Einstein’s photon theory of light brought up a question that most physicists believed had been answered in the debates between Christian Huygens and Isaac Newton… is light a particle or a wave? In Physics 20 you discussed light as a wave… always. But now serious questions had been raised as to whether or not this was true.
Before we decide if light is a wave or particle, let’s examine the properties of the photoelectric effect based on each.
To keep one variable a constant, we will assume that we are testing monochromatic light (one color, one frequency).
· A higher intensity of light (like a brighter bulb) means a higher amplitude of wave
· A higher amplitude means that the wave has more energy.
· We would expect that if light is a wave, as the intensity of the light increases more electrons with higher Ek should be ejected.
· The frequency should not affect photoelectric effect, since it has nothing to do with the energy of the wave.
Example: You’re at the water park at West Edmonton Mall. The wave machine is going so that it is making small waves (small amplitude). You could stand up to your waist in the water all day without getting moved, because the waves just don’t have enough energy to move you. Even if they increase the frequency of the little waves (so that more hit you every second), the waves are still too small to push you around. It’s only when they increase the amplitude (the size of the waves) that you can get pushed off your feet.
You represent the surface electrons on the metal, and the water waves are light waves.
We will still assume that we are dealing with monochromatic light, but now we assume light is a particle (the photons in Einstein’s theory).
· Since E = h¦, monochromatic light will be made up of photons that all have the same energy.
· This is the energy that could eject electrons and give the Ek.
· The energy of the light will only increase if the frequency of the light increased.
· Increasing the intensity of the light only causes more photons of the same energy to hit the surface, so more electrons would drop off, but they all have the same maximum kinetic energy.
Example: You are so bored studying physics that you decide to go practice your forehand in tennis. You go to a court that has one of those automatic ball machines that shoots balls at you. Unfortunately the wiring is all fried and it starts shooting balls at you! Getting hit with one ball every ten seconds isn’t too bad, but then it cranks up the frequency and hits you with ten balls every ten seconds… ouch! That hurts! So you run off the court. Even though each ball was still being shot at you with the same velocity (intensity), they were hitting you a lot more frequently.
You represent the surface electrons on the metal, and the tennis balls are photons of light.
Well, you’ve already seen that Einstein’s use of Planck’s ideas to explain the photoelectric effect depend heavily on the frequency of light, but is there any other evidence that supports the particle theory of light?
Several years earlier Philipp Lenard had discovered that the intensity of light didn’t change the kinetic energy of emitted electrons. Although Lenard did win the Nobel Prize for Physics in 1905, it wasn’t for this discovery, but instead for his work with cathode rays.
Millikan performed further experiments with the photoelectric effect in 1913-14 and found that at any frequency less than the threshold frequency (fo) no electrons are emitted, no matter how great the intensity of the light. An increase in the intensity of the light only means more electrons are emitted, but since the energy of each photon hasn’t changed, the maximum Ek of each electron stays the same.
So, now we have evidence that light behaves as a particle, at least when it comes to explaining the photoelectric effect. But we still have to recognize that light has some properties (diffraction, interference) that are better explained using a wave model… so both models are right! The current model of light is usually referred to as “wave-particle duality”. We now recognize that light is both a particle and a wave. This means that sometimes a wave acts like a particle!