1. What is visible light?
Visible light is the small portion of the electromagnetic spectrum that the human eye can detect. Even though it occupies a tiny part of the full spectrum, it contains all the colours we see in everyday life—from red to violet.
Visible light behaves just like any other electromagnetic wave, with electric and magnetic fields oscillating perpendicular to each other and to the direction of travel.
1.1. Why only this region is visible
The eye’s photoreceptor cells respond only to wavelengths between roughly 400–700 nanometers. Waves outside this range (infrared, ultraviolet, etc.) cannot trigger the photoreceptors.
1.2. Position in the spectrum
Visible light lies between infrared (longer wavelengths) and ultraviolet (shorter wavelengths). As wavelength decreases within visible light, colour shifts gradually from red to violet.
2. Wavelength and frequency range
Visible light covers a specific band of wavelengths and frequencies. Each colour corresponds to a particular wavelength range.
2.1. Typical wavelength range
- Red: ~700 nm
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet: ~400 nm
2.2. Frequency values
The frequency range is roughly:
- \(4 \times 10^{14} \, \text{Hz}\) (red)
- to
- \(7.5 \times 10^{14} \, \text{Hz}\) (violet)
3. Colours of the visible spectrum
Visible light can be split into its colours using a prism or a diffraction grating. When white light passes through a prism, each wavelength bends differently, forming a spectrum of colours commonly remembered by the acronym ROYGBIV.
3.1. Order of colours
- Red
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet
3.2. Why dispersion happens
Different wavelengths travel at slightly different speeds in glass. Shorter wavelengths (violet) slow down more and bend more, while longer wavelengths (red) bend less. This spreading of colours is called dispersion.
4. Interaction of visible light with matter
Visible light interacts with objects in various ways—absorbed, reflected, refracted, or scattered. These interactions determine the colour and appearance of objects.
4.1. Reflection
Objects appear coloured because they reflect certain wavelengths and absorb others. A red apple reflects red light and absorbs most of the other colours.
4.2. Absorption
If an object absorbs most wavelengths of visible light, it appears dark or black to the eye.
4.3. Refraction
Light bends when it passes from one medium to another because its speed changes. This bending produces effects like the apparent bending of a straw in water.
4.4. Scattering
The sky appears blue because shorter wavelengths (blue and violet) scatter more strongly in the atmosphere than longer wavelengths (red and yellow). Blue dominates because our eyes are more sensitive to it.
5. Sources of visible light
Visible light comes from both natural and artificial sources. Different sources produce different intensities and colours of light.
5.1. Natural sources
- The Sun (main source)
- Fire
- Lightning
5.2. Artificial sources
- LED bulbs
- Incandescent bulbs
- Lasers
- Fluorescent lamps
6. Uses of visible light
Visible light is essential not only for sight but also for many technologies and scientific applications.
6.1. Everyday uses
- Seeing objects and colours
- Photography and filming
- Lighting homes and workplaces
6.2. Scientific and technological uses
- Optical microscopes
- Fiber-optic communication
- Laser applications (cutting, scanning, measurement)
7. Example: Rainbow formation
A rainbow forms when sunlight enters water droplets in the atmosphere. The light slows down and bends (refraction), splits into colours (dispersion), reflects inside the droplet, and exits toward the observer. Each droplet sends out just one colour in a specific direction, and together many droplets create a multicoloured arc.