Luminescence, or the emission of light, may be due to causes other than heat [thermoluminescence]. [p. 28]
Illumination. It is often used as a general term that refers to the quantity and quality of light. The illumination of a scene may be bright or dim, harsh or soft, and perhaps even cold or warm. These terms refer loosely to the amount, contrast, and hue (color) of the light. In a narrower sense, illumination is the amount of light received on a specified surface area..... The human eye cannot distinguish the component wavelengths of a light beam, nor can it detect small changes in spectral distribution. Neither is the eye equally sensitive to all wavelengths.... [p. 29]
Brightness is a purely psychological concept. It is a sensation of the observer and cannot be measured by instruments. The ability of the eye to judge absolute values of brightness is very poor due to its great powers of adaptation. The eye is a very sensitive detector of brightness differences, however, provided the two fields of view are presented simultaneously. The measurement of light by visual comparision is the basis of the science of photometry . . . . Brightness is associated with the amount of the light stimulus. It is the visual sensation corresponding to the perception of luminance. [p. 32]
The intensity of light depends on the total amount of light emitted and on the smallness of the conical solid angle in which it is emitted [consider differences in shape of floodlight and spotlight emission]. Stated simply, it is the amount of light emitted in a given direction. [pg. 32]
Luminance, the intensity of light per unit area, is a psychophysical property and can be measured. Luminance and light-source intensity, often confused, are best described by examples. [pg. 32]
A point light source radiates its energy uniformly so that light rays spread out from it in all directions. Illumination at a point on a surface varies with the intensity and shape of the light source and the distance of the surface from it. The amount of light falling on a unit area (the illuminance) decreases with the square of the distance (the inverse square law).
Shadows are formed when light cannot pass through an opaque body in its path. Illumination of the area behind the body is cut off. A small (pinpoint) or distant source of light casts a sharp shadow. A near, large, or diffuse light source produces a fuzzy shadow with a central dark area that receives no light (umbra) and a light outer area (penumbra) which receives some light from part of the source.
Lightness is a term used by an observer to distinguish between lightness and darkness of colored objects, as between light blue and a dark blue paint. It should not be confused with brightness. The observer's perception of lightness is also a recognition of a difference in whiteness or grayness between objects. It is a comparative term referring to the amount of diffusely reflected light coming to the observer's eye from a surface. The surface will appear white if it is a good non-selective diffuse reflector and is well illuminated by white light. If it is a poor reflector or if it receives little or no illumination, the surface will appear gray, grayer, or even black. Black, then, is the perception of an area from which the light is insufficient for detailed vision. White is the perception of a well-illuminated surface whose reflectance is high and non-selective. Gray is the perception of a surface between these extremes..... The Perception of grayness is influenced by, among other things, the illumination of the surrounding area...... [pg. 34-35]
Light behavior. This 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. [pg. 36]
Light travels so fast that for many years scientists thought that its speed was infinite. The first observations and measurements which gave a finite value to the speed of light were made by the Danish astronomer Olaf Roemer in 1675...... He calculated that light took about 22 minutes to travel a distance equal to the diameter of the earth's orbit about the sun..... We now know that the time required is nearly 16.67 minutes, or about 1,000 seconds. Since the diameter of the earth's orbit is about 186,000,000 miles, the speed of light is calculated to be about 186,000 miles per second. [pg. 37]
Reflection. It 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 mirrorlike. 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 angels. 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 A), most reflection is diffuse.... Viewed microscopically, all reflection is regular. The appearance of diffuse reflection is due to the many different angles that light rays encounter when they strike a rough surface. The reflection of each single ray is regular--that is, it is reflected at the same angle at which it strikes the surface. A fairly smooth surface, such as that of a glossy vinyl raincoat, shows both diffuse and regular reflection, the relative proportions depending on the angle of the incident light. But a rough surface, such as that of a tweed coat, shows only diffuse reflection. It has no "shiny" surface. [pg. 39]
