# Properties of Light and the Electromagnetic Spectrum

Light can behave as both a wave and a particle. The energy and frequency of light are directly related to each other but inversely related to the wavelength.

Electromagnetic radiation is a wave of energy produced by the movement of photons through space. Visible light, microwaves, X-rays, and radio waves are all forms of electromagnetic radiation. Early attempts to understand the nature of electromagnetic radiation, specifically light, were difficult because contemporaneous experiments focused on defining light as a particle or as a wave. A wave is a periodic oscillation of energy or matter.

A wave has a specific wavelength, amplitude, and frequency. Wavelength (represented by $\lambda$) refers to the distance between two identical parts of a wave. The wavelength of electromagnetic radiation determines its energy, with shorter wavelengths having more energy than longer ones. Amplitude is the height of a wave, measured as the distance from the point of equilibrium to a crest or trough. Frequency is the number of oscillations of a wave that occur in a given period of time, usually a second, measured in hertz (Hz). Frequency is represented by $\nu$. The frequency of an electromagnetic wave is directly proportional to a photon's energy. An increase in energy means an increase in frequency but also a decrease in wavelength.

#### Wave Properties

Light has properties of both particles and waves, which is called wave-particle duality. The particle nature of light is demonstrated by the photoelectric effect. The photoelectric effect is the emission of electrons when light strikes a material. When light strikes a surface (for example, a metal), it causes electrons to be ejected, provided the light is above a specific frequency (energy). This happens because a fundamental particle that has a fixed amount of energy stored as electromagnetic radiation, called a photon, hits the electron, causing the emission of the electron. The wave nature of light is demonstrated by the double-slit experiment. In this experiment, light is passed through two very small slits in an opaque material, and transmitted light is absorbed on a screen a short distance behind the slits. The image created on the screen is not two bright slits. Instead, it is a large bright spot in the middle, surrounded by decreasing bright spots on either side.

#### Double-Slit Experiment

Because any single source of light has the same wavelength, amplitude, and frequency, the double-slit experiment shows repeating bright spots because of interference. In the bright spots on the screen, the waves of light from the two slits interfere constructively. The wave crests hit the screen at the same time, amplifying the light to create bright spots. In the dim spots between them, the light from the two slits interferes destructively. The crest of one wave hits the screen at the same time as the trough of the other, and they cancel each other out. The electromagnetic spectrum encompasses the entire range of electromagnetic waves, defined by their energy, wavelengths, and frequencies. At the side of the spectrum with the highest energy, highest frequencies, and shortest wavelengths are gamma rays. At the side with the lowest energy, lowest frequencies, and longest wavelengths are radio waves. The visible light spectrum falls in the middle, from about 400 nm (violet light) to about 750 nm (red light). The electromagnetic spectrum is a continuous spectrum containing wavelengths of electromagnetic radiation for an infinite number of possible frequencies with no gaps between them. Electromagnetic radiation exists at all wavelengths within it.