Accelerating charges emit electromagnetic waves. We have seen that a changing electric field generates a magnetic field and a changing magnetic field generates an electric field. This is the principle behind the propagation of electromagnetic waves, because electromagnetic waves, unlike sound waves, do not need a medium to travel through. EM waves propagate when an electric field oscillating in one plane produces a magnetic field oscillating in a plane at right angles to it, which produces an oscillating electric field, and so on. The propagation of electromagnetic waves can be described as mutual induction.
These mutually regenerating fields travel through empty space at a constant speed of 3×108m·s13×108m·s1, represented by cc.
Although an electromagnetic wave can travel through empty space, it can also travel through a medium (such as water and air). When an electromagnetic wave travels through a medium, it always travels slower than it would through empty space.
Since an electromagnetic wave is still a wave, the following equation still applies:
Except that we can replace vv with cc (if we're dealing with an electromagnetic wave travelling through empty space):
Calculate the frequency of an electromagnetic wave with a wavelength of 4,2×1074,2×107 m
 Step 1. Decide which formula to use and solve the problem :
We use the formula: c=fλc=fλ to calculate frequency. The speed of light is a constant 3×1083×108m/s.
c
=
f
λ
3
×
10
8
=
f
×
4
,
2
×
10

7
f
=
7
,
14
×
10
14
Hz
c
=
f
λ
3
×
10
8
=
f
×
4
,
2
×
10

7
f
=
7
,
14
×
10
14
Hz
(3)
An electromagnetic wave has a wavelength of 200 nm 200 nm . What is the frequency of the radiation?
 Step 1. To calculate the frequency we need to identify the
wavelength and the velocity of the radiation. :
Recall that all radiation travels at the speed of light (cc) in vacuum.
Since the question does not specify through what type of material the wave
is traveling, one can assume that it is traveling through a vacuum.
We can identify two properties of the radiation  wavelength(200 nm )wavelength(200 nm ) and speed (cc).
 Step 2. We can use the equation c=fλc=fλ to find the frequency since the wavelength is given. :
c
=
f
λ
3
×
10
8
=
f
×
200
×
10

9
f
=
1
.
5
×
10
15
Hz
c
=
f
λ
3
×
10
8
=
f
×
200
×
10

9
f
=
1
.
5
×
10
15
Hz
(4)