measuring focal length steps

Question 1

how to measure the focal length

Answer 1

1. connect ray box to power supply
2. place object grid in holder, and place that in front of the ray box, forming an illuminated object.
3. place a converging lens and as mirror in holders such that the light that passes through the lens is sent back by the mirror. lens and mirror should face illuminated object.
4. move the lens until a sharp image of the object is formed alongside the object. image is sharpest when its the same size as the object
5. the measure the distance between the optical center of the lens and the grid object. this is the focal length.

REPEAT AND CALCULATE AVERAGE FOCAL LENGTH.