The title of this Student Text is “Are you coming or going?”

The Austrian scientist Christian Doppler discovered the Doppler Effect in the nineteenth century. Many type0s of waves, including sound waves, light waves or even waves on water undergo the Doppler Effect. You may have experienced the Doppler effect if you have ever listened carefully to the sound of a siren or horn of an automobile as it approached and then moved away from you. You probably noticed that the pitch of the sound increased as the source of the sound approached you and then decreased as the source moved away.

This is a description of the figure on Contact Card One. Have your card available so you can follow the description.

In Contact Card One, use the heavy solid horizontal line as a base line. There is a dotted line to track. You will find that it forms a smooth curve rising into a crest and then down into a trough and then starts up again to repeat the pattern. One crest and one trough is one wave, so this curve represents three complete waves.

The waves shown in this figure are shaped like the water waves that you can feel when you are in a bathtub or swimming pool. If you are very still in the water, the level of the water remains constant on your body. The heavy solid horizontal line on Contact Card One represents this level. But when water is disturbed, the water waves travel in crests and troughs, so you feel the water level moving up and down. When the water waves crest, the water level comes up higher on your body than it usually does. At a trough, the water is lower than its normal level.

Keep Contact Card One available so that you can compare it with the next contact cards.

This is a description of the figure on Contact Card Two. Place this card to the left of Card One as you follow the description.

Again you will feel a solid horizontal base line. Track the curved dotted line on Contact Card Two. These waves represent compressed sound waves. The wave crests are more closely spaced than the crests on Contact Card One, so there are five complete waves shown on this card. This means that these waves have a higher frequency than those on Card One, because there are more waves in a given distance. If these were sound waves, these waves would have a higher sound than those with a frequency like those on Contact Card One.

This is a description of the figure on Contact Card Three. Place this card to the right of Card One as you follow the description.

Again you will feel a solid horizontal base line. Track the curved dotted line on Contact Card Three. These waves represent rarefied sound waves. The wave crests are farther apart than the crests on Contact Card One. There are only two complete waves on this card. These waves have a lower frequency than the waves on Contact Card One. The sound of these waves would have a lower sound than those have spacing like those on Contact Card One.

As a sound source, such as a siren, approaches, the waves ahead of the source get shoved together like those on Contact Card Two. The decrease in space between the wave crests increases frequency and the sound you hear has a higher pitch. As the sound source moves away from you, the waves grow farther apart, like those on Contact Card Three, and sound drops in pitch.

The same thing happens when you move quickly toward a stationary sound source. Because of your motion, the frequency with which the sound waves strike your eardrums would increase and you would hear a higher pitch sound. If you were moving away from the sound source, the pitch would lower.

These changes in wave frequency and pitch sound are called Doppler Shifts.