to the type and size of the image produced.

Ans:

Plane mirror:

• The image formed in a plane mirror is virtual and erect.
• And the size of the image produced is same as the size of the object.

Concave mirror:

• The image produced by the concave mirror may be real and inverted or virtual and erect which depends on the position of object with respect to the concave mirror.
• The size of the image obtained from the concave mirror may be magnified, diminished and sometimes of same size of the object which depends on the position of the object placed with respect to the mirror.

Convex mirror:

• The image produced by convex mirror is always virtual and erect.
• And the size of the image produced in convex mirror is always diminished.

b) Describe the positions of the source of light with respect to a concave mirror in torch light, projector lamp, flood light.

Ans:

Torch light:

In torch light concave mirror is used and the source of light is placed at the focus of the concave mirror so that after reflection we can get the parallel beam of light. The parallel beam produced helps in seeing the objects which are more distant apart also and it is having high intense beam also.

Projector lamp:

In projector lamp concave mirror is used and the source of light is placed at the centre of curvature of the concave mirror so that we can see the image produced on the projector clearly.

Floodlight:

In floodlight also concave mirror is used and the source of light is placed beyond the centre of curvature of concave mirror so that we get the bright and intense beam of ligh.

c) Why are concave mirrors are used in solar devices?

Ans:

• We know that, when a parallel beam of light which may parallel to principal axis is incident on the concave mirror then after reflection the light rays converges at focus or at a point in the focal plane.
• Due to the converging property of concave mirror they are mostly used in solar devices.
• In solar devices, the parallel beam of rays which are coming from the sun are converged and focused at a point in the focal plane of concave mirror. Due to which at that point large amount of heat is generated which is used for many purposes such as to produce electricity, for cooking also.
• Hence, due to the converging property of concave mirror they are used in solar devices.

d) Why are the mirrors fitted on the outside of the cars are convex?

Ans:

• We know that, convex mirror always produces virtual and diminished image of the object which covers the wider field of view.
• As the mirrors fitted outside the cars are convex, hence driver can see the wider view of road so that it can confirm which vehicles are behind him in which direction. So he can drive safely by seeing in the mirror of the car.
• Because of this reason the mirrors outside the cars are convex mirror.

e) Why does obtaining the image of the sun on a paper with the help of a concave mirror burn the paper?

Ans:

• We know that, when a parallel beam of light which may parallel to principal axis is incident on the concave mirror then after reflection the light rays converges at focus or at a point in the focal plane.
• When the parallel beam of rays which coming from the sun are converged and focused at a point in the focal plane of concave mirror, if we focused this rays on the paper then at that point the rays will be focused and large heat is generated due to which paper burns.

f) If a spherical mirror breaks, what type of the mirrors are the individual pieces?

Ans:

• After breaking of the spherical mirror there will be no change in the nature of the mirror.
• That means if the concave mirror is broken into pieces then each piece also acts as a concave mirror.
• While if convex mirror is broken into pieces then each piece of it acts as the convex mirror only.

2.) What sign conventions are used for reflection from a spherical mirror?

Ans:

According to the Cartesian sign conventions in spherical mirror, the pole of the mirror is taken as origin and the principal axis is considered as the X-axis.

The following are the sign conventions used:

• In spherical mirrors, the object is always placed on the left side of the mirror. And the distances taken which are parallel to principal axis are measured from the pole of the mirror always.
• The distances which are taken on the right side of the spherical mirror are taken as positive, while the distances taken on the left side of the spherical mirror are taken as negative.
• Also, the distances taken vertically upward from the principal axis are taken as positive.
• While the distances taken vertically downwards from the principal axis are taken as negative.
• The focal length of concave mirror is taken as negative and the focal length of convex mirror is taken as positive.

The following figure shows the Cartesian sign conventions used in spherical mirrors.

3.) Draw the ray diagram for the cases of images obtained in concave mirror as described in the table on page 122.

Ans:

a) When the object is placed at infinity:

When a beam of rays parallel to principal axis is incident on the concave mirror then after reflection the rays pass through focus and the image of the object will be formed at the focus of concave mirror which is point sized as shown in following figure.

b) When the object is placed behind the centre of curvature:

When the object is placed behind the centre of curvature of a concave mirror then the image will be formed between focus F and centre of curvature C of the mirror as shown in following figure.

