# Reflection in Physics

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Reflection in Physics

When the two mirrors are placed parallel to each other then,
n = (360 / zero) = ∞ (infinite)

The least size of the mirror required is half the height of the observer.

The least size of the mirror required is one – third the height of the wall of a room in which an observer at the centre of the room can see the full image of the wall behind him.

If the image is erect and of the same size as the object and it appears to be formed behind the mirror, the reflecting surface is plane.

If the image is erect and formed behind the mirror but diminished in size for all positions of the mirror, the mirror must be convex.

If the image is erect, formed behind the mirror and enlarged, then the mirror is concave.

The image can be seen from any position, provided the line joining the eye to the image cuts the surface of the mirror.

Twilight is an example of diffused reflection of sunlight from clouds, dust particles and other floating particles in the air.

Convex mirror always form virtual image smaller than the object.

If the mirror moves by a distance a towards or away from the object, the image will also move by distance 2a in same phase.

If the object is displayed a distance a towards or away from the mirror, then the image will be displayed by a distance a towards or away from the mirror.

A fish inside the water cannot see the entire surface. It sees only a circular path of radius r.

This fact is based on total internal reflection.

Refraction at a Plane Surface

When a ray of light passes from one medium to other it suffers a change in direction at the boundary of separation of two media.

This phenomenon is called refraction.

When a ray of light travels from one medium to another the wavelength and velocity of light change but the frequency does not change.

If v1 and v2 are the velocities of light in two media and λ1 and λ2 are the corresponding wavelengths in these media then,
v1 / λ1 = v2 / λ2

Twinkling of Stars

The refractive index of atmosphere changes with height. Even at the same level the refractive index of air varies periodically.

The rays of light from a star are sometimes concentrated at a point when it appears bright; next moment the concentration of rays decreases and the star appears faint, the planets being nearer, the amount of light received from them is greater and so, the variation of brightness is not appreciable.

Memorable Points

• When a light ray travels from denser to rarer medium, it deviates away from normal while if it is traveling from rarer to the denser medium, it is deviated towards the normal.
• Refractive index is the optical property of the medium.
• Refractive index of vacuum is one while the refractive index of air is 1.0003, but for simplicity it is taken to be one.
• At sunset and sunrise, sun appears above horizon while actually it is below horizon, due to atmospheric refraction.
• Rivers appear shallow, coin in a beaker filled with water appears raised, pencil appears broken, due to refraction.
• The duration of day appears to be increased by nearly 4 min. due to atmospheric refraction.
• Brilliance of diamonds is due to their high refractive index, low critical angle and hence easy total internal reflection.
• The air bubbles in glass paperweight appear silvery white due to total internal reflection.
• A test tube blackened from outside appears silvery white due to total internal reflection when dipped in water.
• Dispersion takes place because the light rays of different colours are deviated at different angles, so an angular separation is produced between them.
• If the lens is immersed in a medium having refractive index more than that of lens, then the nature of the lens changes, i.e., convex behaves as concave and vice – versa. The focal length of the lens may increase, decrease or even remain the same.
• If the lens is immersed in a medium having refractive index equal to that of the lens, then
• The lens becomes invisible.
• The lens behaves as a plane glass plate.
• The focal length becomes infinite.
• An air bubble in water behaves as a concave lens.
• The nearest distance up to which eye can see clearly is called least distance of distinct vision. For normal eye this distance is 25 cm.

Optical Instruments Physics

Telescopes and microscopes are the aids to increase the apparent size of objects by increasing visual angle.

Very distant objects (such as sun and moon), although very big in size appear very small because they subtend very small visual angle at the eye.

To see them bigger, we cannot decrease their distance.

But if with the help of proper lenses, a small image of the distant object be formed close to the eye then its image will subtend a large visual angle at the eye and the object will appear large. Telescope is based on this principle.

Very small objects subtend small visual angle due to their smallness.

We can increase the visual angle by bringing these objects closer to the eye, but we cannot do so beyond a certain limit (25cm) because then the objects will not be seen distinct.

If with the help of proper lenses, a large image of small object is formed, then this image will subtend a large visual angle at the eye and the object will appear large. Microscope is based on this principle.

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