... Editor's note: This activity is both on the Astrocappella web site and the CD-ROM featured in this article. ... Every student can demonstrate the Doppler effect! ... splash out' ball** . ... Twist the 'splash out' ball open. ... This effect can occur with both sound and light, because both sound and light reveal wave- like behavior. ... The number of waves reaching an observer in one second is called the frequency. ... What has the Doppler shift told astronomers about the expansion of the universe? ...