Question

cm
The focal lengths observed in three cases are:
f1 =________ cm
f2=________cm
f3=________cm​

Answer 1

Answer:

We know lens formula

f1 =

v1 − u1

For first lens: f=+10 cm, u=−∞ (parallel beam is incident on lens).

10 1 = v1 − −∞1101 = v1

v=10

Rays meet at 10 cm right of 1st lens and we know 2nd lens is 15 cm right of lens 1.

so for second lens u=15 cm−10 cm= 5 cm left of lens.

u=−5 cm and f=−20cm

−20

1 = v1 −−51−201 = v1 + 5120−1−4 = v14−1 = v1

v=−4

Image formed at 4 cm left of lens 2.

For third lens object distance u=5 cm+4 cm= 9 cm left of

lens.u=−9 cm and f=−9 cm91 = v1 − −91−201 = v1 + 9191−1 = v10= v1

v=∞ which means parallel rays.

So, final rays will not converge at any point and will travel in a parallel beam.