Question

A teacher gives a convex lens and a concave mirror of focal length about 20 cm each to her student and asks her to nd their focal length by obtaining the image of a distant object. The student uses a distant tower as the object and obtains its sharp image one by one on a white screen. The distances l1 and l2 between the lens/mirror and the screen in the two cases and the nature of their respective images are likely to be


(a)(20 cm, 40 cm) and (erect, erect)
(b) (20 cm, 40cm) and inverted, inverted)
(c) (20 cm, 20 cm) and inverted, inverted)
(d) (22 cm, 40 cm) and (erect, inverted)

Answer 1

Since the object is distant, the nature of the image on the screen in case of both convex lens and concave mirror will be real, inverted, highly diminished and point sized.

If the object is distant, then we get its image at focus on the other side of convex lens. Therefore, d1 will be 20 cm.

In case of concave mirror, if the object is distant, then we get its image at focus in front of concave mirror. Therefore, d2 will be 20 cm.

Answer 2

Answer:

(c) (20cm, 20cm) and (inverted, inverted)

Explanation:

As object is at infinity, image formed at Focus

so, Image distance = focal length in both cases

Therefore, Image distance d1 = Image distance d2 = 20cm

Also because convex lens and concave mirror are given, both will form a highly diminished and inverted image at focus.

So, option (c) is absolutely correct.

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