# Doppler Shift – An Astronomy Experiment

### Purpose

To discover the Doppler Shift.

### Materials

• glass of water
• 8 ½” x 11” black construction paper
• flashlight

### Procedure

Place the flashlight on one side of the glass of water and place the black construction paper on the opposite side. Have students make a prediction about what colour will appear on the backside of the construction paper. Then, turn the flashlight on and look at the backside of the construction paper.

### Prediction (check one)                  Result

blue __________                                          blue __________
orange __________                                    orange __________
red __________                                            red __________
green __________                                       green __________
yellow __________                                     yellow __________
white __________                                       white __________

### Teacher Information

Red will appear on the back side of the black construction paper because red has the longest wavelength. Look at a rainbow: The top colour, and thus the longest colour, is red.

Astronomers compute mathematically how fast stars are moving away from us based upon their red shift (the light moving toward the red of the spectrum).

If stars are moving toward us, their frequency increases and the light shifts to the blue spectrum.

The same is true for sound waves. It is called the Doppler Shift, after Christian Andreas Doppler. Doppler observed that as a train is approaching us and we are stationary, the sound of the train gets louder as the frequency of the waves increases. Once the train goes by, the sound gets gradually softer because the wave length frequency is more spread out.

This information is recommended for use with the Ontario Curriculum, Grade 6: Understanding Earth and Space Systems  and Grade 9: Astronomy.

Stan Taylor is a retired elementary school teacher. He currently does science workshops for Scientists in School.

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