Question

question:
when an object is placed at a distance of 15 cm feom a convex lens a real image is forced at a distance of 30 cm . what is the focal length of lens?

doubt
why is 'u' negative here (-15)​

Answer 1

For the lens ,

u=−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

f=10 cm,

Have a nice day mate ❤️