here - Department of Physics, HKU
here - Department of Physics, HKU
here - Department of Physics, HKU
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
CHAPTER 1. PRE-SPECIAL RELATIVITY PHYSICS 6<br />
glass<br />
l<br />
01<br />
01<br />
01<br />
01<br />
01<br />
01<br />
01<br />
01<br />
toothed wheel<br />
Figure 1.3: Measure the speed <strong>of</strong> light.<br />
using conservation <strong>of</strong> momentum and energy in S. Hence, energy is also<br />
conserved in S ′ .<br />
1.3 Light<br />
Light is usually understood in terms <strong>of</strong> Maxwell’s electromagnetic theory.<br />
It is a kind <strong>of</strong> wave, the oscillations <strong>of</strong> electric and magnetic fields. More<br />
precisely, light is electromagnetic waves within certain range <strong>of</strong> frequencies.<br />
Other electromagnetic waves include radio waves, X-rays, etc. They have all<br />
the properties <strong>of</strong> waves, like diffraction and interference.<br />
One fundamental property <strong>of</strong> light is its speed, c. We found by experiments<br />
that all electromagnetic waves have the same speed. One <strong>of</strong> the first<br />
measurements <strong>of</strong> speed <strong>of</strong> light was done by Armand Fizeau in 1849. In<br />
Fig. 1.3, light is entering from the left, passes through the teeth <strong>of</strong> the wheel<br />
and travels a long distance l to a mirror and is reflected back. If the wheel is<br />
turning slowly, the reflected light will hit the wheel and it will be dark if we<br />
look up from below the glass. Only when the wheel is turning in the correct<br />
speed, we can see the reflected light. If the wheel has n teeth and is turning<br />
r turns per second, then the time taken for the wheel to turn for one tooth<br />
is equal to that <strong>of</strong> the round trip <strong>of</strong> the light,<br />
1<br />
nr = 2l<br />
c , (1.17)<br />
or c = 2nrl. For l = 8km, the wheel has to turn hundreds <strong>of</strong> times per second<br />
for us to see the reflected light.<br />
Nowadays, the speed <strong>of</strong> light is a defined value, c = 2.99792458 ×10 8 m/s.<br />
We define the length <strong>of</strong> one meter by this value and the definition <strong>of</strong> time<br />
(which is defined by the properties <strong>of</strong> some atom). Gravitational wave is<br />
believed to have the same speed. It is, in fact, the ultimate speed <strong>of</strong> nature.<br />
Only because we discover it as speed <strong>of</strong> light that we call it the speed <strong>of</strong> light.
Change language