A student focused the Sun rays using an optical device X on a screen S as shown. From this it may be concluded that the device X is a (select the correct option)

Convex lens of focal length 10 cm.

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Convex lens of radius of curvature 20 cm.

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Convex lens of focal length 20 cm.

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Concave mirror of focal length 20 cm.

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Solution

The correct option is C Convex lens of focal length 20 cm.
Light rays from infinity converge at the focal point of the convex lens. Therefore, the optical device X is a convex lens. Since the distance between the lens and screen where the image is converged is $20 c m$ , the focal length of the convex lens is $20 c m$ .
Hence, the correct option is C.

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Similar questions

A student focused the image of a distant object using a device 'X' on a while screen 'S' as shown in the figure. If the distance of the screen from the device is 40 cm, select the correct statement about the device

Q. A student used a device (X) to obtain/focus the image of a well illuminated distant building on a screen (S) as shown below in the diagram. Select the correct statement about the device (X).

(a) This device is a concave lens of focal length 8 cm.
(b) This device is a convex mirror of focal length 8 cm.
(c) This device is a convex lens of focal length 4 cm.
(d) This device is a convex lens of focal length 8 cm.

Q. Study the given ray diagrams and select the correct statement from the following :

(a) Device X is a concave mirror and device Y is a convex lens, whose focal lengths are 20 cm and 25 cm respectively.
(b) Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 10 cm and 25 cm respectively.
(c) Device X is a concave lens and device Y is a convex mirror, whose focal lengths are 20 cm and 25 cm respectively.
(d) Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 20 cm and 25 cm respectively.

Q. A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. The focal length of this combination of lenses will be:
(a) +10 cm
(b) +20 cm
(c) −10 cm
(d) −20 cm

Q. In an optical instrument a convex lens of focal length 20 cm is used and kept in contact with a concave lens of focal length 40 cm, What is the power of this combination?

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