Parallax

Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. Nearby objects show a larger parallax than farther objects when observed from different positions, so parallax can be used to determine distances.

no parallax

Here's how parallax works. Hold up the index fingers of your two hands so that one is directly in front of the other as you "sight" along them. Notice that if you keep your fingers still and move your head only, your fingers appear to move. This phenomenon is called PARALLAX - apparent relative motion of two objects caused by motion of the observer. You have observed parallax many times. For instance, have you noticed that as you drive down the street, a tree in front of a house appears to move across the front of the house as you drive by? Both the tree and the house were stationary, of course; their apparent motion was caused by your actual motion.

Now notice that as you move your fingers farther apart, the "parallax motion" is larger when you move your head by the same amount, and when you bring your fingers closer together, the apparent motion is smaller when you move your head.

We will make use of the fact that parallax causes apparent motion when the observer moves, to locate the image of a nail in a plane mirror. The idea is that if the image of a nail and a real nail are not in the same place, parallax will cause them to appear to move relative to each other when you move your head. When the real nail and the image are in the same place, however, there will be no parallax - and no apparent motion.

Parallax and its removal 
(a) Parallax: The relative side way shift between two objects, at unequal distance from the eye, when the eye is moved sideways, is called parallax.
The distant object moves slowly in same direction in which the eye is moved.
The near object moves rapidly in opposite direction to which the eye is moved.
(b) Removal: In laboratory, the parallax is to be removed between the tip of real inverted image of object needle and the tip of erect image needle. It is done as described below.
Heights of image needle (I) and object needle (O) (See Ray Diagram Experiment 1 : Section B) are so adjusted that their tips be on a horizontal straight line passing through the pole (P) of the mirror. One eye (preferably left) is closed and open (right) eye is moved to a position from where the tips of the inverted image and the erect image needle are seen in a line touching each other. The eye must be at a distance of about 30 cm from the tips.
The eye is moved towards right. If the two tips get separated, there is parallax.
If the tip of the image needle moves towards left (or right) with respect to the image tip, image needle is nearer to (or away from) the eye. The image needle is moved towards (or away from) the mirror. The parallax (relative shift) is reduced. The image needle is moved little by little till the shifting of the tips has stopped.
When this has been done, we say that the parallax has been removed ‘tip to tip’

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