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.
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