Waves and Obstacles

You saw in the lesson on Wave Motion that light, sound, water waves and other waveforms follow the same rules and laws. This is also true for how they respond to obstacles in their path.

Examples of the effect of an obstacle on a waveform include echoes and how water waves seem to go around a breakwater. Observing similar examples can teach you about properties of waves when confronted with an obstacle.

Questions you may have about this are:

• What can happen when a wave strikes an obstacle?

• What about when it goes right through a material?

• How does the wave act near the edge of an obstacle?

When a wave meets an object

When a wave runs into an object, several things happen to the wave:

• Part of the wave passes through the object and is transmitted

• Part of the wave bounces off the object and is reflected

• Part of the wave is absorbed in the material

This is true for all waveforms, including water waves, sound, light and electromagnetic waves.

Transmission

When a wave traveling in one material meets another material, some of the wave motion may go through the other material, provided that material responds in the same way as the original. The rest of the wave will be reflected and/or absorbed.

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. 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.

Reflection

When a wave runs into an object or a different material, much of the wave is reflected. The rest of the wave is transmitted and/or absorbed.

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.

Likewise, when sound strikes a wall, some of it is reflected as an echo, some of the sound is transmitted right through the wall, and some is absorbed in the wall material.

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.

When a 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. What happens right at the edge is that some of the wave wraps around the edge. This interesting characteristic of waves is called diffraction.

Ray theory

Since waves travel in a straight line, scientists have designated them as rays. It is more convenient to simply draw a ray or straight line to indicate where a wave is going. Ray theory is used in optics to determine the path of light in lenses and other optical devices.

Unfortunately, the method of using rays falls apart when a wave is cut by an 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.

Demonstrate diffraction

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.

In conclusion

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.