Page 72 - Physics - XII

1 1 1
2. Graph between and : Choose a suitable, but the same scale to represent along negative X-axis
u v u
1 1 1
(X'-axis) and along Y-axis. Take O as the origin. By sign convention, is negative while is
v u v
1 1
positive. Plot the various points for the diff erent sets of values of and from the observation table.
u v
1 1
The graph between and comes out to be straight line as shown in Fig. 3.5.
u v
The straight line PQ cuts the two axes OX' and OY at an angle 45° at points P and Q, and making equal

intercepts OP and OQ on the two axes. Measure the distance OP and OQ, then,

1 1
Focal length, f = = = _____ cm
OP OQ
2
Mean value of f = = _____ cm
OP OQ+

Scale used on: Y
Negative x-axis (X′-axis) :1 cm = ... cm of – 1 Q
–1
u
Y-axis : 1 cm =... cm of 1
–1
v

1/v (cm )
–1

X′ P 1/u (cm ) X
–1
Y′

Fig. 3.5: Graph showing relation between 1 and 1 for a convex lens
u v
Result

The focal length of the given convex lens as determined from
1. u, v graph is _____ cm.

2. 11
, graph is _____ cm.
uv
Precautions
1. Tips of the object needle and the image needle should lie at the same height as the centre of the lens.
2. The uprights should be rigid and vertical.

3. Parallax should be removed tip to tip by keeping the eye at a distance at least 30 cm away from the
needle.

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