Experiment to measure the focal length of a concave mirror

Apparatus:concave mirror, screen, metre stick, light source with cross threads.

Method:1. Find a rough value for the focal length of the mirror by finding

the image of a distant object into a screen. The distance from the screen to the mirror is then the rough value of focal length.

2. Set up the apparatus as in the diagram keeping the distance from the screen to the mirror greater than the rough value of focal length. Adjust the position of the screen until a sharp image if the cross threads is seen. Then measure the image distance v and the object distance u.

3. Use the formula 1/f=1/u + 1/v to find f.4. Repeat and find an average.

Another method involves plotting 1/u against 1/v and the value of 1/f is where the graph cuts the x and y axis.

Results

PrecautionsAvoid parallax when measuring distances.Repeat and average.

u/cm v/cm f/cm