The Photoelectric Effect

By: Megan, Mark & Andrew

The Photoelectric Effect

The Photoelectric Effect was in fact discovered by accident while Heinrich Hertz (1857-1894) was trying to prove Maxwell’s theories of Electromagnetism.

Heinrich had built an electric circuit with an oscillating current causing sparks to go between the gap of the electrodes as we see here

He discovered that the electromagnetic energy was being transmitted across the room, by placing a small coil or wire with a tiny gap when the transmitter made sparks he seen the sparks were also forming in the gap of the receiver coil on the other side of the room.

He proved Maxwell’s theories of Electromagnetism; Meaning that waves travel at the speed of light and can be reflected and refracted.

Hertz also noticed that sparks increased when the metal electrodes were exposed to ultraviolet light… which at his time was too difficult to explain.

10 years later… Joseph John Thomson discovered the

electron, meaning that physicists could explain Hertz’s observations. They suggested that ultraviolet light caused electrons to be ejected from metal electrodes creating a conducting path for the sparks to follow.

This process was later named THE PHOTOELECTRIC EFFECT!

The Photoelectric Effect…

Is the ejection of electrons by ultraviolet light

1902 Philipp Lenard wanted to see whether or not photoelectrons were actually ejected from the “emitter”.

Lenard used this apparatus for his experiment

Lenard’s experiment

Electrodes are sealed in an evacuated glass tube with a quartz window so that ultra violet light cannot penetrate it. A sensitive galvanometer detects any current passing through the circuit; the variable power supply that is connected can be made so that either electrode is positive or negative. When the emitter was exposed to ultraviolet light, they were attracted to the collector and then passed through the circuit therefore he discovered what was recently hypothesized was in fact true.

Lenard continued with his experiments and also discovered

these things:

That as the potential difference was increased the current gradually decreased until it finally stopped flowing entirely

Lenard also realized that ultraviolet light with constant intensity ejected electrons with most energy but that there was always a maximum kinetic energy.

Important term…

Stopping Potential: the potential difference that stopped all

photoelectrons

Lenard also used a prism for another experiment to direct different

frequencies of light onto an emitter… basically he observed the following

things:

The intensity of light striking the emitter increases as the number of electrons ejected increases

The maximum kinetic energy of electrons ejected from the metal emitter is determined by only the frequency of light and the intensity doesn’t affect it.

The Photoelectric effect in Action!

Solar panels

Automatic doors…

Homework Questions

Who discovered the photoelectric effect?

What is the photoelectric effect? Why is a glass tube used? What is the stopping potential What is the name given to the

cathode? The anode?