MOTION MOUNTAIN

98 3 what is light?
spark transmitter
receiver
F I G U R E 54 The simplest radio transmitter possible, a gas lighter and a wire, together with the simplest
radio receiver possible, built from a battery pack, a light emitting diode, and a simple coherer made
from a ball pen housing, two screws and some metal powder (© Guido Pegna).
follows as the product of the two.
If you are not convinced, you can measure the speed directly, by telephoning a friend
on another continent, if you can make sure of using a satellite line (choose a low cost
provider). There is about half a second additional delay between the end of a sentence
and the answer of the friend, compared with normal conversation. In this half second,
the signal goes up to the geostationary satellite, down again and returns the same way.
This half second gives a speed ofc≈4⋅36000 km/0.5 s≈3⋅10 5 km/s, which is close to
the precise value. Radio amateurs who reflect their signals from the Moon can perform
a similar experiment and achieve higher precision.
In summary, electromagnetic waves exist and move with the speed of light.
Light as a wave
The electromagnetic wave equation has more interesting stories to tell. Above all, the
wave equation confirmed earlier predictions thatlight itself is an electromagnetic wave,
albeit with a much higher frequency and much shorter wavelength than radio waves.
We check this in two steps: we first show that light is a wave and then show that it is
electromagnetic.
The first to suggest that light is a (kind of) wave was, around the year 1678, the important
physicist Christiaan Huygens.* You can confirm that light is a wave with your
own fingers. Simply place your hand one or two centimetres in front of your eye, look
towards the sky through the gap between the middle and the index finger and let the two
fingers almost touch. You will see a number of dark lines crossing the gap.These lines are
the interference pattern formed by the light behind the slit created by the fingers. Figure
55 showsan example. Interference is thename given to the effect and the amplitude
* Christiaan Huygens (b. 1629 ’sGravenhage, d.1695Hofwyck) was one of the main physicists and mathematiciansofhistime.Huygensclarifiedtheconceptsofmechanics;healsowasoneofthefirsttoshowthat
lightisawave.Hewrote influential booksonprobability theory, clockmechanisms,optics andastronomy.
Amongotherachievements,HuygensshowedthattheOrionNebulaconsistsofstars,discoveredTitan,the
moon of Saturn, and showed that the rings of Saturn consist of rock. (This is in contrast to Saturn itself,
whosedensityislower than that ofwater.)
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net