Coherent & Incoherent Light: Definition & Sources

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  • 0:03 Coherent & Incoherent Light
  • 1:11 Properties of Light
  • 3:33 Lesson Summary
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Lesson Transcript
Instructor: Marc Chiacchio

Marc has taught Bachelor level students climate science and has a PhD in climate science.

In this lesson, you'll learn the difference between coherent and incoherent light and consider some familiar examples, including the light we can observe from a flashlight and a laser.

Coherent & Incoherent Light

Have you ever noticed how clearly defined the ray of light is when a cashier at the grocery store uses a scanner? The laser from the scanner produces a steady, focused stream of light. This is an example of coherent light. Its photons, or particles of light energy, possess the same frequency and its waves are in phase with one another. When you use a flashlight to light up a dark basement, on the other hand, the source of light is diffuse and does not resemble the steady stream of light produced by a laser. Here the photons do not have the same frequency and the waves are not in phase with one another. We call this a form of incoherent light.

These types of light, or electromagnetic radiation, are just one form of light that we can see with our eyes. There are also forms of light that are invisible to our eyes and possess different wavelengths, including gamma rays, x-rays, ultraviolet rays, infrared light, microwaves, and radio waves. With this in mind, we'll explore some important terms and properties of light surrounding the concepts of coherent and incoherent light.

Properties of Light

The term light as it is commonly used refers to the light we see with our eyes, but this only comprises one part of the electromagnetic spectrum. This spectrum includes many forms of light which are distinguished from one another by their wavelengths. Light is also composed of magnetic and electrical fields, hence the term ''electromagnetic.''

In addition to visible light, you're probably familiar with another part of the electromagnetic spectrum: x-rays. We usually experience x-rays at the dentist, where they can assist in the detection of cavities and gum disease. It is also at the low end of dosage we receive in our daily lives. We can refer to any of these light waves in the spectrum as electromagnetic radiation, or energy emanating from a certain point in space. And depending on their source, they can either be described as coherent or incoherent light.

These different light sources share some properties. All light waves contained in the spectrum travel at the speed of 300 million meters (186,000 miles) per second; the scientific symbol for the speed of light is c. This is true whether the waves are coherent or incoherent. Moreover, the speed of light is constant throughout the known universe, a characteristic which was first described by Albert Einstein in his Special Theory of Relativity in 1905.

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