Bending Light: Refraction and the Electromagnetic Spectrum
By Joan Whetzel
When was the last time you were startled by the unexpected beauty of a rainbow appearing in the wake of a storm? Or enjoyed peering into a kaleidoscope to witness its ever changing show of colors and shapes? Or wondered at the bright colors sprawling outward from a prism? All of these displays of color appear as a result of the refraction and reflection of light, breaking down the white light into its component colors in the visible light spectrum.
Snell's Law of Refraction
According to Snell's Law of Refraction, when light passes through air or some other medium (i.e. glass prism, kaleidoscope, or water droplets) the medium cause the light to bend and change speed. Each of the colors in the visible spectrum has a different wavelength (they move at different speeds) which causes them to bend a different angles. This difference in speed and bending at different angles is what separates the individual colors. Snell's law also states that light beams passing from a substance that has a high index of refraction into a low index substance can be completely reflected. In other words, the light gets trapped inside the substance or material until it finally appears out the other end. This is the phenomenon that creates the sparkle in diamonds.
The Electromagnetic Spectrum and the Visible Spectrum
The electromagnetic spectrum (EM) is like a number line, listing the various wavelengths for a series of forms of radiation. Some of the wavelengths move too fast or too slow to be visible to the human eye. The wavelengths falling into the visible light wavelengths range from violet at a shorter wavelength of 4,000 angstroms to red at a longer wavelength of 7,000 angstroms. (One angstrom equals a wavelength of 10-10 meters.) The Hubble Space Telescope is an example of a visible observatory. It can pick up images far more clearly than visible observatories on the Earth's surface as well as collecting light in the ultraviolet range. This is because the Hubble Space Telescope outside the Earth's atmosphere, which distorts the images and absorbs the ultraviolet end of the light spectrum.
Rainbows occur when sunlight (coming from behind the observer) is refracted by the surface of a raindrop (in the exiting storm front in front of the observer). The refraction causes the sunlight to bend inside the raindrop, then be reflected off the opposite side of the raindrop, which aims it back toward the observer. It is refracted a second time as it leaves the raindrop causing the light to bend even further. What causes the lightshow to bend into the rainbow's arch - or bow - is because the raindrops are round, so when the light refracts, it spreads outward from a round center (the round raindrop).
Newton's Prism (108)
Newton discovered how rainbows were refracted light by his experiments with a prism. By placing a light behind side of a triangular pyramid shaped prism resting on its base, the light was refracted inside the prism and emitted out the opposite side of the prism as the separated colors of white light. Most of the other scientists of the day thought that this phenomenon was produced by a mixture of light and darkness, and the thought that the prisms were merely adding the color to the light, rather than separating the colors from the light. But Newton, as the independent thinker that he was, wasn't convinced by the prevailing thought of his contemporaries, and set out to prove his theory of diffraction. He took a second triangular pyramid shaped prism and, standing it on its point next to the first prism, shown the light through both prisms. The first prism separated the light's colors. When the diffracted light entered the second prism it bent the colors a second time, emitting a white beam of light out the other side. So the first prism separated the white light into its individual colors and spread them out into several wide multicolored beams, while the second prism pushed the colors back together into a single beam of white light.
Check out "Newton's Prism Experiments" at: http://micro.magnet.fsu.edu/primer/java/scienceopticsu/newton/
This site has an interactive display showing how the double prism experiment separates and recombines the light's colors.
Scottish physicist, Sir David Brewster, found that when viewing reflected light at 90 degree angle, it appeared polarized. This is due to the fact that light waves consist of two fields, one electric and the other magnetic, and that the oscillation of these two fields moved perpendicular to each other. He found that one method of polarizing the light was to reflect the light between two types of media (e.g. air and glass). During the process of working on his light polarizing experiments, he invented the kaleidoscope. It passes light through two plates of glass with pieces of crystals sandwiched between them. The light then entered a tube with a set of mirrors placed at specific angles. The mirrors reflected the light, polarized it, and re-directed it to the observer's eye at the other end of the tube. His purpose was to explain the relationship between the incident angle of the light and reflecting material's (the mirror's) refractive index.
Florida State University, Molecular Expressions. Newton’s Prism Experiment. Downloaded: 12/14/2011.
Florida State University, Molecular Expressions. Sir David Brewster. Downloaded 12/14/4911.
NASA Goddard Space Flight Center. Measuring the Electromagnetic Spectrum. Downloaded 12/14/3011. http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html
The National Center for Atmospheric Research. About Rainbows. Downloaded 12/14/3100.
Pickover, Clifford A. The Physics Book: From the Big Bang to Quantum Resurrection, 250 Milestones in the History of Physics. New York: Sterling Publishing, 2011. Pp 66, 86, 108, 160, 228.
The University of British Columbia, Canada, Mathematics Department. The Law of Refraction. Downloaded: 12/14/2911.