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Diffraction: Relation to Sound & Light and Effects of Wavelength

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  • 0:06 Introduction to Diffraction
  • 1:32 Diffraction of Sound
  • 2:20 Effects of a Wavelength
  • 4:12 Diffraction of Light
  • 5:44 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

This lesson explores diffraction as one of the many behaviors of waves. Learn how diffraction occurs in sound and light waves and how it is affected by the wavelength of a wave. Find out how animals use diffraction to communicate and how scientists use it to study molecules.

Diffraction is a Unique Wave Behavior

Types of wave behaviors
Wave Behaviors

Learning about waves always requires a thorough understanding of wave behaviors. By behaviors, I mean all the interesting things that waves can do when they interact with media. Waves can travel through objects, reflect off surfaces, resonate with atomic particles, and bend from one medium to the next. Reflection, refraction, and other wave behaviors explain a lot of the mysteries behind how we perceive everyday waves like sound and light. But, there are other, less familiar wave behaviors that we should learn about.

Diffraction is one of those less-obvious wave behaviors that play a big role in our perception of waves. Diffraction describes the change in a wave's direction as it travels between or around barriers. It's similar to reflection and refraction in that it involves a change in the direction of waves when they encounter a change in medium. Reflection describes how waves bounce off surfaces. Refraction describes how waves bend as they pass through the boundary between two different media.

Diffraction refers to the change in wave direction as it travels between or around barriers.
Diffraction

Diffraction is different. In diffraction, waves actually bend around objects that they encounter in their path or bend through openings in between two barriers. You may have seen diffraction occur when water waves travel through a gap in a wall or a jetty. The waves bend outward from the opening in the wall and fan outward from the gap. To see how diffraction really works, we'll first take a look at sound waves.

Diffraction of Sound

It's easy to imagine sound waves bending around obstacles. Have you ever tried to speak with someone who's standing in an adjacent room? Even if that person isn't in your line of sight, they can usually hear you speaking at a reasonable volume. That's because your sound waves bend around the edges of walls and doorways until they travel toward that person. The same thing happens when that person speaks back to you.

Diffraction of sound waves is one big reason that animals can communicate over long distances. Think about the places that most animals live. Forests, mountains, prairies, and swamps all have tons of vegetation and land features that block visual communication. Animals can still get in touch with each other because their vocalizations get past all that. Their sound waves bend around those obstacles and travel toward their intended audience.

Effects of Wavelength

Some animals are better at long-distance communication than others. Elephants, for example, can communicate over miles of land in order to keep their herds together while they're traveling. People haven't always known about elephant communication, because they vocalize at such a low frequency that we can't even hear it. Elephants use infrasound, or sound waves with frequencies of less than 20 Hz. These low frequency, long-wavelength sounds actually diffract around objects to a higher degree than other, higher-pitched sounds. In fact, the amount of diffraction that occurs in any wave is dependent upon the wavelength of that wave.

Let's think for a minute about why this might be true. In order for a wave to bend around an obstacle, the wavelength of the wave must be larger than that obstacle. The same is true for waves traveling through an opening. The wavelength must be larger than the opening if it is to pass through the opening and come out on the other side. For any given obstacle or opening, waves with longer wavelengths bend more than waves with shorter wavelengths. If the wavelength is smaller than the obstacle or opening, then diffraction barely happens at all.

Some animals don't want diffraction to happen to their sound waves. Bats, for example, need their sound waves to come back to them so that they can tell where they are in the dark. They use echolocation to navigate and to find the flying insects that they eat. For this reason, bats need to make sound waves with very small wavelengths, or very high frequencies. These sounds are too high-pitched for humans to hear, and they're called ultrasound. They tend to have frequencies of more than 20,000 Hz, and so they have wavelengths of such a small size that they don't diffract around objects like trees and flying insects. Instead of diffraction, bat sounds are reflected back to the bats' ears, so essentially they can see in the dark.

Diffraction of Light

People have known about diffraction in sound waves for a long time, but what about light waves? Scientists used to think that light wasn't capable of diffraction at all. They could hear sound waves bending around obstacles, but they couldn't see light waves doing the same thing. It turns out that because light waves have such tiny wavelengths, they can only diffract when they pass around obstacles or openings that are less than 1,000 nm wide. This is because wavelengths of visible light are between 380 and 760 nm. So, we don't see diffraction of light waves nearly as often as we do sound waves.

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