To discover the Doppler Shift.
• glass of water
• 8 ½” x 11” black construction paper
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 __________
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.
Resource: http://www.astro.ucla.edu/~wright/doppler.htm .
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.