MOTION MOUNTAIN

what is light? 117
Challenge 116 e
Challenge 117 e
Challenge 118 s
F I G U R E 71 The tail of comet McNaught, photographed in Australia in 2007 (© Flagstaffotos).
Like each type of electromagnetic field, and like every kind of wave, light carriesenergy;theenergyflowTpersurfaceandtimeis
T= 1 μ 0
E×B giving an average ⟨T⟩= 1
2μ 0
E max B max . (67)
Obviously, light also has a momentumP. It is related to the energyEby
As a result, the pressurep exerted by light on a body is given by
P= E c . (68)
p= T (1+r) (69)
c
where for black bodies we have a reflectivityr=0 and for mirrorsr=1; other bodies
have values in between. What is your guess for the amount of pressure due to sunlight on
a black surface of one square metre? Is this the reason that we feel more pressure during
the day than during the night?
If lasers are not available, rather delicate equipment is needed to detect the momentum
or the radiation pressure of light. Already in 1619, Johannes Kepler had suggested
in De cometis that the tails of comets exist only because the light of the Sun hits
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net