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Light and the Quantum Theory

In the 1700's there were two theories on the nature of light. One theory, proposed by Sir Isaac Newton, suggested that light consisted of a stream of tiny particles called corpuscles. The other theory, proposed by Christiaan Huygens, explained that light consisted of waves. Who was right? Well most of the evidence pointed to waves, but there was more to the story.

By the 1800's most scientists accepted the wave theory of light because of the work done by Thomas Young and James Clerk Maxwell. Both men were able to show that light diffracts, and Maxwell developed four famous formulas that explained the behavior of light as a wave. Thus the wave theory became the predominate theory.

However, in the early 1900's the work of Max Planck, Arthur Compton, and Albert Einstein showed that light has particle properties as well. Today we accept a wave-particle duality concept of light.

Light as a Wave

When you light a fire in a fireplace you see and feel electromagnetic radiation. The fire gives off light (visible radiation) and heat (thermal radiation). Both types of radiation exist in the form of electromagnetic waves and each has energy particles. Therefore, we must understand particle and wave properties.

Phenomena such as colors in soap bubbles, oil film and the rainbow are best explained when light is considered to be a wave. So, let's look at the components of a wave.

An electromagnetic wave is composed of an electric wave and a magnetic wave traveling together at right angles to each other. The distance between adjacent maxima of the electromagnetic wave is the wavelength (λ) and half the distance from a maximum to a minimum is the amplitude. The number of cycles (maximum) that pass through a point in a given amount of time is known as the frequency (ν) and a hertz (Hz) is defined as a cycle per second.

The Spectrum

Unlike other types of waves, electromagnetic waves do not require a medium to travel and electromagnetic waves can move through a vacuum at 3.00 x 108 m/s, "the speed of light". It is known that the speed of any wave is the product of the wavelength and frequency and since the speed of light is constant, the wavelength (λ) and frequency (ν) must be inversely proportional.

  • the shorter the wavelength the higher the frequency
  • the longer the wavelength the lower the frequency

The range of electromagnetic radiation wavelengths is called the electromagnetic spectrum and ranges from very short wavelengths (cosmic rays) to very long wavelengths (thermal waves). Visible electromagnetic radiation (white light) is a very small region of the electromagnetic spectrum that spans from 750 nm to about 350 nm.

Visible light can be broken into its component wavelengths by passing it through a prism. The prism bends the light (refraction) as the light passes through and produces a complete array of colors (wavelengths) called a continuous spectrum.

All light can be separated into component wavelengths using a spectrometer, but not all radiation create a continuous spectrum. Many types of radiation have certain wavelengths missing and create a line spectrum. A line spectrum is a spectrum with bright lines appearing at certain wavelengths only.

Light as a Particle

Though the wave theory of light seemed to answer many of the questions concerning light, there were certain phenomena that could not be explained by this idea. Phenomena such as the photoelectric effect and the Compton Effect pointed to the possibility of light being a particle.

Then in 1900, a German physicist, Max Planck, suggested that light was not a continuous flow of energy, but consisted of small packets of energy (quanta) that are used in whole number amounts (quantized).

Planck developed an equation to support his quantum theory using data he collected from studying the frequency and energy of various wavelengths. By comparing the frequencies and energies of the wavelengths, Planck not only realized they were directly proportional but he was able to calculate the value of the proportionality constant (Planck's constant).

Planck's theory was not well accepted however, until a young Swiss patent clerk successfully used the quantum theory in his explanation of the photoelectric effect.

The Photoelectric Effect

The photoelectric effect was first described by Heinrich Hertz in 1887.  Hertz noticed that when certain short wavelengths of light were shone onto a shiny metal surface in a vacuum tube electrons were emitted.  It was also noted that the intensity of the light didn’t matter as long as the energy was high enough (short wavelength).

In 1905, Albert Einstein used the quantum theory to help explain the photoelectric effect and show that electromagnetic radiation has particle properties as well. Starting with his own equation E = mc2 and then substituting Planck's equation in for energy (E), Einstein was able to show that a quantum of energy has mass. In fact the higher the energy the greater the mass and the more it was like a particle. Therefore, Einstein called the quantum a photon.

Wave-Particle Duality

Quanta, now called photons, give light its particle properties. The photon is a specific "hunk" of energy directly proportional to its frequency, inversely proportional to its wavelength and can only be absorbed or released in whole integer amounts (quantized). When the energy is high the wavelength is short and the photon acts as a particle, but when the energy is low the wavelengths are long and the photon acts as a wave.

Therefore, light has a wave - particle duality.