PROPERTIES
Light is a transverse, electromagnetic wave that can be seen by humans.
In 1678, Christiaan Huygens (1629–1695) published Traité de la Lumiere, where he argued in favor of the wave nature of light. Huygens stated that an expanding sphere of light behaves as if each point on the wave front were a new source of radiation of the same frequency and phase.
Thomas Young (1773–1829) and Augustin-Jean Fresnel (1788–1827) disproved Newton's corpuscular theory.
- The wave nature of light was first illustrated through experiments on diffraction and interference.
- Like all electromagnetic waves, light can travel through a vacuum.
- The transverse nature of light can be demonstrated through polarization.
- Light is sometimes also known as visible light to contrast it from "ultraviolet light" and "infrared light".
- Other forms of electromagnetic radiation that are not visible to humans are sometimes also known informally as "light"
In 1678, Christiaan Huygens (1629–1695) published Traité de la Lumiere, where he argued in favor of the wave nature of light. Huygens stated that an expanding sphere of light behaves as if each point on the wave front were a new source of radiation of the same frequency and phase.
Thomas Young (1773–1829) and Augustin-Jean Fresnel (1788–1827) disproved Newton's corpuscular theory.
SOURCES
There are two general sources of light
_ Natural Sources : Our most important natural source of light is the sun. Nearly all the natural light we receive comes from the sun; moonlight is sunlight reflected from the surface of the moon. Distant stars provide an extremely small amount of light.
Artificial Sources : There are several ways of producing artificial light. In general, artificial light source can be divided into three categories.
_ Natural Sources : Our most important natural source of light is the sun. Nearly all the natural light we receive comes from the sun; moonlight is sunlight reflected from the surface of the moon. Distant stars provide an extremely small amount of light.
Artificial Sources : There are several ways of producing artificial light. In general, artificial light source can be divided into three categories.
- Thermal Sources : Example of thermal source are incandescent lamp, burning candle, etc. When object is heated until it glows or becomes incandescent, it emits all visible wavelengths along with large quantity of infrared radiation. Hence, as producers of visible radiation (i.e. luminous energy), they have a low efficiency. Generally, the efficiency of such light sources improves as the operating temperature is increased.
- Gas Discharge Sources : Example of gas discharge source are neon lamp, sodium lamps, etc. In this case, light is obtained by maintaining electric current in a gas at low pressure. Such a source emits only a few wavelength. The color and intensity of light of light depends upon the nature of gas or vapor only. It may be noted that in case of light emitted by a thermal source, the spectrum is continuous. However, when light is obtained from a gaseous discharge, the spectrum is discontinuous i.e. it consists of one or more colored lines. For examples, in the case of sodium lamp, the spectrum consists mainly of two yellow lines very close together with wavelengths of 5890Å. These wavelengths are so close to each other that light from a sodium lamp is said to be monochromatic i.e. a light having only one wavelength.
- Luminescent Sources : The familiar example of such a source is the fluorescent tube. A fluorescent tube consist of a thin-walled glass tube with fluorescent substance coated on the inside of the tube. An electric current is maintained in mercury vapors at low pressure. It emits visible radiation as well as ultraviolet radiations (invisible). The fluorescent material absorbs ultraviolet radiation and re-emits them at longer wavelengths of the visible spectrum.
Characteristics
- The speed of light depends upon the medium through which it travels.
- The speed of light in a vacuum is a universal constant in all reference frames.
- All electromagnetic waves propagate at the speed of light in a vacuum.
- The speed of light in a medium is always slower the speed of light in a vacuum.
(The difference is usually negligible when the medium is air.) - The speed of anything with mass is always less than the speed of light in a vacuum.
(The speed of light in a vacuum is the universal speed limit.) - The speed of light in a vacuum is fixed at 299,792,458 m/s by the current definition of the meter.
- The amplitude of a light wave is related to its intensity.
- Intensity is the absolute measure of a light wave's power density.
- Brightness is the relative intensity as perceived by the average human eye.
- The frequency of a light wave is related to its color.
- Color is such a complex topic that it has its own section in this book.
- Monochromatic light can be described by only one frequency.
- Laser light is very nearly monochromatic.
- There are six simple, named colors in English (and many other languages) each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue, and violet.
- Polychromatic light is composed of multiple frequencies.
- Every light source is essentially polychromatic.
- White light is very polychromatic.
- A graph of relative intensity vs. frequency is called a spectrum (plural: spectra).
Although frequently associated with light, the term can be applied to many phenomena.- A continuous spectrum is one in which every frequency is present within some range. Black body radiators emit a continuous spectrum.
- A discrete spectrum is one in which only a set of well defined and isolated frequencies are present.
(A discrete spectrum is a finite collection of monochromatic light waves.) The excited electrons in a gas emit a discrete spectrum.
- The wavelength of a light wave is inversely proportional to its frequency.
- Light is often described by it's wavelength in a vacuum.
- Light ranges in wavelength from 400 nm on the violet end to 700 nm on the red end of the visible spectrum.
- Wavelengths slightly shorter than 400 nm are said to be ultraviolet.
(They are "beyond violet" in terms of frequency.) - Wavelengths slightly longer than 700 nm are said to be infrared.
(They are "below red" in terms of frequency.)
- Wavelengths slightly shorter than 400 nm are said to be ultraviolet.
- Phase differences between light waves can produce visible interference effects.
(There are several sections in this book on interference phenomena and light.)