Question

A student determines the focal length of a device 'X’ by focusing the image of a distant object on a screen placed 20 cm from the device on the same side as the object. The device 'X' is : (a) Concave lens of focal length 10 cm (b) Convex lens of focal length 20 cm (c) Concave mirror of focal length it) cm (d) Concave mirror of focal length 20cm

Answer 1

Answer:

Option (c) is correct.

Explanation:

The image of the object placed at a very large distance compared to the focal length of a lens or a mirror is formed at the focus, which is at the distance of the focal length of the mirror or the lens.

Let's check all the option:

a) Concave lens of focal length 10 cm:

A concave lens always forms virtual images and the virtual images can not be obtained on a screen. Thus, it is not possible for the device 'X' to be a concave lens.

(b) Convex lens of focal length 20 cm:

Though a convex lens forms real images, but it always forms images on the opposite side of the lens from where the object is placed and since the image on the screen from the device 'X' is obtained on the same side as the object, therefore, it is again not possible for the device 'X' to be a convex lens.

(c) Concave mirror of focal length it) cm:

Concave mirror forms real images of the objects and also it forms images on the same side where the object is placed, it is possible that the device 'X' could be a concave mirror.

But the object is placed at a large distance, therefore its image will formed at the focus of the mirror, i.e., at the distance of the focal length of the mirror.

Since, the screen is placed at distance 20 cm where the image of the object is obtained therefore, the focal length of the mirror should be 20 cm.

Thus, this mirror can also not be the device 'X'.

(d) Concave mirror of focal length 20 cm:

Due all the reasons mentioned in part (c), concave mirror of focal length 20 cm is the device 'X'.

Answer 2

Answer:

the correct option is C

Explanation: