Laws of refraction:

1. Incident and refracted rays lie in the same plane.

2. When a ray of light passes at an angle into a denser medium, it is bent towards the normal, hence the angle of refraction is smaller than the angle of incidence.

Scattering is the random deflection of light rays by fine particles. When sunlight enters through a crack, scattering by dust particles in the air makes the shaft of light visible. Haze is a result of light scattering by fog and smoke particles.

Absorption of light as it passes through matter results in the decrease in intensity. Absorption, like scattering, may be general or selective. Selective absorption gives the world most of the colors we see. Glass filters which absorb part of the visible spectrum are used in research and photography.

Diffraction is the bending of waves around an obstacle. It is easy to see this effect for water waves. They bend around the corner of a sea wall, or spread as they move out of a channel. Diffraction of light waves, however, is harder to observe. In fact, diffraction of light waves is so slight that it escaped notice for a long time. The amount of bending is proportional to the size of light waves – about one fifty-thousandth of an inch (5,000 Å) – so the bending is always very small indeed.

When light from a distant street lamp is viewed through a window screen it forms a cross. The four sides of each tiny screen hole act as the sides of a slit and bend light in four directions, producing a cross made of four prongs of light. Another way to see the diffraction of light waves is to look at a distant light bulb through a very narrow vertical slit. Light from the bulb bends at both edges of the slit and appears to spread out sideways, forming an elongated diffraction pattern in a direction perpendicular to the slit.

Light can be imagined as waves whose fronts spread out in expanding concentric spheres around a source. Each point on a wave front can be thought of as the source of a new disturbance. Each point can act as a new light source with a new series of concentric wave fronts expanding outward from it. Points are infinitely numerous on the surface of a wave front as it crosses an opening.

As new wave fronts fan out from each point of a small opening, such as a pinhole or a narrow slit, they reinforce each other when they are in phase and cancel each other when they are completely out of phase. Thus lighter and darker areas form the banded diffraction patterns.

A pattern of waves will move outward, forming concentric circles, if small pebbles are dropped regularly from a fixed point into a quiet pond. If a board is placed in the path of these waves, they will be seen to bend around the edges of the board, causing an interesting pattern where the waves from the two edges of the board meet and cross each other. When an obstruction with a vertical slit is placed in the pond in the path of the waves, the waves spread out in circles beyond the slit.

D iffraction patterns are formed when light from a point source passes through pinholes and slits. A pinhole gives a circular pattern and a slit gives an elongated pattern. A sharp image is not formed by light passing through because of diffraction. As the pinhole or slit gets smaller, the diffraction pattern gets larger but dimmer. In the diffraction patterns shown below the alternate light and dark spaces are due to interference between waves arriving from different parts of the pinhole or slit.

Fig.1.

Interference is an effect that occurs when two waves of equal frequency are superimposed. This often happens when light rays from a single source travel by different paths to the same point. If, at the point of meeting, the two waves are in phase (vibrating in unison, and the crest of one coinciding with the crest of the other), they will combine to form a new wave of the same frequency. The amplitude of the new wave is the sum of the amplitudes of the original waves. The process of forming this new wave is called constructive interference.

If the two waves meet out of phase (crest of one coinciding with a trough of the other), the result is a wave whose amplitude is the difference of the original amplitudes. If the original waves have equal amplitudes, they may completely destroy each other, leaving no wave at all. Constructive interference results in a bright spot; destructive interference producing a dark spot.

Partial constructive or destructive interference results whenever the waves have an intermediate phase relationship. Interference of waves does not create or destroy light energy, but merely redistributes it.

Two waves interfere only if their phase relationship does not change. They are then said to be coherent. Light waves from two different sources do not interfere because radiations from different atoms are constantly changing their phase relationships. They are non-coherent.

Interference occurs when light waves from a point source (a single slit) travel by two different paths (through the double slit). Their interference is shown by a pattern of alternate light and dark bands when a screen is placed across their path.

Fig.2.

Fig.3.