Light Waves

FIELDS OF STUDY: Optics; Electromagnetism

ABSTRACT: The nature of light as both particles and electromagnetic waves is presented. The interaction of light with matter is the basis of the science of optics and has many applications. Visible light is just a small portion of the electromagnetic spectrum, which is the range of wavelengths that make up electromagnetic radiation.

PRINCIPAL TERMS

• diffraction: a change in direction (bending) of a light ray as it passes around an obstruction or through an opening.
• interference: the meeting of two waves traveling in the same medium, causing their properties to interact.
• photoelectric effect: a phenomenon that describes the emission of electrons from matter (typically metal) upon exposure to electromagnetic radiation.
• photon: an elementary particle of light that moves and has energy but lacks mass and electrical charge.
• polarization: the process by which the motion of electromagnetic waves, which is perpendicular to the direction of energy transfer, is brought into alignment in the same plane.
• reflection: the bouncing back of waves from surfaces that do not absorb those particular wavelengths.
• refraction: the change in direction of a light ray as it passes from one medium to another.

The Nature of Light

Nuclear fusion reactions within the sun produce vast amounts of energy that are emitted into space from the sun’s surface. Most of the energy emitted is in the form of electromagnetic waves. These waves cover the entire range of the electromagnetic spectrum, from extremely long, low-frequency waves such as radio through extremely short, high-frequency waves such as x-rays. Visible light is a very narrow range of frequencies within this overall spectrum. Electromagnetic energy is transmitted in the form of photons that have characteristic behaviors of both particles and waves. A photon can be thought of as a particle of energy moving through space like a wave moves through water.

Light is typically referred to by either frequency or wavelength. The frequency is defined as the number of complete wavelengths that occur, typically in one second of time. The wavelength is the distance between two peaks or two troughs of the wave. White light, such as that from the sun or an electric light, is composed of electromagnetic waves of many different wavelengths. These waves are detected by the eye as different colors when they become separated by diffraction, as when sunlight passes through a prism. The colors change continuously from red, with the longest visible wavelength, to violet, with the shortest visible wavelength. Each wavelength corresponds to a different specific energy of the photons that make up the light of a particular wavelength. Wavelengths longer than visible red are called infrared, and wavelengths longer than visible violet are called ultraviolet.

Matter that allows light to pass through it is said to be transparent to that wavelength. The speed of light is affected by the density of the medium, causing the light rays to change speed and direction when they pass from one medium to another. This is refraction, and it is readily seen when an object that is partly in air and partly in water appears to change direction. When light strikes a surface, it can either be absorbed or reflected. When light is absorbed, the material accepts the energy of the photons into its atoms and molecules. In reflection, light waves bounce off of the surface and the energy of the photons is not absorbed. When only light of certain frequencies is absorbed, the other frequencies are reflected and may be seen as colors. Leaves and grass appear green because their material absorbs light in the red, orange, yellow, blue, and violet parts of the visible spectrum and reflects the green waves.

When electromagnetic waves overlap, their amplitudes add together to produce either a greater amplitude or a lesser amplitude. Constructive interference produces a waveform with a greater amplitude than the sum of the two component amplitudes. Destructive interference produces a waveform in which the amplitude is reduced.

Wave-Particle Duality

Light behaves as though it is composed of particles (photons). At the same time, it behaves as though it is composed of waves. This is the essential feature of wave-particle duality. The "double slit" experiment carried out by Thomas Young (1773–1829) in 1801 demonstrated the behavior of light as a wave. When light passes through two closely-spaced narrow slits, the light that emerges exhibits a pattern of light and dark bands that are the result of constructive and destructive interference. This is a characteristic behavior of waves that is demonstrated by the interference of waves in the water of a ripple tank. In 2015, for the first time, scientists were able to take an image that showed light behaving as both a wave and a particle at the same time.

Wave behavior cannot account for the photoelectric effect, however. The photoelectric effect was identified in 1887 by Heinrich Hertz (1857–94) and was studied extensively by Robert Millikan (1868–1953). Millikan, and others, observed that when light fell on a metal surface, it could induce the metal to generate an electric current through space in a specially constructed vacuum apparatus. Eventually, it was determined that the metal was in fact emitting electrons and that monochromatic light (light having just one wavelength) produced the effect rather than the entire range of wavelengths in white light. Albert Einstein (1879–1955) explained the photoelectric effect by positing photons having specific energies corresponding to specific frequencies. The energy of the photon (E) is equal to the product of the frequency (f) and the Planck constant (h), written as:

E = hf

Light intensity and distance are inversely related by the inverse square law. The intensity of light (I) decreases as the square of the distance (d) between the light source and the detector of the light. This can be described by the following equation:

I = 1/d²

Therefore, when the distance between the light source and the detector is doubled, the intensity of the light that is detected is just one-quarter of its original value.

Other Properties of Light

Normally, light waves do not have a uniform plane of vibration. Instead, the vibrations are radially distributed about the direction of motion. Photons of light can be thought of as pulsating balls of energy traveling through space. When reflected from a flat surface such as a paved highway or passed through an appropriate filter, the light can be altered in such a way that only vibrations parallel to one plane go on. Such polarization is used to restrict the intensity of light without altering its wavelength. Polarizing sunglasses and window coatings are perhaps the most common applications of this phenomenon.

In addition, astronomers can use the light waves of distant stars to measure distances between celestial objects. This is possible through the Doppler effect, or Doppler shift. (The Dopper shift refers to a perceived change in frequency as a wave source moves farther from or closer to an observer.) Scientists can use spectroscopy to study the movement of s bodies according to the shifting of their wavelengths of light.

Bibliography

Goldstein, Dennis H. Polarized Light. 3rd ed. Boca Raton: CRC, 2011. Print.

Hillesheim, Heather E. Sound and Light. New York: Chelsea House, 2012. Print.

Keller, Ole. Light: The Physics of the Photon. Boca Raton: CRC, 2014. Print.

Lynch, David K., and William Livingston. Color and Light in Nature. 2nd ed. New York: Cambridge UP, 2001. Print.

Roychoudhuri, Chandrasekhar, A. F. Kracklauer, and Katherine Creath, eds. The Nature of Light: What Is a Photon? Boca Raton: CRC, 2008. Print.

Walmsley, Ian. Light: A Very Short Introduction. New York: Oxford UP, 2015. Print.

Wolfe, William L. Optics Made Clear: The Nature of Light and How We Use It. Bellingham: SPIE, 2007. Print.

Woo, Marcus. "The Weird Quantum Behavior of Light, Captured in a Lab." Wired. Condé Nast, 4 Mar. 2015. Web. 29 July 2015.