SATHEE: Optics 3 Question 18

Optics 3 Question 18

18. The size of the image of an object, which is at infinity, as formed by a convex lens of focal length $30 c m$ is $2 c m$ . If a concave lens of focal length $20 c m$ is placed between the convex lens and the image at a distance of $26 c m$ from the convex lens, calculate the new size of the image. (2003, 2M)

(a) $1.25 c m$

(b) $2.5 c m$

(d) $2 c m$

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Answer:

Correct Answer: 18. (b)

Solution:

Image formed by convex lens at $I_{1}$ will act as a virtual object for concave lens. For concave lens

$\begin{aligned} or \frac{1}{v} - \frac{1}{u} & = \frac{1}{f} \\ or \frac{1}{v} - \frac{1}{4} & = \frac{1}{- 20} \\ v & = 5 c m \end{aligned}$

Magnification for concave lens

$m = \frac{v}{u} = \frac{5}{4} = 1.25$

As size of the image at $I_{1}$ is $2 c m$ . Therefore, size of image at $I_{2}$ will be $2 \times 1.25 = 2.5 c m$ .

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Optics 3 Question 18

18. The size of the image of an object, which is at infinity, as formed by a convex lens of focal length 30cm is 2cm. If a concave lens of focal length 20cm is placed between the convex lens and the image at a distance of 26cm from the convex lens, calculate the new size of the image. (2003, 2M)

Answer:

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