Wave Interactions

Waves interact with matter in several ways. The interactions occur when waves pass from one medium to another. The types of interactions are reflection, refraction, and diffraction.

Comprehension Check: Copy this table into your notes. Leave room for a diagram and examples. Fill it in as we go.

Description Diagram Examples

Reflection

Refraction

Diffraction

Reflection:

An echo is an example of wave reflection. Reflection occurs when waves bounce back from a boundary that separates two different media. Reflection can happen with any type of waves, not just sound waves. For example, light waves can also be reflected. In fact, that’s how we see most objects. Light from a light source, such as the sun or a light bulb, shines on the object and some of the light is reflected. When the reflected light enters our eyes, we can see the object.

Comprehension Check: Play with a slinky and demonstrate reflection.

Reflected waves have the same speed and frequency as the original waves before they were reflected because they do not go into the medium. However, the direction of the reflected waves is different. When waves strike an object head on, the reflected waves bounce straight back in the direction they came from. When waves strike an obstacle at any other angle, they bounce back at the same angle, but in a different direction. The original angle is the angle of incidence. The new angle is the angle of reflection. Both angles are measured relative to a line that is perpendicular to the wall. This line is called the normal line.

Comprehension Check:

If a wave traveling at 20 m/s hits a mirror at 30o, at what angle and speed will it be

reflected?

Refraction:

Refraction is another way that waves interact with matter. Refraction occurs when waves bend as they enter a new medium at an angle. Light bends when it passes from air to water or from water to air. The bending of light traveling from the fish to the man’s eyes causes the fish to appear to be in a different place from where it actually is.

Comprehension check:

Where would the fish appear to be if the man looked down

at it from straight above its actual location?

Waves bend as they enter a new medium because they start traveling at a different speed in the new medium. For example, light travels more slowly in water than in air. This causes it to refract when it passes from air to water or from water to air.

Think of riding a skateboard along the sidewalk. If one of the wheels veers into the grass growing along the side, the wheel turns slower and causes the skateboard to change coarse (and the rider to fall off!)

Comprehension Check:

Would a sound wave moving through atmospheric air go faster or slower

when it passes through helium? Why?

Would the light waves in the photo above change speed as they go

through the glass?

Comprehension Check:Using the evidence from the picture above, infer which wavelengths of visible light are refracted the most – long wavelengths or short wavelengths.

Explain Why.

Magic with Reflection

and Refraction

Diffraction:

Did you ever notice that you can hear sounds around the corners of buildings even though you can’t see around them? That’s because sound waves spread out and travel around obstacles. This is called diffraction.

All waves may be diffracted, but it is more pronounced in some types of waves than others.

Comprehension Check:

Would a sound wave also travel THROUGH the

building?

Diffraction also occurs when waves pass through an opening in an obstacle. For a given type of waves, such as sound waves, how much the waves diffract depends on the opening in the obstacle and the wavelength of the waves.

There is less diffraction when the length of the opening is greater than the wavelength. There is more diffraction when the length of the opening is less than the wavelength.

Comprehension Help:

Go to Interactive Waves Lesson and play with the modules.

Doppler Effect

Has this ever happened to you? You hear a siren from a few blocks away. The source is a police car that is racing in your direction. As the car approaches, zooms past you, and then speeds off into the distance, the sound of its siren keeps changing in pitch. First the siren gets higher in pitch, and then it suddenly gets lower. Do you know why this happens?

The answer is the Doppler Effect.

The Doppler Effect is a change in the frequency of sound waves that occurs when the source of the sound waves is moving relative to a stationary listener. (It can also occur when the sound source is stationary and the listener is moving.)

The sound waves from the police car siren travel outward in all directions. Because the car is racing forward, the sound waves get bunched up in front of the car and spread out behind it. As they propagate outward they retain their circular shape. Sound waves that are close together have a higher frequency and sound waves that are farther apart have a lower frequency.

Comprehension Check:

The changed pitch of the Doppler effect is due to a change in…

Wave speedWave frequencyWave amplitude

Wave sound

Doppler Effect

Explained

Doppler, Shock Wave, Sonic Boom

What is redshift?