TS EAMCET 9-Sep-2020 Shift 1 Solved Paper

Given, lens combination and position of object is as shown belowFor first lens, u=−30cm,f=10cm
By lens formula,
‌

1

v

−‌

1

u

=‌

1

f

⇒‌

1

v

=‌

1

f

+‌

1

u

⇒‌‌‌

1

v

=‌

1

10

−‌

1

30

=‌

3−1

30

⇒‌‌‌

1

v

=‌

1

15

or v=15cm
Now, image of first lens acts like object for second lens.
For second lens.u=+10cm,f=−10cm
Again, using lens formula, ‌

1

v

=‌

1

f

+‌

1

u

=‌

1

−10

+‌

1

10

⇒‌‌‌

1

v

=0 or v=∞
This means light rays becomes parallel after refraction from the second lens.
When these parallel light rays passes through third lens, their image is formed at focus. Hence, distance of image from third lens is 30cm.