Theory of Light

The visible spectrum is a very small part of the electromagnetic spectrum with the limits extending from 380 to 780 nanometres (nm). A nanometre is equal to one billionth of a metre (1x10^-9) or one millionth of a milimetre. This range represent the extreme limits of the visible light spectrum, 380nm being near Ultraviolet (UV) and 780nm being near Infrared (IR). Typically, the range of human vision is from 380nm to 760nm, with peak sensitivity at 555nm (Green), although this range will differ between people, and therefore an approximation only.

To provide some comparison, light at a wavelength of 780nm, is very dim and barely perceivable. The infrared light emitting diodes (LED) in a TV remote control is typically 940nm, well into the infrared spectrum. The laser diode in a CD player emits light at a wavelength of 780nm, and classified as infrared. At the opposite end, Ultraviolet light produced by the common blacklight tube (as found at parties etc) produce a peak emission at around 371nm.

The electromagnetic spectrum is the range (or gamut) of all possible frequencies of electromagnetic radiation, including, radio waves ( AM, FM, SW ), microwaves, infrared, visible light, ultraviolet, X-Rays, and Gamma Rays.

As we move through the spectrum, increasing frequency (and inversely decreasing wavelength), the electromagnetic radiation becomes more energetic. From ultraviolet frequencies through to gamma rays are forms of ionising radiation. Gamma Rays have the shortest wavelength of all of the waves, therefore are the most energetic and have the abillity to penetrate through the smallest of gaps including subatomic sizes.

White Light as produced by sources such as the sun and incandescent lamps is composed of all of the wavelengths in the visible light spectrum. These colours are known as the spectral colours.

Although the visible spectrum has been divided into defined colour names, the spectrum is continuous and there are no clear and distinct boundaries between colours.

The table below provides approximate wavelength bands of the visible spectrum, and associated frequencies. Visible light has frequencies in the terahertz (THz) region (1x10¹²).

Colour	Wavelength Band	Frequency Band
Red	780 - 620 nm	384 - 483 THz
Orange	620 - 597 nm	483 - 502 THz
Yellow	597 - 577 nm	502 - 520 THz
Green	577 - 492 nm	520 - 609 THz
Blue	492 - 455 nm	609 - 659 THz
Violet	455 - 380 nm	659 - 789 THz

The speed of light in a is a universal constant, about 300,000 km/s or 186,282 miles per second. The exact speed of light in a vacuum is: 299,792,458 m/s.

It takes approximately 8.3 min for light from the sun the reach the earth ( 150,000,000 / 300,000 / 60 = 8.3 ) Taking the distance of the sun from Earth into account, which is 150,000,000 km, and the fact that light travels at 300,000 km/s, it shows in someway how fast light actually travels.

With the use of the SI units for wavelength (λ), frequency (v) in Hz and speed of light (c), we can derive some simple equations relating to wavelength, frequency and speed of light:

v = c / λ
or inversely,
λ = c / v

In the next section, we will cover the brief history of light and the wave-particle duality.

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