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Light. Reflection, absorption, and transmission of light. Shadows. Umbra, penumbra. Eclipse of the sun and moon.
What is light? What optical phenomena can we observe that will give insight into what it is? How is light created?
We see a shaft of sunlight coming through a break in the clouds. What is this thing coming through the clouds? Is it a substance? If not, what is it? It appears to be rays of some type coming from the sun. We know it has the power to warm. On a warm day we feel its warmth when it hits us. It appears to always travel in a straight line. When it is blocked by an object, a shadow of the object is created.
The question of what light is has been speculated on by philosophers from ancient times. Plato had his ideas and Aristotle had his. Christian Huygens observed that the laws of reflection and refraction could be explained on the basis of a wave theory i.e. a wave motion analogous to the out-moving waves created by a pebble cast into a pond. Sir Isaac Newton considered the possibility of light being either a stream of particles emanating out radially from a luminous body or a wave motion. He came to the opinion that it was a stream of particles. His opinion resulted in a corpuscular theory of light. The corpuscular theory dominated until some experiments by Thomas Young and Augustin Fresnel on interference in the mid 1800's suggested a wave nature for light. In 1873 James Clerk Maxwell showed that an oscillating electrical circuit should radiate electromagnetic waves and that the velocity of these waves should be 3×108 m/sec. Within experimental limits, this figure corresponded to the measured velocity of light. It was consequently speculated that light consisted of electromagnetic waves of extremely short wavelength. Fifteen years later Heinrich Helmoltz succeeded in producing short wavelength waves (microwaves) and showed that they possessed all the properties of light waves. They could be reflected, refracted, focused by a lens, polarized, etc. This would appear to indicate that light is an electromagnetic wave. Yet there are optical phenomena that cannot be explained by the wave theory and require a corpuscular theory. When light travels from one place to another, it acts as though it were a wave. But when it is emitted by matter or absorbed by matter it acts as though it were a stream of particles. As a consequence, the question of just what light is has still not been resolved. The present pragmatic approach is to consider it to be wave-like in some cases and particle-like in others or to, in some way, view it as being both wave-like and particle-like at the same time (a very interesting feat of mind and logic).
Light as an electromagnetic wave. Light waves correspond to only a very narrow band of wavelengths in the electromagnetic spectrum. This span consists of wavelengths that run from violet at 400 nm to red at 700 nm. See Fig. 1.
Light waves don’t need an elastic medium for propagation. They will travel through a vacuum. They are transverse waves in contrast to sound waves which are longitudinal. The speed at which light travels in a vacuum is 186,000 miles/sec.
How light is produced. The most common way of producing light is by heating substances to incandescence. The sun and stars consist of incandescent gases. The flame of a candle or wood fire consists of an incandescent gas produced by combustion. The electric light bulb gives light by heating a filament to incandescence by passing electricity through it. There are other ways in which light can be produced. The light in a neon sign is produced by bombarding the
molecules of a gas with electrons. The light of a fluorescent tube is produced by the action of invisible ultra-violet light on chemicals with which the tube is coated. The light of a fire-fly is produced by complex chemical reactions.
Reflection, absorption, and transmission of light. Whether light is absorbed, reflected or transmitted by a substance depends on
1) the color of the substance.
2) the angle at which the ray of light strikes the surface
3) the chemical makeup of the substance
Color of the substance. Dark-colored objects absorb light readily. Black objects absorb nearly all the light they receive. Light-colored objects tend to reflect light.
Angle at which the ray of light strikes the surface. Rays of light striking a surface at an oblique angle are more likely to be reflected than those striking it vertically. Only about 8 percent of light falling perpendicularly on a sheet of glass is reflected (half of this from the rear surface), but at large angles of incidence almost all the incoming light is reflected at the front surface.
Chemical makeup of the substance. Depending on molecular makeup, some substances, like coal, absorb light; others, like glass, transmit it.
Transparent, translucent and opaque substances. Some substances such as air, glass and water transmit light easily. They are said to be transparent. So much light is transmitted that it is easy to distinguish objects through them. Other substances transmit some light, but so much of the light is scattered or diffused by them that the objects seen through them cannot be identified. Such substances are said to be translucent. Substances that do not transmit any light are called opaque.
Role played by the frequency (or wavelength) of light. When a light wave of a particular frequency strikes an object,
1. It can be absorbed, in which case its energy is converted to heat.
2. It can be reflected.
3. It can be transmitted.
It is a rare case in which light of a single frequency is incident on an object. The usual case is that of light of many frequencies impinging on an object. In this case objects have a tendency to selectively absorb some of the frequencies, reflect others, and possibly transmit some.
Types of reflection. There are two types of reflection:
1) Specular reflection (also called regular reflection)
2) Diffuse reflection.
See Fig. 2.
Specular reflection is the type of reflection that occurs when light is reflected off a smooth surface such as a mirror, shiny metal, or a calm body of water. In this type of reflection the waves are all reflected off the surface in the same direction (the reflected waves are all parallel). Diffuse reflection is the type that occurs when light strikes a rough surface such as clothing or an ink blotter. When light strikes a rough surface it is scattered in all directions. Most surfaces are rough at a microscopic level and diffuse reflection is the norm for most objects. This is very fortunate since this diffuse reflection is the reason most of the objects about us can be seen. Light striking them is reflected in all directions and, as a consequence, we can see them from any direction from which we look.
Shadows. Umbra, penumbra. Consider Fig. 3 in which a ball is located between a point source of light S and a screen. The region within that shaded truncated cone shown lying between the ball and the screen is in shadow. No point within this region receives any light and is in total darkness. This area is called the umbra. At any point within the umbra, the point source cannot be seen.
Let us now consider the case when the source of light is larger than a point i.e. it has real dimensions as all light sources do. See Fig. 4. In this case the shadow thrown by the ball consists of two parts:
1) the umbra
2) the penumbra
The umbra is the part within the black cylinder shown between the ball and the screen. It is in total darkness and receives no light from the source. No part of the source is visible from any point in the umbra.
The penumbra is the region shown slightly shaded between the ball and the screen. In this region the source of light can be seen partially, but not completely. It receives some of the light from the source but not the entire light.
The concepts of the umbra and penumbra are important in connection with eclipses of the sun and moon.
Eclipse of the sun. Sometimes it happens that the moon, in its monthly trips around the earth, comes directly between the sun and the earth, putting part of the earth in its umbra and penumbra. When this happens, people located in the umbra will observe a total eclipse of the sun and people in the penumbra will observe a partial eclipse of the sun. See Fig. 5.
Eclipse of the moon. There is always an umbra stretching out from the earth on the side opposite the sun. When our locality passes into that umbra it becomes night. In its monthly orbit around the earth, the moon sometimes passes through this umbra projected by the earth. When this happens there is an eclipse of the moon. See Fig. 6.
Dull, Metcalfe, Brooks. Modern Physics
Sears, Zemansky. University Physics
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