What is Light?

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  • 0:02 What is Light?
  • 0:45 Key Concepts of Light
  • 2:22 Key Properties of Light
  • 4:40 Dividing Up Light
  • 5:32 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will define and describe what light is from both technical and more common-use perspectives, discuss what the different kinds of light are, and explain why it's called radiation.

What is Light?

It can be both visible and invisible. It can be short and long. It is a piece of information and radiation at the same time. It can be dangerous, but it does not have to be.

It is not the most interesting man in the world. It is electromagnetic radiation, a kind of energy known as light. But visible light, the stuff you see, is only one form of electromagnetic radiation. There's also gamma rays, X-rays, ultraviolet light, infrared, microwaves, and radio waves. We'll explore some of the important concepts related to light in this lesson

Key Concepts of Light

Let's begin this lesson by discussing some important concepts related to light. As per the intro, light technically refers to all forms of electromagnetic radiation. However, do keep in mind that the word light is often used colloquially to refer to the electromagnetic radiation we can see with our eyes, that is to say, visible light. Other than visible light, all other forms of electromagnetic radiation are invisible to the unaided human eye.

Also, when I say radiation, as in electromagnetic radiation, I don't want you to freak out or anything. In the context of this lesson, radiation refers to something radiating out, or spreading outwards from a certain point. This is what light does. That's why it's a form of radiation. Sure, X-rays can be dangerous in high quantities, but visible light, not so much. In any case, both are forms of radiation and they are electromagnetic because the energy is conveyed in the form of changing electric and magnetic fields.

This electromagnetic radiation spreads out through space at a set speed. That speed, the speed of light, symbolized by a lowercase c, is 300 million meters per second (m/s). All forms of electromagnetic radiation travel at this speed. At such a speed, electromagnetic radiation can circle the equator 7.5 times in one second.

Properties of Light

Electromagnetic radiation has two key properties. It has wavelike and particle-like properties. When you see a rainbow, you see visible light behaving as a wave, a wave that carries energy.

Like visible light, all forms of electromagnetic radiation have something known as a wavelength, the distance between two successive peaks of a wave. It's typically represented by the letter 'lambda' and measured in meters (m).

If you're unsure of what I mean by a wavelength, imagine throwing a rock into a pond and watching the ripples spread across the water. If you could pause such a scene and use a ruler to measure the distance between the peaks of the wave, that would be the wavelength.

Wavelength is inversely related to the frequency of a wave, the number of cycles of a wave that passes specific points in 1 second. It is usually represented by the letter 'nu' and measured in cycles per second, or Hertz (Hz). The higher the frequency of a wave, the shorter the wavelength, and vice versa.

In addition to behaving like a wave, electromagnetic radiation can also behave like particles. This is the property of light that cameras depend on to take a picture. A particle of electromagnetic radiation, sort of like a small packet of waves, is known as a photon and sometimes called a light quantum.

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