The Basics Facts about Photons

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.

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

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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