Question

An object is located $4m$ from the first of two thin converging lenses of focal lengths $2m$ and $1m$, respectively. The lenses are separated by $3m$. The final image formed by the second lens is located from the source at a distance of:

• A. $8.0m$
• B. $7.5m$
• C. $6.0m$
• D. $6.5m$

Answer 1

The correct option is B $7.5m$

We know the lens equation $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$.

Applying this for first lens with $u=-4m$ and $f=2m$, we get $v=4m$.

Hence, first image is at a distance fo 1m from the second lens towards its right. This will work as an object for the second lens.

Applying the equation again with $u=1m$ and $f=1m$, we get $v=0.5m$.

Hence total distance of final image from object is $4+3+0.5=7.5m$