Wave-Particle Duality A
Up to now, we have been treating light as if if were a wave. And in fact, that's OK, because light does act like a wave. (Whether "acting like a wave" means it IS a wave is a philosophical question.) What are all the ways that light acts like a wave?
- Light exhibits the phenomenon of interference, and revealed by thin slits and diffraction gratings. Sound waves and water waves do the same thing.
- Refraction, the bending of a wave when it goes into a medium where its speed is different, happens to water waves, sound, and light.
- Light exhibits polarization, which not only demonstrates that it's a wave, but specifically a transverse wave.
Before 1800, there was much debate about the nature of light. Is light made of particles shooting through space, or is it some kind of a wave, akin to sound or water waves on the ocean. Some people (most notably Isaac Newton) thought it was most likely a particle. Others, such a Christian Huygens (discoverer of the equation for centripetal force and inventor of the pendulum clock, among many other accomplishments) thought that light acts was a wave. Once Thomas Young observed diffraction and interference by sending light through two adjecent slits (in 1801) then the issue was resolved. Light was a wave, not a particle.
Two developments at the end of the nineteenth century further reinforced the view of light as a wave. James Clerk Maxwell, in the early 1870's, packaged together the laws of electricity and magnetism that had been previously developed by Faraday and Gauss and others, and made one crucial addition. He also used these equations to show that light consists of waves traveling through electric and magnetic fields. Heinrich Hertz, in 1887, first created artificial radio waves, and Guglielmo Marconi around the turn of the century developed the first practical long-distance radio communication equipment.
But wait! Physicists around this same time were doing experiments exploring the nature of atoms and electrons. In particular, the photoelectric effect. When light is shined on a metal surface, it can liberate electrons. The details of these experiments could only be explained by thining of light as a particle. The energy of a wave is governed by the amplitude of the wave (actually, the square of the amplitude). So, if light is a wave, having a low brightness light should eject no electrons, but once bright enough, electrons would be liberated. Turning up the brightness even more should cause electrons to be ejected more and more energetically. But this isn't what happens. Instead, there is a color cutoff. As long as the color is blue enough (in wave terms, short enough wavelength) electrons will be ejected. Even if the brightness of the light is very low. And there is no time delay, which would be expected if light were a wave. If the color of the light is made even bluer, the ejected electrons come out with higher energy.
So, these results point toward a particle model of light. We call light particles photons, and the energy of photons is greater the bluer the light. This can be summarized by the following equation:
where h is a constant that is determined experimentally to be 6.626x10-34 J/Hz = 4.14x10-15 eV/Hz.
Take a moment to really look at this equation. Notice that even though we are saying this is the energy of a photon, a particle of light, right there isn the equation is f, the frequency, a wave concept. This equation, seemingly a simple equation, encapsulates the necessity to think of light as both a wave and a particle simultaneously.
Activities & Practice
to do as you read
Play with this PhET simulation of Radio Waves and Electromagnetic Fields.
Play with the Photoelectric Effect.
Practice Questions
1. The ionization energy of hydrogen --- that is, the energy needed to strip its electron completely away --- is 13.6 eV. What frequency photon would just barely ionize hydrogen? What wavelength does that correspond to? What part of the electromagnetic spectrum is that?