Question

An object of certain height is placed in front of a concave mirror of focal length 10 cm. What would be the image distance if the magnification is (-1)?

Answer 1

Answer:

(a) In concave mirror, using sign convention

u=−10cm

R=−15cm

f=

2

−15

cm

Using mirror’s relation

v

1

+

u

1

=

f

1

v

1

=

f

1

−

u

1

v

1

=−

15

2

+

10

1

v

1

=

30

−1

v=−30cm

Image is real and it forms at a distance of 30 cm in front of mirror.

Magnification

m=

u

−v

m=

−10

30

=−3

This shows image is inverted and 3 times magnified.

(b) again using sign convention

u=−5cm

f=

2

−15

cm

v

1

=

f

1

−

u

1

v

1

=

15

1

v=+15cm

Image is formed at a distance of 15 cm behind the mirror.

Magnification

m=

u

−v

=

−5

−15

=3

+3 magnification shows image is virtual and erect.