Vision: sense of light, mediated by the eye and brain of animals
Photosynthesis: conversion of solar energy into biochemical energy by green plants and certain microbes
Ultraviolet effects: biological effects of ultraviolet radiation, such as damage of DNA
Circadian rhythms: biological rhythms that have periods of about 24 hours and can be altered by exposure to light
Bioluminescence: emission of light from biochemical reactions within living organisms, such as the flash of a firefly
What exactly is light?
Light can be defined as "electromagnetic radiation that is visible". We’ll define electromagnetic radiation soon. Let's start by saying that light from the sun or a simple light bulb is called "white" light. When a beam of white light passes through a simple prism, it is dispersed into different colors, called the color spectrum. A rainbow shows the same dispersion of colors.
Why does a prism or a rainbow separate white light into different colors?
The white light from the sun or a light bulb is really a mixture of many different colors or wavelengths of light. In fact, light has many of the same characteristics as waves of water.
What does “electromagnetic radiation that is visible” mean?
Visible electromagnetic radiation has a wavelength between about 0.000000400 meters (400 nanometers, nm) and 0.000000700 meters (700 nm). The wavelength of light is typically expressed in nanometers simply because it's much easier to write "400 nm" than "0.000000400 meters". Light with a wavelength of 400 nm is violet light and light with a wavelength of 700 nm is red light. The wavelength of light can also be expressed using "scientific notation".
0.000000400 meters = 400 nm = 400 x 10-9 m = wavelength of violet light
0.000000700 meters = 700 nm = 700 x 10-9 m = wavelength of red light
But electromagnetic radiation can have a wavelength shorter than 400 nm and longer than 700 nm, even though we cannot see outside this range. As shown in the picture below, visible light constitutes only a very small part of the overall electromagnetic spectrum.
The Electromagnetic Spectrum (adapted from NASA)
The TOP TWO ROWS show the wavelengths that penetrate the earth's atmosphere. You can see that radiowaves and visible light easily penetrate the earth's atmosphere. Microwaves and Infrared radiation also penetrate the atmosphere, although to a lesser extent. You can also see that light (visible electromagnetic radiation) constitutes a very small part of the electromagnetic spectrum.
The SECOND TWO ROWS show some common objects of different sizes within this range. A large office building can be 102 m (100 meters) high. A bacterium is about 10-6 m (0.000001 meters) in diameter. The nucleus of an atom is about 10-12 m (0.000000000001 meters) in diameter
The BOTTOM ROW shows the frequency that is associated with each wavelength in the top row. The frequency is simply the number of wavelengths that pass a point in a second and is expressed in Hertz (Hz), in honor of the German physicist Heinrich Rudolph Hertz (1857-1894). One MegaHertz (MHz) is equivalent to one million Hertz (1 Mhz = 106 Hz).
You can see that frequency increases from left to right, whereas the wavelength decreases from left to right. In fact, since the speed of radiation is known (299,792,458 meters per second, or 186,282.397 miles per second), you can always determine the frequency if you know the wavelength. Just use this equation: Speed of Light / Wavelength = Frequency or more simply: c / λ = f
