Question

a teacher gives a convex lens and a concave mirror of foal length 20 cm each to his student and asks him to find their focal length by obtaining the image of a distant object. The student uses a distant tree as the object and obtains it's sharp image,one by one,on a screen. The distances d1 and d2 between the lens/mirror and the screen in the two cases and the nature of their respective sharp images are likely to be. Give reason for your answer.​

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

The image will be inverted in both cases because it can be obtained on screen.

The distances will be 20cm in both cases