MEASUREMENT USING LIGHT

Principles of Light

What we commonly refer to as light is electromagnetic radiation in the visible range. Every substance with a temperature above absolute zero emits electromagnetic radiation. This radiation is characterized by a frequency and wavelength that are related by the speed of light: frequency = speed of light ÷ wave length. (c = speed of light = 3 × 10⁸ m/sec or 186,400 miles/sec or 7.5 times around the earth's circumference in 1 second.) High energies are associated with high frequencies and short wavelengths, such as those of gamma rays and x-rays. As wavelength increases to the micron range, ultraviolet radiation proceeds to visible light, and at larger wavelengths, infrared is then followed by microwaves and radio waves with wavelengths in the kilometer range.

Electromagnetic waves and sound waves have some important differences. The particles in motion in sound waves are in the same direction as the propagation (longitudinal waves), whereas electromagnetic waves are perpendicular to the direction of propagation (transverse waves). Sound waves can propagate only through matter, whereas electromagnetic waves propagate through a vacuum without attenuation. The speed of light is about 1 million times faster than the speed of sound in air. If an observer is moving relative to a sound source, measurement of the speed of sound depends on the observer's own motion, but the speed of light is the same to any observer and any frame of reference. This statement is a basic premise of Einstein's special theory of relativity. At the high-frequency end of the electromagnetic spectrum are two forms of ionizing radiation: x-rays and gamma rays. These high-frequency waves are capable of knocking electrons out of their orbits and can thereby cause cell injury and death or ontogenesis. Gamma rays are commonly emitted by decaying radioactive nuclei.

Visible light and infrared light demonstrate several properties common to all electromagnetic radiation. Light represents a form of energy that when passing through matter, may be reflected, transmitted, or absorbed. Although light itself cannot be stored, it can be converted into some other form of energy such as electricity, chemical energy, and heat. In addition, light can be generated from other forms of energy, including heat (incandescent), electrical (gas discharge), and chemical (photoluminescent) energy.