Light exhibits a wave-particle duality, which means it can act like a wave or like a particle. Light is a form of electromagnetic radiation. As a wave, it has an electric field component perpendicular to a magnetic field component, and the wave propagates through space. Although both light and sound act as a wave, light travels much faster than sound. The speed of light in a vacuum is constant, traveling 3 times 10 to the eighth meters per second. The speed of sound is only about 340 meters per second. This is why you can see lightning before you hear thunder. Waves have particular properties. For example, the amplitude of a wave is the distance between the midpoint and the crest (which is the highest point on the wave). The wavelength is the distance between two consecutive points on the wave: for example, between two consecutive crests. The wave also has a frequency. If you're standing still and a wave is propagating through space past you, the frequency is the number of cycles of the wave that pass by you in a given amount of time. Frequency is typically measured in cycles per second or hertz. Often the cycles are left out of our units, so frequency is just measured in seconds to the negative first power. Frequency and wavelength are inversely proportional according to the equation ν equals c divided by λ. The Greek letter "nu" represents frequency, "c" represents the speed of light, and "lambda" represents wavelength. When light acts as a wave, the color of light corresponds to the wavelength or frequency of the wave, while the brightness of light corresponds to the amplitude of the wave. When we say "light," we're referring to a very narrow band in the electromagnetic spectrum, which human eyes are tuned to detect. There are multiple other bands in the electromagnetic spectrum; in general, these bands are categorized by how they're used. The lowest energy electromagnetic radiation has the lowest frequency or the longest wavelength. Radio waves are used to transmit a signal over long distances. Microwaves are absorbed by water molecules, so they can be used to heat food. Your skin and organs are transparent to x-rays, but x-rays interact differently with your bones and metal atoms. You can create a mnemonic to remember the order of the electromagnetic spectrum. I use, "Good Xylophones Ultimately Vibrate Readily In Melodious Ranges," but you can create your own if you'd like. To remember the order of colors in the visible spectrum, you can use ROY G BIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet. Another property of light when it acts as a wave is interference. When two waves cross paths in the same space, they can undergo constructive or destructive interference. If the waves are in phase so that the crest of one wave lines up with the crest of the other, their crests add together in constructive interference. If the two waves are out of phase so that the crest of one wave lines up with the trough (or low point) of the other wave, they undergo destructive interference and can cancel each other out. This is the idea behind noise-canceling headphones. Sound is a wave. Noise-canceling headphones use a microphone to detect the exterior sounds and attempt to create the same sound wave but one that is out of sync with the exterior sounds. They generally work best with continuous repetitive noises like the hum of an airplane engine because those waves are simpler. Waves also undergo diffraction. If you send a wave at a barrier with a slit on the scale of its wavelength, it will bend as it leaves the slit. If you send particles at the slit, the particles will pass through without changing course or hit the barrier and be reflected. If you have multiple slits, you can produce a diffraction pattern. With multiple slits, the light wave can simultaneously enter both slits, diffract, and interfere with itself on the other side. If you use a film on the other side to detect the light, you'll see an alternating pattern of light and dark regions where the waves underwent constructive and destructive interference. This diffraction pattern is a clear example of light acting as a wave.