Reflection varies with the type of material. Polished metal reflects most of the light that falls on it, absorbs only a little, and transmits practically none. Paper is made up of partly transparent fibers. Light striking paper may penetrate several fibers, being partly reflected at each surface. The light that finally reaches your eyes and lets you know you are looking at paper has been reflected and transmitted many times. All the rest of the light has been absorbed and added to the heat energy of the molecules of the paper. Most materials are quite selective in the way they absorb and reflect the different wavelengths of light. A purple dye will transmit blue and red light but will absorb green light. Gold and copper metals reflect red and yellow wavelengths more strongly than blue. Silver reflects all colors and therefore appears almost white. Metallic reflection is an example of pure surface color. Nearly all "object colors" are due to selective reflection and absorption of light. Object colors are an attribute of the object, though the color seen at any time depends also on the color of the illumination. Total absorption of light makes an object look black..... Reflected light reveals the color and texture of woven cloth. What we normally consider as reflection involves selective absorption, selective reflection and refraction of light that partially penetrates the surface. [pg. 40]
Laws of Reflection
1. Angle of reflection equals angel 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 though the point of incidence. [pg. 41]
Refraction. It 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 fastest 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 pane of glass perpendicular to the surface (above), 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 angel, it changes direction as well as speed. Inside the glass, the ray ends 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. [pg. 42] [Try the following Tutorial: Refraction]
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 toward the normal, hence the angle of refraction (r) is smaller than the angle of incidence (i)...
3. The index of refraction of any medium is the ratio between the speed of light in a vacuum (or in air) and its speed in the medium. [pg. 43]
The Index of Refraction [n] determines the amount of bending of a light ray as it crosses the boundary from air into the medium. [pg. 43]
Internal Reflection occurs whenever a light ray strikes the surface of a medium whose refractive index is less than that of the medium in which the light is traveling. The amount of light that is reflected depends on the angle at which it hits the surface. Light from a point source (above) hits the surface at many angles. [pg. 44]
Dispersion is the separation of light into its component wavelengths. One method of dispersing a light beam is to pass it through a glass prism--a thick piece of glass with flat non-parallel sides (below). The refractive index of all materials depends slightly on the wavelength of the light. For glass and other transparent materials the refractive index is larger for the short (blue) wavelengths than for the longer (red) ones. Thus, when a beam of white light is passed though a prism, the blue rays will be bent more than the red rays--that is, the light spreads out to form a spectrum. The colors in the spectrum appear in the order of increasing wavelength: violet, blue, green, yellow, orange, and red. Sir Isaac Newton first explained the spectrum. He showed that, contrary to popular belief, the prism did not create the beautiful colors, but only made visible the components of white light.
Scientists make use of dispersion in the analysis of light emitted or absorbed by various materials both on the earth and on other bodies in space.[pg. 45]
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 A)--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 conceal each other when they are completely out of phase. Thus lighter and darker areas form the banded diffraction patterns.... Diffraction patterns are formed when light passes through pinholes and sits. A pinhole gives a circular pattern and a slit gives and elongated pattern. A sharper 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. [p. 46-47]
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. This process is called destructive interference. 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 produces 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 than 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 (see lasers....). [pg. 48-49]
Iridescent colors, which change the appearance with the angle of viewing and the direction of the illumination, are due to interference. The delicate hues of soap bubbles and oil films, the pale tints of mother-of-pearl, and the brilliant colors of a peacock's tail are all iridescent colors..... A soap bubble appears iridescent under white light when the thickness of the bubble is of the order of a wavelength of light. This occurs because light waves reflected from front and back surfaces of the film travel different distances. A difference in phase results that may cause destructive interference for some particular wavelength, and the hue or color associated with that wavelength will be absent from the reflected light. If the missing hue is red, reflected light appears blue-green, the complement of red. If film thickness or direction of illumination changes, interference occurs at different wavelengths and the reflected light changes color. [pg. 49]
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.
