Nature of Light and Color

We know the world through our senses: sight, hearing, touch, taste, and smell. Each sense responds to particular stimulus, and the sensations we experience give us information about our surroundings. Sight is the most important of the senses. Through sight we perceive the shape, size, and color of objects, also their distance, motions, and relationships to each other. Light is the stimulus for the sense of sight - the raw material of vision.

To understand the fascinating story of light, let us explore its nature, its behavior in lenses and prisms, and then its uses in science and art.

Nature.Electromagnetic waves carry energy in all directions through the universe. All objects receive, absorb, and radiate these waves which can be pictured as electric and magnetic fields vibrating at right angles to each other and also to the direction in which the wave is travelling. Light is one form of electromagnetic wave. All electromagnetic waves travel in space at the same speed - the speed of light.

Electromagnetic waves show a continuous range of frequencies and wavelengths. Frequency is the number of wave crests passing a point in one second. Electromagnetic wave frequencies run from about one per second to over a trillion per second. For light, the frequencies are four to eight hundred trillion waves per second. The higher the frequency, the shorter the wavelength.

Visible light is that portion of the electromagnetic spectrum that normally stimulates the sense of sight. Electromagnetic waves exhibit a continuous range of frequencies and wavelengths. In the visible part of the spectrum these frequencies and wavelengths are what we see as colors. The wavelengths of light range from 3,500 A to 7,500 Å. The wavelengths of infrared rays (7,500 Å - 10,000,000 Å), longer than light rays, are not detected by the eye, and do not appreciably affect ordinary photographic film. They are also called heat or thermal rays and give us the sensation of warmth.

Light behavior includes transmission, absorption, reflection, refraction, scattering, diffraction, interference and polarization. Transmission, absorption and reflection account for all the light energy when light strikes an object. In the course of transmission, light may be scattered, refracted or polarized. It can also be polarized by reflection. The light that is not transmitted or reflected is absorbed and its energy contributes to the heat energy of the molecules of the absorbing material. The modification of light through these processes is responsible for all that we see.

Reflection is of two kinds - diffuse and regular. Diffuse reflection is the kind by which we ordinarily see objects. It gives us information about their shape, size, color and texture. Regular reflection is mirror-like. We don't see the surface of the mirror; instead, we see objects that are reflected in it. When light strikes a mirror at an angle, it is reflected at the same angle. In diffuse reflection, light leaves at many different angles. The degree of surface roughness determines the proportion of diffuse and regular reflection that occurs. Reflection from a smooth, polished surface like a mirror is mostly regular, while diffuse reflection takes place at surfaces that are rough compared with the wavelength of light. Since the wavelength of light is very small (about 5,000 Å), most reflection is diffuse.

Laws of reflection:

1. Angle of reflection equals to angle of incidence.

2. Incident and reflected rays lie in the same plane.

3. Incident and reflected rays are on opposite sides of the normal - a line perpendicular to the reflecting surface and passing through the point of incidence.

Refraction is the bending of a light ray when it crosses the boundary between two different materials, as from air into water. This change in direction is due to a change in speed. Light travels faster in empty space and slows down upon entering matter. Its speed in air is almost the same as its speed in space, but it travels only 3/4 as fast in water and only 2/3 as fast in glass. The refractive index of a substance is the ratio of the speed of light in space (or in air) to its speed in the substance. This ratio is always greater than one.

When a beam of light enters a plane of glass perpendicular to the surface, it slows down, and its wavelength in the glass becomes shorter in the same proportion. The frequency remains the same. Coming out of the glass, the light speeds up again, the wavelength returning to its former size.

When a light ray strikes the glass at some other angle, it changes direction as well as speed. Inside the glass, the ray bends toward the perpendicular or normal. If the two sides of the glass are parallel, the light will return to its original direction when it leaves the glass, even though it has been displaced in its passage. If the two sides of the glass are not parallel, as in the case of a prism or a lens, the ray emerges in a new direction.

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 actas anew 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 edgesof 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.

Diffraction 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 createor 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.