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Einstein Year - a year celebrating physics - Photoelectric Effect
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did you know... Photoelectric Effect

Photoelectric Effect

In 20th century physics two ideas stand out as being totally revolutionary: relativity and quantum theory. Although Einstein is best known for his theory of relativity, he also played a major role in developing quantum theory. And it was his contribution to quantum theory - explaining the photoelectric effect - which won Einstein his Nobel Prize in 1921.

The photoelectric effect is the name given to the observation that when light is shone onto a piece of metal, a small current flows through the metal. The light is giving its energy to electrons in the atoms of the metal and allowing them to move around, producing the current. However, not all colours of light affect metals in this way. No matter how bright a red light you have, it will not produce a current in a metal, but even a very dim blue light will result in a current flowing. The problem was that these results can't be explained if light is thought of as a wave. Waves can have any amount of energy you want - big waves have a lot of energy, small waves have very little. And if light is a wave, then the brightness of the light affects the amount of energy - the brighter the light, the bigger the wave, the more energy it has. The different colours of light are defined by the amount of energy they have. If all else is equal, blue light has more energy than red light with yellow light somewhere in between. But this means that if light is a wave, a dim blue light would have the same amount of energy as a very bright red light. And if this is the case, then why won't a bright red light produce a current in a piece of metal as well as a dim blue light? Einstein realised that the only way to explain the photoelectric effect was to say that instead of being a wave, as was generally accepted, light was actually made up of lots of small packets of energy called photons that behaved like particles. Einstein wasn't the first person to use the idea of photons, but he was the first to make it the starting point of an explanation rather than a convenient fiddle to explain away odd results.

With light as photons, Einstein showed that red light can't dislodge electrons because its individual photons don't have enough energy - the impacts are just not large enough to shift the electrons. However, blue light can dislodge electrons - each individual photon has more energy than the red photon. And photons of ultraviolet light, which have yet more energy, will give electrons enough energy to whizz away from the metal altogether. A good way to think of the photoelectric effect is like a full car park with lots of really bad drivers. There is a car parked in a space, and lots of other drivers want that space. To get it they can try knocking the parked car out of the way, but they can only manage to hit it one car at a time. A tiny red mini just won't have the energy to knock the parked car out of the parking space, but a big blue van will. And imagine hitting the parked car with a big ultraviolet lorry - the parked car is most likely going to move far enough to collide with something else. Returning to light and electrons, there is never really just one photon of light at a time. A bright light emits lots of photons, but it doesn't matter how bright a red light gets; red photons will still not be able to budge a single electron. This is like having a car park full of red minis each randomly hitting a parked car in turn - there will be a lot of dents but the parked car will remain where it is. However, even a dim blue light will shift some electrons - we know that even one blue van will be able to move the parked car.

Einstein's explanation of the photoelectric effect was just the start of an avalanche of discoveries that became quantum theory. In this theory, light is not just a particle and not just a wave: it can be one or the other, depending on how it is measured. And it was discovered later that even the electrons are not just particles but are waves too.

Photoelectric effect links

Physics 2000

Explaining quantum physics and how we can see Einstein's work in many household items. This site is for everyone.

The Particle Adventure

A wonderful site for kids - answering questions like "What is the world made of?" and "How do we know any of this?".


Play Fermilab's particle games and find out how to smash particles apart.

Photoelectric Effect

For more on the photoelectric effect have a look here: or here:

Photoelectric effect

Why was Einstein's work such a breakthrough? This site gives a great account of the problems with the photoelectric effect before Einstein's work.

If you've got questions about the photoelectric effect, this is the website for you. Just enter your level of knowledge, then type in your question and will find the best information on the internet for you.