Question

a 36. An object is placed at a distance x from a convex lens when a real image is formed at a distance of 15cm from the lens. If focal length of lens is 10cm, calculate the value of X?

(a) 10cm (b) 20cm (c) 25cm (d) 15cm​

Answer 1

Explanation:

=−15 ;f=+10

v

1

−

u

1

=

f

1

v

1

−

15

−1

=

10

1

v=+30

Distance between mirror and lens is 10 cm then the image is formed at 20cm from the mirror , this becomes object distance for the mirror and the image formed by the mirror serves as the object for lens.

Final image formed by lens is same at the initial position as that of the object

for lens,

v=+15 ; f=+10

v

1

−

u

1

=

f

1

15

1

−

u

1

=

10

1

u=−30 , this is the image for the mirror

now for mirror , u=+20 ;v=+20

v

1

+

u

1

=

f

1

20

1

+

20

1

=

f

1

hence option a 10cm is the correct answer