In the following diagram, an object ‘O’ is placed 15 cm in front of a convex lens L1 of focal length 20 cm and the final image is formed at ‘I’ at a distance of 80 cm from the second lens L2. Find the focal length of the lens L2.
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In the following diagram, an object ‘O’ is placed 15 cm in front of a convex lens L1 of focal length 20 cm and the final image is formed at ‘I’ at a distance of 80 cm from the second lens L2. Find the focal length of the lens L2.
In the following diagram, an object ‘O’ is placed 15 cm in front of a convex lens L1 of focal length 20 cm and the final image is formed at ‘I’ at a distance of 80 cm from the second lens L2. Find the focal length of the lens L2.
For lens L1
u = −15cm
f = +20 cm
using lens formula
\(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\)
\(\frac{1}{v}=\frac{1}{u}+\frac{1}{f}\)
\(\frac{1}{v}=-\frac{1}{15}+\frac{1}{20}=-\frac{1}{60}\)
v = −60 cm
This image will act as object for lens L2 lens
u = −20 − 60 = −80cm
v = + 80 cm
\(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\)
\(\frac{1}{80}+\frac{1}{80}=\frac{1}{f}\)
\(\frac{1}{f}=\frac{1}{40}orf=40cm\)