Briefly describe how we could find the focal length of a concave mirror.

To determine the focal length of concave
mirror.

We require a concave mirror preferably mounted on a
stand, a lighted candle and a white screen to catch the image after
reflection

Keep the mounted concave mirror on a plane
surface (say a long table). Keep in front of the reflecting surface a bright candle at
sufficient distance from the concave mirror  on the principal axis so that we can get a
clear image somewhere. Now adjust the screen ,candle and the mounted concave mirror so
that they are all on the principal axis. Move the screen back and forth and determine
the position for the brightest clearest  image.

Note down
for the clear image distance v and the corresponding object distance u from the pole of
the mirror along the principal axis. Then, if f is the the focal length of the mirror,
solve for f = uv/(u+v) which determines the focal length f on one single
trial.

You make the following table of observation for
repeated trials varying object distance u and the image distance
v:

Trial no:  Dist u of object from the pole . Distance v
of image from the pole. Focal legnth f = uv/((u+v)

After
doing different trials you can find the average focal length from the last
column.