Light

Electromagnetic radiation transfers energy from one place to another, even through the vacuum of empty space. It is self-perpetuating, like a perfect pendulum, that swings back and forth forever. And just like two puppies playing tug of war with a toy, repeatedly pulling on it at the same time, because each senses the pull from the other, the electric and magnetic fields oscillate. When the electric field gets stronger, the magnetic field instantly strengthens too, and when the electric field weakens, so does the magnetic field. Get the picture? With EMR, there is motion too, as light travels in a straight line, away from the source. The red and blue wave animation below helps us to understand this. Do realize though, that these are fields of energy and magnetism. Once a light wave is created from some source, it travels on forever, until it is either absorbed by some gases it meets, or is reflected off some atoms or molecules.

Now, in your mind, picture the most beautiful rainbow you have ever seen. Maybe you were lucky and saw a double one! You may already know that the colors of the rainbow are caused by water droplets acting like prisms, splitting sunlight into its component colors.

The picture below shows how we can separate a beam of white light into a rainbow of these basic colors by passing it through a prism. In principle, the original beam of white light could be produced once again by passing the entire red-to-violet range of colors—called a spectrum (plural, spectra)—through a second prism to recombine the colored beams. This experiment was first reported by Isaac Newton over 300 years ago.

Animation of light bending through a prism, courtesy of Dr. Alexander Churenkov

The refraction index of any material is the property that causes light to bend when the light hits its surface at any angle, other than perpendicular. This index depends upon the wavelength of the light. This fact can be used to resolve the light beam into the spectral components it consists of. One of the tools used for spectrum analysis of light is the glass prism.

Play with it yourself, by loading this Java Applet!

Image of light through a prism, courtesy of Dr. Eric Chaisson

When passed through a prism, white light splits into its component colors, spanning red to violet in the visible part of the electromagnetic spectrum. The slit immediately in front of the flashlight narrows the beam of radiation. The image on the screen is a series of colored images of the slit. Human eyes are insensitive to radiation of wavelength shorter than 400 nm or longer than 700 nm.

What does EMR look like?

The particle of light is called the photon. It has characteristics of both a wave and a particle. And it has no mass! That means it weighs absolutely nothing!
	Here, 'E' is the electrical energy field and 'H' is the magnetic field. They are always at right angles to one another ('orthogonal'). A principle of electro-magnetic wave propagation is that the vectors E and H oscillate in phase, i.e. they achieve the maximum value in the same points of the space. In this animation, the blue electric field is at maximum at the top of its cycle and the red magnetic field is at its maximum at the right-most point of its cycle. One point that helps to understand light, is in realizing that the wave is created when a specific amount of energy, called a 'photon', is expelled from an atom. This happens when an electron falls from one excited state (orbital) to a lower one. Another field of physics, called 'quantum mechanics' is needed to explain how this works. Todd Stedl's web site provides a good introduction to these concepts.
	Once the photon is released, the chain of oscillating electric and magnetic fields travels away from the atom. It does not leave a trail as it goes. It travels like a bullet from a gun. Images such as this wave animation tend to confuse our understanding, by implying that the light wave is a solid thing. This is strictly for visualizing, like this time exposure photo of night-time traffic lights. Just like the car lights and the bullet, a light wave is only at one point in space at any moment in time. When we see a star, we are seeing a series of light waves, which were emitted in sequence and arrive at our eyes in that same sequence.

Light can be thought of as a wave which travels at a certain velocity, the velocity of light. Like an ocean wave, or a sound wave, the height (called the amplitude) of a light wave changes with time and place. The distance between crests or troughs of the wave is called the wavelength. If you stand in one place and watch crests pass, the speed of the wave is the wavelength times the number of crests that pass per time interval. The frequency tells you how many crests pass during each time interval. If the wavelength of a wave is 2 cm, and the frequency is 1 per second, then one crest passes each second. Since crests are 2 cm apart, the velocity of the wave is 2 cm/sec. Light is unique among waves, in that it does not need a medium in which to propagate. Light can travel in a vacuum because it is a self-perpetuating wave. Sound cannot travel in a vacuum - it needs atoms and molecules to affect. A vacuum is empty of these, and therefore quiet. The velocity of light is the ultimate speed limit: Nothing in our universe (that we know of, at least) can go any faster. To visualize what a wave of light looks like, in a simplistic way, Jos Bergervoet has produced some animations of EMR on a half-wavelength dipole antenna, which you may learn about here:

What determines the color of a beam of light?

RGB.gif, courtesy of Alexander Churenkov

According to the Young-Helmholtz theory of color vision, the sensation of any color can be achieved by the superposition of pure red, green and blue colors. This fact was proved experimentally and indicates that in the eye there are three types of receptors, which are sensitive separately to red, green and blue light. These receptors are excited in proportions that correspond to the color of the visible light. Red light excites only the red light receptors, green light excites the receptors responsible for green light, and blue light receptors of blue light. If all receptors are excited to an equal degree, we have the sensation of white light, and if the receptors are not excited, the sensation of darkness. For this reason, the overlapped spots of the red, green and blue light shown in the figure look like a white spot. Additionally, the superposition of red and blue lights appears magenta, superposition of the green and blue lights appears cyan, and superposition of red and green colors appears as a yellow color.

For more information about how colors are produced using these three colors, check out the 'Einstein's Legacy, TV Screens' link at this University of Colorado site.

The answer is its wavelength (or, equivalently, its frequency)—we see different colors because our eyes react differently to electromagnetic waves of different wavelengths. Red light has a frequency of roughly 4.3 × 10¹⁴ Hz, corresponding to a wavelength of about 7.0 × 10⁷ m. Violet light, at the other end of the visible range, has nearly double the frequency—7.5 × 10¹⁴ Hz—and (since the speed of light is the same in either case) just over half the wavelength—4.0 × 10⁷ m. The other colors we see have frequencies and wavelengths intermediate between these two extremes.

Astronomers often use a unit called the nanometer (nm) when describing the wavelength of light. There are 10⁹ nanometers in 1 meter.

Astronomers often discuss wavelengths in units of Angstroms (named for a Swedish physicist). An Angstrom is about the size of an atom. There are 10 Angstroms in a nanometer (10Å = 1nm).

Thus, the visible spectrum covers the wavelength range from 400 to 700 nm (4000 to 7000 Å). The radiation to which our eyes are most sensitive has a wavelength near the middle of this range, at about 550 nm (5500 Å), in the yellow-green region of the spectrum. It is no coincidence that this wavelength falls within the range of wavelengths at which the Sun emits most of its electromagnetic energy—our eyes have evolved to take greatest advantage of the available light.

There's much more to learn about EMR, so let's go!