two clean shaven men passing by each other

Surely it would be one blue-stubbled man being left trailing by a red-hot shaving kit?

We’re back to maths school again!

http:// http://www.ncsa.uiuc.edu/Cyberia/Bima/doppler.html

http:// http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Doppler.html