Question

Suppose you have focused on a screen the image of candle flame placed at the farthest end of the laboratory table using a convex lens. If your teacher suggests you to focus the parallel rays of the sun, reaching you laboratory table, on the same screen, what you are expected to do is to move the:
A. Lens slightly towards the screen
B. Lens slightly away from the screen
C. Lens slightly towards the sun
D. Lens and screen both towards the sun

Answer 1

Hint:Here we have to see how an image is formed on a convex lens.
The real image is created at the focus as an object is positioned at infinity. The image scale is a lot smaller than that of the object. The real image is created between the centre of curvature and focus while an object is located behind the centre of curvature. Compared to that of the object, the scale of the image is the same.A virtual image is created when an object is positioned between the focus and the pole. The image's dimension is greater than that of the object.

Complete answer:
When an object is placed an infinity image is formed at the focus of the lens and when object is placed at twice the distance of the focus, image is formed at twice the distance of the lens. It means that as we move from infinity to twice the focal length of the lens image will shift from the from focus to the twice to the focus and vice versa.

Since candle is placed at the farthest end of the laboratory still it is not at infinity so image formed by the convex lens will be somewhere between focus and centre of curvature of the lens but parallel rays will converge at the focus so distance between lens and screen should be reduced to get the image of the sun.And to do that we need to move the lens towards the screen or screen towards the lens or screen towards the sun(assuming incident rays from the sun are towards lens to the screen.)

Hence correct answer is option A.

Note: Candle is placed at the farthest end of the laboratory still it is not at infinity so image formed by the convex lens will be somewhere between focus and centre of curvature of the lens but parallel rays will converge at the focus so distance between lens and screen should be reduced to get the image of the sun.