What is light? How does it damage fabrics?

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The electromagnetic nature of light

So what is light?

Light is just one form of energy known as electromagnetic radiation. The same stuff as radio, TV and microwaves, just a different frequency. The light that we can see encompasses only a very small part of the entire electromagnetic radiation spectrum. This spectrum includes everything from low frequency radio waves up to extremely high frequency (and very dangerous) gamma radiation: Electromagnetic spectrum

Visible light covers a frequency range from around 5 x 10¹⁴ for red light to 7.5 x 10¹⁴ Hz for violet light. That's between one-half to three-quarters of a million billion Hz! These very high frequencies are more conveniently described in the form of wavelengths. To obtain the wavelength of a particular frequency (or color) of light, divide the speed of light (approx. 300,000,000 meters per second) by its frequency. Red light would then have a wavelength of approx. 0.0000006 meters or 600 nm (billionths of a meter) in length. Visible light has a wavelength range of approx. 400nm to 700nm.

Outside of this range are the two well-known forms of invisible "light" - infrared ("less than" red) and ultraviolet ("more than" violet). Infrared is invisible but can be felt as radiant heat. Ultraviolet light is also invisible but has much more energy than visible light - as much as seven times the energy. (The scale above is very compressed spanning from 1Hz to 1 million billion billion Hz! Each graduation is 100 times that of its neighbor.)

But why does light damage fabric?

This is where we need to take a quick look at quantum physics! Don't worry - this is obviously not the place to cover such a heavy topic in detail. All we need talk about is a little item called wave-particle duality. It has been shown that light behaves as both wave AND particle. This is difficult to visualize as nothing in our everyday lives behaves this way. We either experience waves, such as on a surface of water or on a vibrating string, or particles such as dust or salt grains etc.. We never see both properties occurring together. As a particle, light can be sub-divided down as far as a single "quanta" of light known as a photon. This is all we need to know for our purposes. These photons are the bullets we mentioned in our analogy. For a given frequency, a photon has a fixed amount of energy. So for a given color of light, there are a whole bunch of photons with the same energy.

The molecules that make up different fabrics are held together with different molecular energies. Molecular glue if you like. Delicate fabrics' molecules are held together less strongly than those of tougher fabrics. If a photon with enough energy comes along and collides with such a molecule, it can impart enough energy to disrupt the molecule, creating two separate parts that are no longer the same material. This is the process that, if repeated often enough, will cause discoloration and possibly even destruction of a delicate fabric. It is also why ultraviolet light damages our skin while visible light has no effect. The energy in visible light is not sufficient to cause molecular disruption in our skin.

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