Question

A student obtained an image of a burning candle, placed at the farther end of a laboratory table, on a screen using a concave mirror. For getting better value of focal length of the mirror, the subject teacher suggested him for focusing a well illuminated distant object.
What should the student do ?

• A. He should move the mirror away from the screen.
• B. He should move the mirror slightly toward the screen.
• C. He should move the mirror as well as the screen toward the newly selected object.
• D. He should move only the screen toward the newly selected object.

Answer 1

The correct option is B He should move the mirror slightly toward the screen.

We know that a concave mirror forms the image of an distant object at focus now if the object is brought closer to centre of curvature, image is formed at centre of curvature.And if we bring object further closer ,that is, between $F$ and $C$ then image is formed beyond C.

From above discussion it is clear that as object distance decreases ,image distance is increasing (image is going farther from mirror) .The reverse is also true if object is taken away from mirror then image gets closer to mirror.

Hence since the student is advised to form the image of a distant object so object distance now increases so image distance will decrease so in order to get the clear image the distance between mirror and screen shoube be reduced , that is, the mirror should be moved towards screen.