In the given figure below, there are $2$ convex lenses $L_{1}$ and $L_{2}$ having focal length $f_{1}$ and $f_{2}$ respectively. The distance between $L_{1}$ and $L_{2}$ will be:

f_{1}

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f_{2}

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f_{1} + f_{2}

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f_{1} - f_{2}

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Solution

The correct option is C $f_{1} + f_{2}$
For $L_{1}$ , rays are coming from infinity and hence, image is formed at focus.

So, image distance, $v_{1} = f_{1}$

For $L_{2}$ , rays after refraction, become parallel to principal axis.
Hence, the object for the second lens is at its focus.

Object distance for $L_{2}, u_{2} = f_{2}$

Therefore, the distance between the lenses will be

$L = v_{1} + u_{2} = f_{1} + f_{2}$

Hence, option $(C)$ is correct.

Why this question ?
For learning the concept of equivalent focal length of combination of different lenses & displaced by some distance $d$ .

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