Light, the Quantum Concept, and the Photoelectric Effect

Pretend that someone placed a lamp with a bare light-bulb in front of you. After doing so, they asked you to hold your hands out in front of you and walk closer and closer toward the light-bulb. What sort of observation could you make about what you were feeling on your hands? If you have ever gotten close to a fire, you know that the closer you are to something bright, the warmer the temperature becomes. The same is true with the light-bulb. That is because light transfers energy as electromagnetic radiation. Different wavelengths of light transmit different levels of energy. These different wavelengths and their associated radiation level can be found on the electromagnetic spectrum.

Because of its unique duality, the study of light has provided insight into atoms and the behavior of electrons. View this presentation to learn more about light, the parts of a wave, and the electromagnetic spectrum. Before you begin, take a look at the world around you and consider how light impacts what you see in the world around you. Then, click the player button to view the presentation.

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Why is the study of light and its behavior important in the study of chemistry? Every element emits light when heated by the passage of electricity through its gas or vapor. The atoms first absorb energy and then release it in the form of light, but the light emitted is of a specific wavelength(s), unlike the continuous spectrum of white light. Thus, each wavelength corresponds to a specific amount of energy being emitted. The spectrum of such a light is called an emission spectrum of the element and is unique to each element.

What is a Photon?

A light particle is called a photon. You can think of a photon as a tiny bundle of electromagnetic energy. The laser in this picture is emitting a tremendous number of photons each second. Photons are particles, but they also have some wave properties. Just like waves have frequency, each photon also has a frequency. The frequency of the photon is directly proportional to its energy.

Max Planck and Planck's Constant

The behavior of light cannot fully be explained by its wave nature alone. Founding the important field of quantum physics, Max Planck proposed that energy was emitted and absorbed in discrete units, or quanta. In this interactivity, click on the next button of each screen to proceed through the information to learn about Max Planck and Planck's constant.

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Albert Einstein and the Photoelectric Effect

Through extensive testing, Albert Einstein discredited the notion of light as simply a wave. He determined that a metal emits an electron when it absorbs a photon In this interactivity, click on each bar in the accordion to learn more about Albert Einstein and the photoelectric effect.

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Millikan's Disagreement

Robert Millikan did not believe Einstein’s work that asserted that the photon theory of light was correct, because there was so much evidence to support the wave theory of light. Millikan spent ten years working on an experiment to test the theory. His results confirmed all of Einstein’s predictions. The intensity was not the most important property of the light, the frequency was. The frequency has to be a certain minimum value for the electrons to be ejected. The frequency had to be increased to increase the kinetic energy of the electrons. Below the minimum threshold frequency, no matter how long you wait, no electrons will be emitted. These experimental observations did not support the classical wave theory at all. They completely supported Einstein’s photon theory.

Light, the Quantum Concept, and the Photoelectric Effect Review

Now that you have explored light, the Quantum Concept, and the Photoelectric Effect, review your knowledge in this non-graded activity. In this interactivity, answer all of the questions by following the directions on each question slide. Then, click SUBMIT to check your responses. Click on the interactivity thumbnail, and then click NEXT to get started.