Question

An image of a candle is placed at the distance of 40 cm from a spherical lens is formed on a screen is placed on the other side of lens at a distance of 40 cm from the lens. Identify the lens and focal length. What will be the nature if the candle is shifted 25 cm towards the lens?

Answer 1

Since, the image can be obtained on a screen , therefore its a real image.
Concave lens lacks the ability to form real image. Real images can be formed only by using convex lens. So , its a convex lens.
The object distance and image distance are equal and in the opposite directions.This is possible only when the object is kept at the centre of curvature of the lens.The image is formed at the centre of curvature on the other side of the lens.
Here , centre of curvature = 40 cm
Thus, Focal Length = centre of curvature / 2 = 40 cm /2 = 20 cm.

Now , if the object is shifted 25 cm towards the lens , the object will lie between Focus and the optical centre.In this case a magnified, upright virtual image will be formed on the same side as the object.