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# Special Theory of Relativity

## Interpret motion between different objects that are moving at constant speeds relative to each other.

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Photoelectric Effect

### Photoelectric Effect

Credit: Laura Guerin
Source: CK-12 Foundation

Discovered by Heinrich Hertz in 1887, electrons can be ejected from a metal surface when struck with light. This result demonstrated that one of the properties of light is that it is quantized.

#### News You Can Use

• The photoelectric effect is investigated by shining light on one of two electrodes that are placed in a vacuum tube. Electrons are ejected from the electrode that the light is incident upon. These electrons with either speed towards the opposite electrode or be repelled by it. The attraction or repulsion between the electron and the opposite electrode is dependent upon the potential difference between the two electrodes.
• When this system is used to study the number and kinetic energies of the electrons ejected, two interesting things are discovered:
1. Increasing the intensity of the incident light only increases the number of ejected electrons, not the kinetic energy of the ejected electrons.
2. If the frequency of the incident light is reduced, there is a point at which no electrons are ejected. This is independent of the intensity used.
• The above two discoveries lead scientist to the conclusion that light is quantized.

#### Explore More

Using the information provided above, answer the following questions.

1. What role does the work function play in determining if an electron will get ejected from a metallic surface?
2. If the energy of the photons is less than the work function of a metal, will electrons be ejected?
3. What is the stopping potential?

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