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Light
Terms and Formulae
Terms
Photon
- A quanta of light. Photons are particles that have no mass or charge and travel only at
speed
c
, regardless of medium or reference frame. They have an energy given by
E
=
hν
where
ν
is the frequency of the light to which they correspond, and
h
= 6.626×10
-34
J.s (Planck's
constant). We can account for the behavior of light by considering it as consisting of a very large
number of photons. In this regime the electromagnetic field appears continuous and the
granularity of the light beam is negligible.
Poynting vector
- Named after John Henry Poynting (1852-1914), this is given by:
This is the unit power per area crossing a surface with normal
. The direction of
is parallel to
the direction of the propagation of the light ray.
Spherical wave
- The linear wave described in
Waves
is not the only solution to the wave
equation. In three-dimensions plane and spherical waves can also exist. In spherical waves, the
disturbance of the medium is a function of

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