Light

Light is a form of electromagnetic radiation. Other types of electromagnetic radiation include radio waves, microwaves, infrared, ultraviolet, x-rays, and gamma rays. All electromagnetic waves possess energy. Moreover, electro-magnetic waves (including light) are produced by accelerated electric charges (such as electrons). Light moves through space in a wave that has an electric part and a magnetic part. That is why it is called an electromagnetic wave.

Speed of Light

Light travels through empty space at a high speed, very close to 300,000 kilometers per second (km/s). This number is a universal constant: it never changes. Since all measurements of the speed of light in a vacuum always produce exactly the same answer, the distance light travels in a certain amount of time is now defined as the standard unit of length. For convenience, the speed of light is usually written as the symbol, c.

Characteristics of Waves

All waves, including light waves, share certain characteristics: They travel through space at a certain speed, they have frequency, and they have wavelength. The frequency of a wave is the number of waves that pass a point in one second. The wavelength of a wave is the distance between any two corresponding points on the wave. There is a simple mathematical relationship between these three quantities called the wave equation. If the frequency is denoted by the symbol f and the wavelength is denoted by the symbol λ, then the wave equation for electromagnetic waves is:

c = f λ

Since c is a constant, this equation requires that a light wave with a shorter wavelength have a higher frequency.

Waves also have amplitude. Amplitude is the "height" of the wave, or how "big" the wave is. The amplitude of a light wave determines how bright the light is.

Wavelength and Color

There is a simple way to remember the order of wavelengths of light from longest to shortest: ROY G. BIV. The letters stand for red, orange, yellow, green, blue, indigo, and violet. (This violet is not the same as the crayon color called violet, which is a shade of purple.) The human eye perceives different wavelengths of light as different colors. Red is the color of the longest wavelength the human eye can detect; violet is the shortest. Red light has a wavelength of around 700 nanometers (nm). (A nanometer is one-billionth of a meter.) Light with a wavelength longer than 700 nm is called infrared. ("Infra" means "below.") Violet light is around 400 nm. Electromagnetic radiation with a shorter wavelength is called ultraviolet. ("Ultra" means "beyond.") It is best not to use the terms "ultraviolet light" or "infrared light," for instance, because the word "light" should be applied only to wavelengths that the human eye can detect.

Refraction and Lenses

What happens when light encounters matter depends on the type of material. Glass, water, quartz, and other similar materials are transparent. Light passes through them. However, light slows down as it passes through a transparent material. This happens because the light is absorbed and reemitted by the atoms of the material. It takes a small amount of time for the atom to reemit the light, so the light slows down. In water, light travels around0.75c or 225,000 km/s. In glass, the speed is even slower, 0.67c. In diamond, light travels at less than half its speed in vacuum, 0.41c.

When a beam of light passes from vacuum (or air) into glass, it slows down, but if the beam hits the glass at an angle, it does not all slow down at the same time. The edge of the beam that hits the glass first slows down first. This causes the beam to bend as it enters the glass. The change in direction of any wave as it passes from one material to another and speeds up or slows down is called refraction. Refraction causes water to appear to be shallower than it is in reality. Refraction causes a diamond to sparkle.

Refraction is also what creates a mirage. Sometimes the air a few centimeters above the ground is much warmer than air a few meters farther up. As light from the sky passes into this warmer air, it speeds up and bends away from the ground. An observer may see light from the sky and be fooled into thinking that it is a lake. Sometimes, even trees and houses can be seen in the mirage, but they will appear upside down.

Refraction of light allows a lens to perform its function. In a converging lens, the center of the beam reaches the lens first and slows down first. This causes the beam to be bent toward the center of the lens. A parallel beam of light passing through a good-quality lens will be bent so that all the light arrives at a single point called the focal point. The distance from the lens to the focal point is called the focal length, f. A diverging lens spreads the beam out so that it appears to be coming from the focal point.

In a slide projector, the lens projects an image of an object (the slide) onto a screen. The distance from the lens to the image and the distance of the lens to the object are related to the focal length by this strange-looking formula (d_i is the image distance and d_o is the object distance):

Interpreting this formula is a little difficult. Remember that the focal length of the lens does not change, so is a constant. If the image distance (d_i) gets larger, the object distance (d_o) must get smaller to make the fractions add to the same constant value.

Reflection and Mirrors