Photons are the elemental units of electromagnetic spectrum. For example, white light is composed of a number of different kinds of photons, some are red photons, some are blue photons, etc. Note that, there is no white photon; instead the combination of the various colored photons when they interact with your eyes, your brain perceives what we call white light.
Photons are not limited to visible light alone. There are also radio frequency photons, infrared photons, microwave photons, and other kinds of photons that our eyes cannot detect.
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Properties of Photons
Photons have no rest mass, but they do carry momentum (energy).
A photon has a distant wavelike character within is electromagnetic bundle.
The wavelength of a photon (horizontal distance between consecutive electrical or magnetic field peaks) depends on the medium in which it travels and on the source that produced it. It is this wavelength that determines the colour of a photon.
The photon’s frequency is related to its wavelength by:
Where v, is the speed of the photon, in free space v is equal to the speed of light given by c = 3.0 x 108 m/s but in other media such as glass, v becomes smaller than the speed of light.
A photon with a large wavelength (or small frequency is less energetic than a photon with a shorter wavelength (or a high frequency). The energy of a photon is equal to E = hf, where h is plank’s constant 6.63 x 10-34 j.s
A photon can be generated by acceleration/decelerating a charged particle e.g. an electron that is made to vibrate back and forth within an antenna will produce radio frequency photons that have very long wavelengths (low energies) when compared with light photons. Visible light is produced when outer shell electrons within atoms are forced to make transition between energy levels, accelerating in the process.
Other frequency photons maybe created by vibrating or rotating molecules very quickly while others, especially those with very high energy e.g. Gamma rays can be created by the charge accelerators within atomic nuclei.
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Electromagnetic Spectrum
The electromagnetic spectrum includes wavelength of electromagnetic radiation varying from short wavelength (high frequency) Gamma rays to long wavelengths (low-frequency) Radio waves.
The figure below shows the breakdown of the electromagnetic spectrum.
Table: Photons physical origin and artificial generation
| Type of Photon | Physical Origin | Artificial generation |
| Gamma rays | Atomic nuclei | Acceleration |
| X-rays | -Inner electrons -Outer electrons | X-ray tubes |
| Ultraviolet | Lasers, X-rays | |
| Light | Outer electrons | Arcs, Lasers, X-rays |
| Infrared | Molecular vibrations and rotations | Sparks, arcs, lamps, lasers, X-rays, Hot bodies |
| Microwaves | -Electron spin -Nuclear spin | |
| Radio frequency | Electron circuits |
Radio Frequency Photons
They extend from a few hertz to about 109 Hz (wavelengths from kilometres to about 0.3 m). They are often generated by alternating currents within powerlines and electric circuits such as radio and television transmitters.
Microwave Photons
They extend from 109 Hz to 3 x 1011 Hz (wavelengths from 30 cm to 1 mm). These photons can penetrate the earth’s atmosphere and hence they are used in space vehicle communication, radio astronomy, and transmitting telephone conversations to satellites. Microwave photons are also used for cooking food. Microwaves are often produced by atomic transitions and by electron and nuclear spins.
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Infrared Photons
These photons extend from about 3 x 1011 to 4 x 1014 Hz. Infrared radiation is created by molecular oscillations and is commonly emitted from incandescent sources such as electric heaters, glowing coals, the sun, human bodies (which radiate photons in the range of 3000 to 10,100 nm), and special types of semiconductors.
Light Photons
They comprise a narrow frequency band from about 3.84 x 1014 to about 7.69 x 1014 Hz and are generally produced by rearrangement of outer electrons in atoms and molecules e.g. in the filament of an incandescent light bulb, electrons are randomly accelerated by applied voltages and undergo frequent collision. These collisions result in a wide range of electron acceleration and as a result a broad frequency spectrum (within the light band) results giving rise to white light.
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Ultraviolet photons
Ultraviolet photons extend from approximately 8 x 1014 to 3.4 x 1016 and are produced when an electron in atom makes a long jump down from a highly excited state. Unfortunately, the frequency of ultraviolet photons tends to react badly with the human cell DNA, which in turn can lead to skin cancer. The sun produces a large output of ultraviolet radiation. The fortunate thing about us is the protective ozone. The molecules in the upper atmosphere can absorb most of this ultraviolet radiation by converting the photons energy into a vibrating motion within the ozone molecules.
X-rays
These are highly energetic photons that extend from about 2.4 x 1016 to 5 x 1019 Hz, making their wavelengths often smaller than the diameter of an atom. You can produce x-rays by rapidly decelerating a high speed charged particle. X-rays can be used in x-ray imagery.
Gamma Rays
These are the most energetic of the photons; whose frequency begins around 5 x 1019Hz. These photons are produced by particles undergoing transitions within the atomic nuclei. The wavelike properties of Gamma rays are extremely difficult to observe.
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