Page 63 - Physics - XII

3. The distances measured in opposite direction to the direction of incident rays are taken as negative.
4. Heights measured perpendicular to the principal axis upwards are taken as positive.

5. Heights measured perpendicular to the principal axis downwards are taken as negative.
Object on left Mirror
Heights
upward
(Positive) Incident ray P

Heights x-axis
downward Distance in opposite Distance along the
(Negative) direction of incident direction of incident
ray (Negative) ray (Positive)

Fig. 1.2: The Cartesian sign convention illustration
Parallax

The relative sideway shift between two objects, at unequal distances from the eye, when the eye is moved
sideways, is called parallax. The parallax is to be removed between the tip of real inverted image of the
object needle and the tip of the erect image needle.
Adjust the heights of object needle (O) and image needle (I) in such way that their tips lie on a horizontal
straight line passing through the pole of the mirror. Keep one eye closed and move open eye to a position
from where the tips of inverted image and erect image needle are seen in a line touching each other. Now,
eye is moved towards right if there is shift between the two tips, there is a parallax. If the tip of image
needle moves towards left w.r.t. the image needle tip, the image needle is moved towards the mirror. The
parallax is reduced when the image needle is moved little by little till there is no relative shift between
the tips. Parallax may be removed either by image needle suitably away from the eye or by moving object
needle towards the eye. When there is no relative shift between the two tips, we say that parallax has been

removed tip to tip.
Opti cal Bench

A metallic optical bench consists of a horizontal metallic bed with a millimeter scale fi xed along its length.
There are four uprights which can slide along the length of the bed. These uprights carry the mirror, the
object needle, the image needle, etc.

Index Correcti on
O x P
The diff erence between the actual distance and the Index needle
observed distance between two uprights is called the index
correction. It is also known as bench-correction. The
readings of two uprights do not give the actual distance
between the tip of the pin O and pole P of the mirror
(Fig. 1.3). Hence, a correction must, therefore, be applied.
It is called the index correction. A B

For the determination of index correction, an index needle A B
is used to measure the distance between two uprights by y
using a metre scale. But before measuring the distance Fig. 1.3: Determination of index correction
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