And the image formed will be real, inverted and diminished.

c) When the object is placed at the centre of curvature:

When the object is placed at the centre of curvature C of concave mirror then image also formed at the centre of curvature and which is real, inverted and of the same size as the object size as shown in following figure.

d) When the object is placed between centre of curvature C and focus F:

When the object is placed between centre of curvature C and the focus F then the image will be formed behind the centre of curvature of concave mirror and which is real, inverted and magnified.

e) When the object is placed at the focus:

When the object is placed at the focus F of concave mirror then then the image will be formed at infinity which is real, inverted and highly enlarged as shown in following figure.

f) When the object is placed between focus F and pole P of the mirror:

When the object is placed between focus F and pole P of the concave mirror then the image will be formed behind the mirror which is virtual, erect and magnified as shown in following figure.

4.) Which type of mirrors are used in the following.

Periscope, floodlights, shaving mirror, kaleidoscope, street lights, head lamp of a car.

Ans:

• In periscope plane mirror is used.
• In floodlights and shaving mirror the concave mirror is used.
• In kaleidoscope also plane mirror is used.
• In street lights convex mirrors are used.
• In head lamps of a car concave mirror is used.

5.) Solve the following examples:

a) An object of height 7cm is kept at a distance of 25cm in front of a concave mirror. The focal length of the mirror is 15cm. At what distance from the mirror should a screen be kept so as to get clear image? What will be the size and nature of the image?

Ans:

Given that,

For concave mirror,

Object height h1= 7cm

Object distance u= -25cm

Focal length f= -15cm

We know that, the mirrors formula is given by,

1/f = 1/u + 1/v

1/-15= 1/-25 + 1/v

Thus, 1/v = -1/15 + 1/25

1/v = -2/75

Hence, v= -37.5cm

Thus, as the image will be formed at a distance 37.5cm from the concave mirror hence, screen should be kept at a distance 37.5cm from the concave mirror.

We know that, the magnification is given by,

m= h2/h1= -v/u

h2/7= 37.5/-25

Thus, h2= -10.5cm

Thus, the size of the image formed will be 10.5cm.

From the above data we can say that the image formed will be real and inverted.

b) A convex mirror has a focal length of 18cm. The image of an object kept in front of the mirror is half the height of the object. What is the distance of the object from the mirror?

Ans:

Given that,

For convex mirror,

Focal length f= 18cm

And h2= h1/2

Hence, h2/h1 = 1/2

We know that, magnification is given by,

m= h2/h1 = -v/u

Thus, -v/u= 1/2

Hence, v= -u/2

We have mirrors formula as,

1/f = 1/u + 1/v

But, v= -u/2

Then,

1/18= 1/u -2/u

1/18= -1/u

Thus, u= -18 cm

Thus, the object distance is found to be 18 cm.

c) A 10cm long stick is kept in front of a concave mirror having focal length of 10cm in a such way that the end of the stick closest to the pole is at a distance of 20cm. What will be the length of the image?

Ans:

Given that,

For concave mirror,

For end A:

Focal length f= -10cm

Object distance u = -30cm

We know that, the mirrors formula is given by,

1/f= 1/u + 1/v

1/-10= 1/-30 + 1/v

Thus, 1/v = -1/10 + 1/30

Hence, 1/v = -2/30

v = -15 cm

For end B:

Focal length f= -10cm

Object distance u= -20cm

We know that the mirrors formula is given by,

1/f = 1/u + 1/v

1/-10= 1/-20 + 1/v

Thus, 1/v = -1/10 + 1/20

Hence, v= -20cm

Thus, the length of the image formed is given by,

20 – 15= 5 cm

6.) Three mirrors are created from a single sphere. Which of the following- pole, center of curvature, radius of curvature, principal axis will be common to them and which will not be common.

Ans

• See the following labelled diagram in which we have named the mirrors as M1, M2 and M3.
• From figure we conclude that, the pole P of all mirrors is not common.
• But, centre of curvature C and radius of curvature R= CP of all the three mirrors are in common.
• Principal axis PA is not also in common.
• And main thing here is that, the radius of curvature R is the radius of sphere here.

Updated: August 18, 2021 — 3:35 pm