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Waves and Obstacles

by Ron Kurtus (revised 5 January 2008)

When a wave meets an obstacle, part of the wave is reflected off the surface of the material, some is transmitted through the material and some is absorbed.

For the part of the wave that is transmitted, the direction of its angle can change if the speed of the wave is different in the material. This is called refraction. Also, the part of the wave that hits the edge of the obstacle is bent due to an effect called diffraction. These reactions of the wave to an obstacle occur in sound waves, light waves, water waves and other types of wave motion.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion



Useful tools: Units Conversion.

Wave reflects off object

When a wave strikes an object, part or even all of the wave will be reflected off the surface of the material. For example, objects you see are a result of light from some source bouncing off or being reflected from the surface of the object. Likewise, echoes are sounds that reflect off a wall at some distance from you.

Wave reflecting off smooth flat surface

Wave reflecting off smooth flat surface

A smooth flat surface will reflect the wave and not change its shape, while a irregular or rough object can scatter the waves.

Wave passes into object

Some of the wave energy is not reflected upon hitting an object. It is transmitted into and even through the object. During that transmission, some of that wave energy can be absorbed within the material.

Transmission

When a wave strikes an object, some of the wave energy may go through the other material, depending on the wavelength and characteristics of the material.

For example, since sound is a vibration in air, it will also pass through most other materials by causing the same type of vibrations in them. Some of the sound from music in another room will pass through a wall. Low frequency notes usually travel through the wall material easier than high frequency notes.

Radio and television electromagnetic waves pass through the walls in your house to reach the device's antenna. Light passes through some materials such as glass, but it is not transmitted in many other materials.

Velocity changes in material

The velocity of a wave in another material is usually different than the original.

Sound usually travels faster in denser material. Sound in water is faster than in air. On the other hand, the speed of light is slower in denser materials. The speed of light in glass is slower than in air.

Refraction

When a wave is transmitted through a material on an angle, its direction is changed. You can see that when light goes through water at an angle. That phenomenon is called refraction.

Light wave refracting in water

Light wave refracting in water

Absorption

Some materials absorb better than others. Sound absorbing materials will reflect very little sound and will transmit even less. Most of the sound energy is absorbed in the material.

Likewise, some materials absorb light waves better than others. Dark materials that don't transmit light are the best for absorbing the light energy. For example, when light strikes a colored piece of glass, some of it is reflected, some of the light passes through the glass, and some of the light is absorbed in the colored material.

Wave meets an edge

When a wave meets the edge of an object, you would think some would go straight ahead, while the blocked portion would be reflected or absorbed. But what happens is that right at the edge of the object the wave starts to wrap around the edge. This interesting characteristic of waves is called diffraction.

Waves diffracting around edge

Waves diffracting around edge

You can see this with a lake or ocean breakwater, where the water waves tend to curve around the edge of the breakwater. It isn't much, but it is noticeable. You can demonstrate this by making waves in a calm pool and interrupting half of the waves with an obstacle. You can easily see the waves slightly curve around the edge of the obstacle.

Diffraction is one reason you can hear around corners, although reflections also play a major role in that.

Summary

When a wave meets an obstacle, some of the wave may be transmitted through the material, some is reflected, and some of the wave energy may be absorbed by the material. When a wave meets the edge of an obstacle, the wave is often diffracted and bends slightly around the edge of the material.


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