Photoelectric Effect


 In 1887, the German physicist Heinrich Rudolf Hertz discovered an interesting property of matter. This property is that physical materials emit charged particles when they absorb radiant energy (eg, light). Of course, not all substances absorb radiant energy, and the ones that don't will not emit charged particles. But it could readily be established that some substances do behave this way.

Hertz initially observed that the minimum voltage required to draw sparks from a pair of metallic electrodes was reduced when they were bathed in ultaviolet (UV) light, such as from a mercury vapor lamp. The more intense the UV light, the lower the required voltage became.

In the broadest sense, the substance in question can be solid, liquid, or gaseous; the radiant energy must be visible light, UV; X-rays; or gamma rays (cosmic radiation). The charged particles can be electrons or ions. However, in general practical use, the substance is a metal plate and the charged particles are electrons.

In any case, a second German physicist, Philipp Lenard, studied this phenomenon using a metal plate, and in 1900 concluded that the charged particles emitted were the same as those found in cathode rays. That is, they were electrons. By 1902, it had been shown that the resulting current (called photoelectric current because it was caused by light) is proportional to the intensity of the light causing it for any given frequency of light energy, and that the maximum kinetic energy imparted to any electron is independent of the intensity of the light, but is directly proportional to the frequency of the light.

In 1905, Albert Einstein worked out the primary equation involved, and by 1912, the requisite measurements could be made with high precision. These experiments confirmed the conclusion that light is not a continuously wave-like phenomenon, but rather involves particle-like "corpuscles" of energy, now called photons, which are the quanta of electromagnetic energy.

Background provide by 

Graphic provided by 

 absolute astronomy

It is a great site for information on quantum mechanics

    The incoming monochromic light (light composed of only one frequency) strikes the cathode and liberates the electron.  The move toward the anode Not because its attracted to it, but because it has kinetic energy and momentum. The Cathode/Anode (which creates a voltage between them) does negative work against the electron.  If it is enough work the electron is repeled.  If it isn't enough work then the electrons hit the anode and sent through a circuit and though a ammeter which measures the photoelectric current.  

    The object of this experiment is to find the minimum voltage need to repel the electron that voltage is called the stoping voltage. That voltage time the charge of the electron is equal to the kinetic energy of the electron. 

 1. Light behaves two ways.  

   The first way is as a wave.  As a wave, light oscillates back and forth, it can move around barriers, and more importantly it could interfere with other light waves (if two pieces of light are in the same space, the amplitude of those two pieces of light are added together) which could make that point brighter or darker (or no light at all) 

        The second way is as a particle.  Particles a hard things that bounce around an collide with each other. Particles don't loop around buildings, they don't cancel each other out.  As a matter of fact if two particles try to occupy the same space the collide and either stick together or bounce off each other.

        Because light can take the property of a wave sometimes (more to the point all the time) and a particle sometimes (once again all the time), it said that light has a wave particle duality, and that it can infer with its self, deliver its energy in an all or nothing fashion like a wave, and slam in electron pushing them off a metal surface like a particle

2. It all or nothing, electrons bound to the surface of a metal need whats called a threshold frequency (or sometime a threshold wavelength) in order to liberated from the surface of the metal.  It would be more appropriate call it a threshold energy because it energy (or work if you will) that forces the electron off the surface. This threshold frequency is also called the work function. The relationship between the frequency and work function is 

               F = hf     where 

                                  F is the work function 

                                   f is the threshold frequency 

 3. The Photoelectric current is effected by the intensity of the light (hopeful this makes sense, more light coming in more electrons come off the surface).  But the kinetic energy each electron has is only effected by the wavelength of the light coming in. 

 4. When a voltage is applied so when the electrons moves from one side to the other the voltage does negative work equal to the amount of kinetic energy it is called the stopping voltage.  It called a stopping voltage because it actually repel the electron not allowing it to get to the other side.