10
2 The Nature of Light
magnetic fields are at right angles to the direction of travel. The energy of the wave
is inversely proportional to its wavelength, with longer waves having lower energies
and shorter waves having higher energies. (In terms of visible light, red light has a
longer wavelength than blue light, and so carries less energy.)
The relationship between frequency and wavelength is given by (2.1), where c is
the speed of light ν is frequency, and λ is the wavelength:
c = νλ
(2.1)
Hence as wavelength increases, frequency decreases.
Conversely, in quantum mechanics, light can be considered to be a particle, the
photon, with energy given by (2.2), where the energy is E, h is Planck’s constant,
frequency is ν, and wavelength is λ:
E = hν =
hλ
c
(2.2)
It is important to realise that light is neither a wave nor a particle; rather it is both
simultaneously, and it is only the method of observation that determines whether it
has the features of a wave or a particle. Hence when, for example, when you are
looking at light’s interaction with optical instruments, its behaviour is wavelike, but
within our cameras, it behaves like a particle.
Within the research community, there is a tendency for different groups to have
each its own idiosyncratic means to describe the light they work with. Those who
work at short wavelengths (such as X-ray astronomers) tend to characterise their
photons by their energies. (This approach is also used with cosmic rays.) Observers
in the UV to IR range tend to refer to their light by wavelength. Lastly, those in the
millimetre and radio regimes tend to describe things in terms of frequency.
The electromagnetic spectrum itself is continuous. For human convenience, we
divide it up into a number of bands, although the exact cutoff locations for the
bands are disputed. Table 2.1 shows these various bands with their typical energies,
wavelengths, and corresponding frequencies.
Table 2.1 Table showing the various bands of the electromagnetic spectrum and their approximate
wavelength, frequency, and energy
Name
λ nm
ν Hz
E eV
Radio
>10 8
3 × 10 8
10 −8
Microwave
10 8 –10 5
3 × 10 9 –10 12
10 −5 –0.01
IR
10 5 –700
3 × 10 1 2–4.3 × 10 14
0.01−2
Visible
7,000−400
4 × 10 14 –7.5 × 10 15
2−3
UV
400−1
7.5 × 10 14 –3 × 10 17
3 − 10 3
X-ray
1−0.01
3 × 10 17 –3 × 10 19
10 3 –10 5
Gamma ray
<0.01
>3 × 10 17
>10 5
2 The Nature of Light
magnetic fields are at right angles to the direction of travel. The energy of the wave
is inversely proportional to its wavelength, with longer waves having lower energies
and shorter waves having higher energies. (In terms of visible light, red light has a
longer wavelength than blue light, and so carries less energy.)
The relationship between frequency and wavelength is given by (2.1), where c is
the speed of light ν is frequency, and λ is the wavelength:
c = νλ
(2.1)
Hence as wavelength increases, frequency decreases.
Conversely, in quantum mechanics, light can be considered to be a particle, the
photon, with energy given by (2.2), where the energy is E, h is Planck’s constant,
frequency is ν, and wavelength is λ:
E = hν =
hλ
c
(2.2)
It is important to realise that light is neither a wave nor a particle; rather it is both
simultaneously, and it is only the method of observation that determines whether it
has the features of a wave or a particle. Hence when, for example, when you are
looking at light’s interaction with optical instruments, its behaviour is wavelike, but
within our cameras, it behaves like a particle.
Within the research community, there is a tendency for different groups to have
each its own idiosyncratic means to describe the light they work with. Those who
work at short wavelengths (such as X-ray astronomers) tend to characterise their
photons by their energies. (This approach is also used with cosmic rays.) Observers
in the UV to IR range tend to refer to their light by wavelength. Lastly, those in the
millimetre and radio regimes tend to describe things in terms of frequency.
The electromagnetic spectrum itself is continuous. For human convenience, we
divide it up into a number of bands, although the exact cutoff locations for the
bands are disputed. Table 2.1 shows these various bands with their typical energies,
wavelengths, and corresponding frequencies.
Table 2.1 Table showing the various bands of the electromagnetic spectrum and their approximate
wavelength, frequency, and energy
Name
λ nm
ν Hz
E eV
Radio
>10 8
3 × 10 8
10 −8
Microwave
10 8 –10 5
3 × 10 9 –10 12
10 −5 –0.01
IR
10 5 –700
3 × 10 1 2–4.3 × 10 14
0.01−2
Visible
7,000−400
4 × 10 14 –7.5 × 10 15
2−3
UV
400−1
7.5 × 10 14 –3 × 10 17
3 − 10 3
X-ray
1−0.01
3 × 10 17 –3 × 10 19
10 3 –10 5
Gamma ray
<0.01
>3 × 10 17
>10 5