3.1 Measurement of the focal length of a concave mirror Outline of experiment
mirror
In this experiment, we will determine the focal length of a concave (converging) spherical mirror. The main measurements required are object distance (u) and image distance (v). Once
these distances have been gathered, the formula 1
be used to calculate focal length. The graph of 1 and 1
__ u against 1
Equipment required Concave mirror, ray box, cross-threads, screen, ruler u Method
1. Before using the equipment in M3.1a, approximate the focal length of the mirror by focusing a distant object onto a screen and measuring image distance (v). This distance will be the approximate focal length (due to distant parallel rays converging at the focal point).
2. Make all distances of u greater than this approximate focal length for the remainder of the experiment (this ensures all images will be real).
3. Set up equipment as shown in M3.1a, making sure to fix the mirror and screen securely. 4. Turn on the ray box and adjust the screen distance until a sharp image of the cross-threads is in focus.
5. Measure u and the corresponding distance v and record it in the table. 6. Repeat for different values of u and v at least four more times. 7. Record the results in a table and plot the graph from the results (make sure that the axes
are 1
__ u and 1
__ v ).
u (cm) v (cm)
__ u
1
__ v
1
__ u + 1
1
__ v = 1
__ f
f (cm)
__ u + 1
__ v = 1
__ f can
__ v can be plotted for each value set
__ f found by averaging where the graph cuts each axis.
M3.1a Measuring the focal length of a concave mirror screen cross-threads object distance (u) ray box
