Light
The electromagnetic spectrum: where visible light comes from
The light an LED screen emits is a form of electromagnetic radiation, just like radio waves or X-rays. Yet our eyes see only a wafer-thin slice of that whole spectrum. Here you will discover which slice, and why.
Everything we call ‘light’ is a kind of wave: a vibration that travels through space. There are many kinds of such waves. We use some to listen to the radio, others to heat food in a microwave oven, and still others let a doctor take a picture of your bones.
Our eyes can see only a very small group of those waves. We call that visible light. Every colour you know, from red to violet, is in that small group. The rest of the waves are still there, but we simply cannot see them.
An LED screen makes exactly those visible waves, in the right colours, so that you see a picture.
Light is electromagnetic radiation: a wave of an electric and a magnetic field travelling together through space. The full electromagnetic spectrum orders all that radiation from long to short wavelength. In the 17th century, Isaac Newton used a prism to show that white light splits into the separate colours of the visible spectrum.
From radio waves to gamma rays
Ordered from long to short wavelength, the regions follow in turn: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Within that range, visible light is only a narrow band between roughly 380 and 700 nanometres (a nanometre = a billionth of a metre). Below and above it the radiation simply continues, but our eyes do not respond to it.
Why this matters for a screen
An LED converts electrical energy into radiation with a specific wavelength, and therefore a specific colour. By combining red, green and blue LEDs, every colour the screen shows is created. Infrared (heat) and ultraviolet fall outside what we see; a well-designed LED puts as much of its energy as possible into the visible band.
Electromagnetic radiation propagates through a vacuum at the speed of light c ≈ 3 × 10⁸ m/s. Wavelength (λ) and frequency (ν) are related by c = λ·ν, and the energy per photon by E = h·ν (with h the Planck constant). The shorter the wavelength, the higher the frequency and the photon energy.
The visible band in context
The visible band (≈ 380–700 nm) is, on a logarithmic view, minuscule compared with the full spectrum, which ranges from kilometre-long radio waves to picometre-short gamma rays.
| Region | Wavelength (order of magnitude) |
|---|---|
| Radio waves | > 1 m |
| Microwaves | 1 mm – 1 m |
| Infrared | 700 nm – 1 mm |
| Visible light | 380 – 700 nm |
| Ultraviolet | 10 – 380 nm |
| X-rays | 0.01 – 10 nm |
| Gamma rays | < 0.01 nm |
Narrow-band emission of LEDs
Unlike an incandescent lamp, which emits a broad continuous spectrum and loses much of its energy as infrared (heat), an LED emits narrow-band light around a dominant wavelength (spectral full width at half maximum typically a few tens of nm). That makes LED colours saturated and energy-efficient: nearly all the emitted energy falls within the visible band. White LEDs achieve white by combining a blue chip with a yellow phosphor, or by mixing red, green and blue chips together.