Page 209 - Physics_Form_2

Refraction and dispersion of light

Table 5.6

1 1 1 uv
-1
-1
f (cm) (cm ) u (cm) (cm ) v (cm) (cm ) uv (cm ) (cm)
-1
2
f u v u + v
FOR ONLINE READING ONLY

Questions
(a) Determine the average value of uv
u + v
(b) Compare the value obtained in (a) above with the focal length of the lens.
1 1
(c) Plot graphing software to plot a graph of against .
v u
(d) Analyse the graph to determine the relationship between object distance and image
distance.
(e) Explain how you can obtain the value of the focal length from the graph.

The average value obtained from the the formula, 1 = 1 + 1 to realize that, at

uv f u v
formula is the focal length of the
1
u + v y-intercept, 1 = 0 . So, 1 = or f = .
v
lens. This is in agreement with the lens u f v
formula. Note that, from the formula, The focal length may therefore be
obtained by using the y-intercept of the
1 1 1 v + u
= + = graph and finding its reciprocal.
f u v uv
1 −1
v (cm )
uv
Therefore, f =
v + u

Alternatively, the value of f may be
1
obtained by plotting a graph of against
v
1 . The graph is a straight line with an O 1 (cm )
−1
u u
intercept on both axes, as shown in
Figure 5.42. From the graph, we may use Figure 5.42: A graph of 1 against 1
v u

203

Physics Form 2 Final.indd 203 25/10/2025 10:28