what are the proofs of the particles nature of light?

Question

what are the proofs of the particles nature of light?

Answer 1

The question of whether or not light is wave like or particle like has an interesting history. Originally, Newton advocated the idea that light was composed of little particles (corpuscles). Later in the late 1700's early 1800's, experiments were preformed revealing that light has some behaviors that are associated with waves. (Light can diffract or bend around corners). In the mid 1800's, a scottish physicist named Maxwell was playing around with the four main equations describing electricity and magnetism. He realized that when he got these equations into a certain form, they looked a lot like the equation that describes the motion of a wave. He reasoned that if these equations were indeed describing some sort of electric and magnetic wave, then the speed of that wave must be around 186,000 miles per second. "What in nature goes that fast?" he must have thought. "Well, light goes that fast, so maybe light is a kind of electric and magnetic wave...". Thus the wave description of light held sway for a few decades. In the early 1900's Max Plank was trying to understand the spectrum of light given off by objects at a particular temperature. (basically, he was trying to predict what color things would glow when you got them hot) In his time, people knew from experiments what the spectrum of light coming off of an object at a specific temperature looked like, but nobody knew why it looked the way it did. Plank managed to explain the experiments by making the assumption that the energy of the vibrating atoms giving off the light could only come in discrete little units he called "quanta". Albert Einstein then used Plank's idea of quanta, to explain the observation that in the photoelectric effect that the energy of ejected electrons from a metal plate with light shinning on it depends upon the frequency of the incoming light and not on the amplitude. If light were a wave, you would expect light with a bigger amplitude (brighter light) to result in ejected electrons with bigger energies. Instead you just get more electrons with lower energies. Einstein suggested that light was composed of little particles called "photons" that interact with the electrons like little pool balls. Brighter light just means more photons and thus more ejected electrons. So light behaves like a wave in some experiments and behaves like a particle in other experiments. Both the wave description of light and the particle description of light are needed to describe all light related phenomena. Neither description alone can be used to explain all experiments. The wave picture and the particle picture are called complimentary concepts for this reason. Perhaps light is neither a wave nor a particle. Maybe these are just human ideas that are used to describe it, but together they do a good job.

Answer 2

Actually the first answer is not entirely correct. Light has momenta p = h/L; where L is wavelength and h in Planck's constant. [See source.] And, as we all know, the change in momentum dp/dt = F, which is force. So as light strikes the rotating mirror in a vacuum, the direction and, therefore, the momentum of light is changed and a force is created that thrusts the mirror around.

But note...p = h/L depends only on wavelength and that is not a quantum or particle. It is in fact just the wave model for light. So the spinning mirror does not prove the particle nature of light at all. It only proves that light has momentum...relativistic momentum because light has no mass.

I think the photoelectric cell and the so-called Compton Effect are the major proofs of the particle nature of light. We all know that certain materials (usually silicon based) create electricity when struck by light. Turns out the effect can only be explained by light particles, waves would not do. Check this out:

"The [Compton] effect is important because it demonstrates that light cannot be explained purely as a wave phenomenon. Thomson scattering, the classical theory of an electromagnetic wave scattered by charged particles, cannot explain any shift in wavelength. Light must behave as if it consists of particles in order to explain the Compton scattering. Compton's experiment convinced physicists that light can behave as a stream of particles whose energy is proportional to the frequency." [See source.]

Answer 3

A device called a radiometer which has clear glass enclosure that looks very much like a light bulb---inside is a vacuum-- there are four square vanes on side black and the other side white. When light is shined on the radiometer the vanes spin around!!!! The brighter the light the faster the vanes spin!!!!!

The vanes spin because the particles of light hit the vanes of the radiometer, causing it to spin around. It is very interesting to watch!!!!

If you want one check where they sell science supplies. I is a standard item used in high school science and physics.

Check out the following website:

http://www.en.wikipedia.org.wiki/Crookes_radiometer

Answer 4

Einstein's Photo-electric effect.

Answer 5

You can show the photo-electric effect. Try googling "photoelectric effect experiment"