Question

An object of height 5 cm is placed 20 cm in front of a concave mirror of focal length 15 cm At
what distance from the minor, should a screen be placed to obtain a sharp Image Calculate
the size of the image formed. Draw a ray diagram for the same.

Answer 1

Explanation:

According to the question:

Object distance, u=−30 cm

Focal length, f=−15 cm

Let the Image distance be v.

By mirror formula:

v1+u1 = f4 [4pt]

⇒ v1 + −30 cm 1

=

−15 cm

1

[4pt]

⇒

v

1

=−

−30 cm

1

+

−15 cm

1

[4pt]

⇒

v

1

=

30 cm

1−2

[4pt]

⇒

v

1

=−

30 cm

1

[4pt]

∴v=−30 cm

Thus, screen should be placed 30 cm in front of the mirror (Centre of curvature) to obtain the real image.

Now,

Height of object, h

1

=2 cm

Magnification, m=

h

1

h

2

=−

u

v

Putting values of v and u:

Magnification m=

2 cm

h

2

=−

−30 cm

−30 cm

⇒

2 cm

h

2

=−1

[4pt];

⇒h

2

=−1×2 cm=−2 cm

Thus, the height of the image is 2 cm and the negative sign means the image is inverted.

Thus real, inverted image of size same as that of object is formed.

The diagram shows image formation.