The photoelectric effect

In the 1800s it was found that when a beam of ultraviolet light is held to a zinc plate on an electroscope, it would cause the electroscope to slowly charge up.  

• The electrons in the metal plate are dislodged by the ultraviolet light and leave the plate, so the plate becomes positively charged.

• This phenomenon is called photoemission.

• The photoelectric effect is the interaction between an atomic particle (electron) and an electromagnetic wave (light). The full understanding of this led Einstein and Planck to the revolutionary quantum physics.

• The photoelectric effect is the emission of electrons by a metallic element when illuminated by electromagnetic radiation. It cannot be fully explained by classical physics. 

• The photoelectric effect occurs when light shining onto a piece of metal causes electrons to be given off. For most metals, ultraviolet light is needed.

See also:

• The Nobel Prize in Physics 1921 - Albert Einstein

• The Nobel Prize in Physics 1923 - Robert A. Millikan

Photoelectric Experiments

When the photoelectric effect was studied in detail, the experimental results were quite different from what was expected. [Pay attention to these differences!]

What was expected (predictions from light as wave):  

1. A brighter light would cause electrons of greater kinetic energy to be emitted.

2. If a very dim light was used, it would take some time before any electrons received enough energy to escape from the metal.

3. The frequency (or colour) of light would not affect the energy of the emitted electrons.

These discrepancies led to the Particle Model for light.

• Albert Einstein was able to explain the photoelectric effect using an idea from Max Planck, a German physicist. 

• Planck had earlier explained the radiation of hot glowing object by proposing that Electromagnetic radiation comes in a fixed 'packets' or quanta of energy, called photons.

Homework: complete the reading and questions on SciPad

• Photoelectric Effect (p.56-76)