When the line cuts the x-axis y 0 , which means v 1
Based on the equation where .
1 u 0
Precautions Carry out the procedure to find the approximate focal length.This will make it easier to find the exact image later. It will allow you to place the ray box outside the focal length which is necessary to produce a real image.This reading is generally inaccurate because of the difficulty in measuring the image distance and also in finding the sharpest image of the distant object.
Take care when using metre stick to avoid parallax error. Take care to find the sharpest image on the screen. Repeat a number of times at each value of u and find an average v.
Note
An object pin can be used instead of the cross threads of a ray box and a search pin instead of a screen. The focal length (f) can be calculated by formula instead of using a graph.
Calculations The focal length can be found without using a graph. For each pair of readings sub the values of u and v into the
formula f u v . Calculate the average to find the focal (f ) of the mirror. Tabulation of Data
uv . Table 12.2
u / cm v / cm f / cm average focal length
1 u 1
f , it is clear that
1 v 0
1 v 0
, since a implies
1 v b
is plotted on the y-axis.3
1 f 1
u the x coordinate where the line cuts
the x-axis. It is equally valid to choose the y coordinate where the line cuts the y-axis as this is
1 / v
1 / v Fig 12.17
3Strictly speaking, approaches 0, as
1 v
1 v 0Q1 0 vQ1 0
1 f 1
u 1 v as
; obviously this cannot happen. By extending
the line of best fit so that it cuts the axes we get a very accurate approximation of as approaches 0. Another way of interpreting the graph is to rewrite the formula The graph represents a line with slope 1
