Question

A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is _____.​

Answer 1

Answer:

−20cm

Explanation:

Given: object size h=10mm, image size h

′

=5mm, image distance

v=−30cm,

To find object distance=? f=?

from the magnification formula =

h

h

′

=

u

−v

or

10mm

5mm

=

u

−30cm

or

u

−30cm

=

2

1

or u=−60

Now, it can be calculated as follows:

v

1

+

u

1

=

f

1

or

−30cm

1

+

−60cm

1

=

f

1

or

60

−2−1

=

f

1

f

1

=−

20cm

1

or f=−20cm (the negative sign confirms that the calculated focal length is of a concavemirror)