Measurement of the Focal Length of a Concave Mirror Apparatus Concave mirror, screen, ray box with cross threads, metre stick
Procedure Find the approximate focal length of the mirror by focusing the image of a distant object (e.g. a window at the far end of the room) on a piece of paper. The distance from the mirror to the paper is the focal length.
Set up apparatus as illustrated in Fig 12.16, with the ray box outside the focal length. Measure the object distance (u) from the cross threads to the mirror using a metre stick.
Concave mirror
Cross threads Ray box
Screen u v Fig 12.16
Move the screen back and forth until the image of the cross threads comes into clear focus. Measure the image distance (v) from the screen to the mirror using a metre stick.
Repeat the process using different object distances so as to get six or seven readings.
Calculations Calculate the reciprocal of the object distance
Tabulation of Data Table 12.1
u / cm v / cm
1 u > cm1
1 v > cm1
Graph Plot a graph of the inverse of object distance , measured in , against the
a
inverse of image distance , measured in , with the inverse of image distance on the y-axis. Extend the line of best fit so that it cuts the x-axis and y-axis. Conclusion
a
1 v b
cm1 a (f )
1 ub
cm1 a
1 ub
and image distance . a
1 v b
The line should cut both axes at the same value.This value is equal to the inverse of the focal length . Invert this value to get the focal length of the mirror.
