SAT Physics Subject Test

Chapter 16 Modern Physics


Light and other electromagnetic waves exhibit wave-like characteristics through interference and diffraction. However, as we saw in the photoelectric effect, light also behaves as if its energy were granular, composed of particles. This is wave–particle dualityElectromagnetic radiation propagates like a wave but exchanges energy like a particle.

Since an electromagnetic wave can behave like a particle, can a particle of matter behave like a wave? In 1923, the French physicist Louis de Broglie proposed that the answer is yes. His conjecture, which has since been supported by experiment, is that a particle of mass m and speed v—and thus with linear momentum p = mv—has an associated wavelength, which is called its de Broglie wavelength.

λ = 

Particles in motion can display wave characteristics, and behave as if they had a wavelength λ = h/p.

Since the value of h is so small, ordinary macroscopic objects do not display wave-like behavior. For example, a baseball (mass = 0.15 kg) thrown at a speed of 40 m/s has a de Broglie wavelength of

This is much too small to measure. However, with subatomic particles, the wave nature is clearly evident.

Questions 6-7

  6. Name or describe an experiment that demonstrates that light behaves like a wave.

  7. Name or describe an experiment that demonstrates that light behaves like a particle.

Here’s How to Crack It

 6.     Young’s double-slit interference experiment shows that light behaves like a wave. Interference is a characteristic of waves, not of particles.

 7.     The photoelectric effect shows that light behaves like a particle, where the energy of the light is absorbed as photons: individual “particles” of light energy.