Apparatus
- White screen with a hole covered with cross-wire
- Concave mirror on a mirror holder
- A candle
Procedure
The candle is placed behind the cross-wire on the screen as shown
One should ensure the candle is at the center of the cross-wire hole.
Arrange the apparatus as shown below
The concave mirror is placed in-front of the screen with cross wire and the distance between them adjusted until a sharp image of the cross wire is formed next to the cross-wire on the screen as shown in figure below. The object is one labelled O and image is the one labelled I.
The object cross-wires O and Image I are coinciding at that position
At that position where the image of cross-wire coincides exactly with the cross-wire object, the distance between the mirror and the image I is the radius of curvature of the mirror. So the image is formed is at the center of curvature of the mirror.
But the focal length is half of the radius of curvature. That is, R=2f.
Therefore Focal length f=R/2.