Electromagnetic Energy

Electromagnetic radiation is the energy produced by the movement of charged particles and has properties of both waves and particles.

Electromagnetic radiation is a wave of energy produced by the movement of particles 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. 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.

The Double-Slit Experiment

To understand why this effect occurs, it is necessary to first understand the properties of a wave. A wave has a specific wavelength, amplitude, and frequency. Wavelength ($\lambda$) 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 a 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). 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

Because any single source of light has the same wavelength, amplitude, and frequency, the double-slit experiment shows repeating bright spots due to 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.

The Electromagnetic Spectrum

When electrons in atoms are excited, they give off light, also called electromagnetic radiation. Using a prism, the light emitted is passed through the prism, separating the light into individual wavelengths. Each element gives off light of specific colors, and thus each element is unique. A line spectrum is a pattern of colors on a dark background produced by an element when it gives off light (emission) or a pattern of dark lines on a background of the electromagnetic spectrum produced by an element when it absorbs light (absorption). These lines, representing photons, have an arrangement that is characteristic of a specific element. Because each element's line spectrum is unique, it can be used to identify the element. Line spectra are used by astronomers to identify the elements in faraway stars and planets.
Atomic line spectra demonstrate a failure of classical mechanics, the study of how macroscopic objects behave. Classical mechanics does not allow for wave-particle duality. In classical mechanics light must be either a wave or a particle but not both at the same time. If light is a wave, as shown by the double-slit experiment, the line spectrum for any element should be continuous. Varying wavelengths and intensities of light should excite electrons to different degrees, yet electrons emit light at distinct wavelengths. Regardless of the intensity of light exciting a hydrogen atom, the atom will only emit light at 410 nm, 434 nm, 486 nm, and 656 nm. The line spectrum for any element is discontinuous. This suggests that the energy carried by photons of light is quantized, which means it exists only as discrete values. The quantized nature of light shows that it must have a particle nature, which contradicts the results of the double-slit experiment. This failure of classical mechanics led to the development of quantum mechanics, the branch of science that deals with subatomic particles, their behaviors, and their interactions.