SATHEE: Optics Question 94

Optics Question 94

Question: The size of the image of an object, which is at infinity, as formed by a convex lens of focal length 30cm is 2 cm. If a concave lens of focal length 20 cm is placed between the convex lens and the image at a distance of 26 cm from the convex lens, calculate the new size of the image

[IIT-JEE (Screening) 2003]

Options:

A) 1.25 cm

B) 2.5 cm

C) 1.05 cm

D) 2 cm

Show Answer

Answer:

Correct Answer: B

Solution:

Convex lens will form image $I_{1}$ at it’s focus which acts like a virtual object for concave lens.

Hence for concave lens $u = + 4 c m$ , $f = 20$ $c m .$

So by lens formula $\frac{1}{- 20} = \frac{1}{v} - \frac{1}{4} \Rightarrow v = 5 c m$ i.e. distance of final image $(I_{2})$ from concave lens $v = 5 c m$ by using $\frac{v}{u} = \frac{I}{O} \Rightarrow \frac{5}{4} = \frac{I}{2} \Rightarrow (I_{2}) = 2.5 c m$

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Optics Question 94

Question: The size of the image of an object, which is at infinity, as formed by a convex lens of focal length 30cm is 2 cm. If a concave lens of focal length 20 cm is placed between the convex lens and the image at a distance of 26 cm from the convex lens, calculate the new size of the image

Options:

Answer:

Solution:

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