Determining the focal length of a concave mirror



Optical bench method of locating images
in a concave mirror.



By using some of the techniques described in the previous sections, it is
possible to determine the focal length of a concave mirror.
The apparatus used is an optical bench, a light box, a concave mirror
mounted so that it can be moved along the optical bench, and a screen.
The light box, screen and mounted mirror can be moved along a groove
on one side of the bench. Figure shows the set-up for such an
experiment.
The light box is placed at one end of the optical bench and the mirror
is moved along the optical bench until the image of the object is
focussed onto the front of the light box.
You will recall from earlier post that, when the object and the image
are at the same position, they are at the centre of the curvature, or 2
focal lengths from the mirror.



A light box suitable for use with an optical
bench.



Measuring the distance between the mirror and the light box will
give the radius of curvature of the mirror. Halving this distance will
give the focal length of the mirror.
Alternatively, the focal length can be obtained by using a distant
object. If the optical bench is held near a classroom window, the mirror
can be used to focus a distant object onto the screen placed along the
optical bench. The distance between the mirror and the screen will be
approximately equal to the focal length because the incoming rays are
parallel.
By obtaining a set of object and image distances with the light box
and screen, the focal length and magnification of the mirror can be calculated
by the mirror formulae, which are derived in an earlier post.
Figure 1
Optical bench method of locating images
in a concave mirror.
Figure 2
A light box suitable for use with an optical
bench.

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  • ISHMAEL MAGOGODI

    i hv just received help out of your post,thanks!!but i will be glad if u can provide me with a different picture for a set up of “DETERMINING THE FOCAL LENGTH OF A CONCAVE MIRROR”