Reflection, diffraction, and interference all play a part in the complex phenomenon of scattering. If the scattering particles are of uniform size and much smaller than the wavelength of light, selective scattering may occur and the material will appear colored, as shown above. Shorter wavelengths will be scattered much more strongly than longer ones. In general, scattered light will appear bluish, while the remaining directly transmitted light will lack the scattered blue rays and thus appear orange or red. Many natural blue tints are due to selective scattering rather than to blue pigments. The blue of skies and oceans is due to this kind of scattering. Blue eyes are the result of light scattering in the iris when a dark pigment is lacking.
Scattering by larger particles is nonselective and produces white. The whiteness of a bird's feather, of snow, and of clouds--all are due to scattering by particles which, though small, are large compared to the wavelength of light. [pg. 50]
Absorption of light as it passes through matter results in a 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. An absorption curve for a filter shows the amount of light absorbed at a particular wavelength. A unit thickness of the absorbing medium will always absorb the same fraction of light from a beam. If the first millimeter thickness of a filter absorbs 1/2 the light, the second millimeter absorbs 1/2 the remaining light, or 1/4 of the total. The third millimeter absorbs 1/2 of the 1/4, so only 1/8 of the light is transmitted through three millimeters of filter..... [p. 51]
Fluorescence and phosphorescence are caused by light striking atoms. In the collision, energy is transferred from the light to the electrons of the atoms. This energy may be re-radiated as light or dissipated as heat. If the emitted light is of the same frequency as the incident light, the effect is a kind of scattering. In many cases, however, the emitted light is of a different (usually lower) frequency than the incident light, and is characteristic of the atom that emitted it. The immediate re-radiation of absorbed light energy as light of a different color is called fluorescence. [The color of the fluorescence depends on the nature of the mineral.]
Some materials continue to emit light for a time after the incident radiation has been cut off. This is phosphorescence, usually a property of crystals or of large organic molecules. Phosphorescence often depends on the presence of minute quantities of impurities or imperfections in the crystal that provide "traps" for excited electrons. These electrons have received extra energy from incident radiation. The electrons remain in the "traps" until shaken loose by the heat vibrations of the atoms in the crystal. Phosphorescent light is emitted as the electrons return to their normal positions. Solid substances that produce light in this way are called phosphors. [p. 52]
Polarized light waves are restricted in their direction of vibration. Normal light waves vibrate in an infinite number of directions perpendicular to their direction of travel. For example, in the head-on view of unpolarized light the lines a, b, c, d, and an infinite number of others are perpendicular to the ray. At a particular instant any one of them might represent the direction of the vibrations. Thus, from moment to moment, the direction of the light vibrations changes in a random fashion. When components of vibration in one direction only are present, the light is plane polarized. [p. 53]
Plane of vibration of a polarized light wave is usually unaffected in passing through a transparent material--it remains polarized in the same plane. Some optically active materials, however, rotate the plane of vibration in either a clockwise or counterclockwise direction. Quartz crystals occur in both clockwise and counterclockwise varieties. Sugar solutions are also optically active. A chemist can determine the concentration of sugar in a solution by measuring the rotation of the plane of vibration when plane-polarized light is passed through the solution. A dextrose sugar solution causes a clockwise rotation; levulose sugar, a counterclockwise one. A device for measuring the angle of rotation of the plane of vibration is called a polariscope. A sacharimeter is a polariscope used in sugar analyses. [p. 54]
Doubly refracting crystals, such as calcite and quartz, break up light rays into two parts, called ordinary rays and extraordinary rays, which are polarized at right angles to each other. Such a crystal has a different refractive index for each of the two rays, and they are bent at different angles when they enter the crystal. This double refraction will form two images when a calcite crystal is placed over a dot on a piece of paper. The dot appears as two dots a small distance apart. Rotating the crystal causes one of the dots to rotate about the other. The dot that remains stationary is the image formed by the ordinary ray. This always lies in the plane of incidence (plane including the normal and the incident ray). The moving dot is the image formed by the extraordinary ray. [pg. 55]
R E F E R E N C E S
[Light and Color, by Clarence Rainwater, Prof. of Physics, San Francisco State College, Original Project Editor Herbert S. Zim, Golden Press, NY, Western Publishing Company, Inc., 1971.]
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