Light has both wavelike and particle-like properties, and how it behaves depends partly on how you treat it. Light is referred to as electromagnetic radiation because it is made up of both electric and magnetic fields. Visible light is only one among many types of electromagnetic radiation. Electromagnetic radiation travels through space at a speed of 186,000 mi/s. The distance between peaks of a wave is called the wavelength. Frequency is the number of waves that pass a stationary point in 1 second. Wavelength x frequency = the speed of light. Light does not require a medium and can travel through empty space. A particle of electromagnetic radiation is called a photon. You can think of a photon as a packet of waves. The amount of energy a photon carries is inversely proportional to its wavelength. Shorter-wavelength protons carry more energy, and longer-wavelength protons carry less energy. Short wavelength, high frequency, and large photon energy together; long wavelength, low frequency, and small photon energy go together. A spectrum is an array of electromagnetic radiation displayed in order of wavelength. The wavelength of visible light ranges from about 400 to 700 nm (4000 to 7000 Å). You sense the wavelength of light as color. Light with wavelengths at the short-wavelength end of the visible spectrum appears violet to your eyes, and light with wavelengths at the long-wavelength end appears red. Your eyes do not detect infrared radiation, but your skin can sense some of it as heat. Beyond the infrared part of the electromagnetic spectrum lie microwaves and radio waves. Electromagnetic waves shorter than violet are called ultraviolet radiation. Electromagnetic waves that are even shorter are called X-rays, and the shortest are gamma rays. The boundaries between these wavelength ranges are defined only by conventional usage, not by natural divisions. Earth’s atmosphere is opaque to most electromagnetic radiation. Only visible light, some short-wavelength infrared radiation, and some radio waves reach Earth’s surface through wavelength bands called atmospheric windows.
Light can be focused into an image in one of two ways. Either a lens refracts (“bends”) light passing through it, or a mirror reflects (“bounces”) light from its surface. Refracting telescopes use a lens to gather and focus light, whereas reflecting telescopes use a mirror.
Cosmic rays are not electromagnetic radiation; they are subatomic particles such as electrons and protons traveling at nearly the speed of light, arriving from mostly unknown cosmic sources.
—February 2023
Lance Jones 