The Wave Theory of Light: Definition & Evidence Video

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  • 0:01 What Is Light?
  • 1:08 The Wave Theory of Light
  • 1:46 Evidence
  • 3:13 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain what light is and why we thought it was nothing but a wave before the 20th century. At the end, test what you've learned with a short quiz.

What is Light?

What is light? This is a question that can be answered very simply, or be made exceedingly complicated. A simple way to answer is to say that light is a type of wave that causes objects to be visible to human eyes. The sun produces light, and that light bounces off objects and into our eyes. This makes it so that we can see things, because the brain can interpret that light and tell us what's out there. Pretty simple.

But what is light actually made of? How does it work? That deeper question is answered by physics. And it took centuries to figure out.

An important step happened in the early 20th century: we discovered that light was BOTH a particle and a wave. That might seem impossible. It might make you scratch your head, and that's certainly the way physicists felt about the idea at first. But thanks to Albert Einstein and Max Planck, we know that light can act like a particle or like a wave, depending on the circumstances. This is called wave-particle duality.

In today's lesson, we're going to talk about light behaving as a wave.

The Wave Theory of Light

The wave theory of light was the way we first understood light. The theory was spread most significantly by Robert Hooke and Christiaan Huygens in the 17th century.

They predicted that if light was a wave, we would see certain things. We would see that light could reflect off shiny surfaces, refract (or bend) when moving from one material into another, and diffract (or spread) around objects or when moving through slits. It should also be possible to see interference, where peaks or troughs of waves add up to create brighter light, and peaks and troughs cancel out to create darker areas.


All of these things were seen in formal experiments by the 19th century. But some of them are easy to see in your own home. It's obvious that light can reflect - you just have to look in a mirror. Light bounces off the mirror and goes into your eye so you can see yourself. It's also obvious that light can refract: All you have to do is put a spoon in a large glass of water and watch how the spoon appears to bend.

That happens because the light is bending as it moves between air and water. Both of these things can be seen even more clearly in a laboratory using beams of light or lasers.

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