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Standard XII

Physics

Lens Maker's Formula

You are given...

Question

You are given three lenses $L_{1}, L_{2}$ and $L_{3}$ each of focal length $20 c m$ . An object is kept at $40 c m$ in front of $L_{1}$ , as shown. The final real image is formed at the focus $^{'} I^{'}$ of $L_{3}$ . Find the separations between $L_{1}, L_{2}$ and $L_{3}$ .

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Solution

The separations between the lenses will be as follows:
Distance between $L_{2}$ and $L_{3}$ doesn’t matter as the image formed by $L_{2}$ is at infinity so that $X_{2}$ can take any value.
Image by $L_{3}$ is formed at the focus. So the object should lie at infinity for $L_{3}$ .
So, $L_{2}$ will form image at infinity and the object for $L_{2}$ should be at its focus.
But wehave seen above that the image by $L_{1}$ is formed at $40$ cm right of $L_{1}$ which is at $20$ cm left of $L_{2}$ .

So, Distance between $L_{1} + L_{2} = 20 + 40 = 60 c m$

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You are given three lenses L1,L2 and L3 each of focal length 20cm. An object is kept at 40cm in front of L1, as shown. The final real image is formed at the focus ′I′ of L3. Find the separations between L1,L2 and L3.

You are given three lenses $L_{1}, L_{2}$ and $L_{3}$ each of focal length $20 c m$ . An object is kept at $40 c m$ in front of $L_{1}$ , as shown. The final real image is formed at the focus $^{'} I^{'}$ of $L_{3}$ . Find the separations between $L_{1}, L_{2}$ and $L_{3}$ .