Question

A thin convex lens (L1) of focal length 80 cm and a thin concave lens (L2) of focal length f are kept co-axially, 20 cm apart as shown in figure below. When a narrow and parallel beam of light is incident on the convex lens, beam emerging from the concave lens is also a parallel beam Find f.​

Answer 1

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Parallel rays after refraction through convex lens will converge at focus. So, convex lens will focus at 80cm. So, object for concave lens will be at 60 cm from it on the right hand side.

We know,

$⇒ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \\ ⇒ \frac{1}{f} = \frac{1}{ \infty } - \frac{1}{60} \\ ⇒f = -60cm.$

So, focal length of concave lens is 60 cm.

Hope it helps. :)

Answer 2

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A thin convex lens (L1) of focal length 80 cm and a thin concave lens (L2) of focal length f are kept co-axially, 20 cm apart as shown in figure below. When a narrow and parallel beam of light is incident on the convex lens, beam emerging from the concave lens is also a parallel beam Find f.

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☆ After refraction the parallel rays will converge at focus

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Focal length = 80cm _____(given)

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∴ The focal length of the concave lens —

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$\dfrac{1}{f} = \: \dfrac{1}{v} - \dfrac{1}{u}$

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$\: ⇒\dfrac{1} {f} = \: \dfrac{1}{ \infty } - \dfrac{1}{60}$

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$f \: = \sf \: 60 \: cm$

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Hence, the length of the focal length of the concave mirror is 60 cm.

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$\pink{\large\mathfrak{hope\: it \:helps \:you}}$

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