Visible Light Spectrum

Visible Light Spectrum Basics

The visible light spectrum is the part of the electromagnetic spectrum that human eyes can detect. It consists of light with wavelengths approximately between 400 nm (nanometres) and 700 nm.

The colours of visible light, in order of increasing wavelength, are:

• Violet
• Indigo
• Blue
• Green
• Yellow
• Orange
• Red

This sequence is often remembered by the mnemonic ROYGBIV.

White light, such as sunlight, is a mixture of all these colours combined. When white light passes through a prism or water droplets, it splits into its component colours, showing the visible spectrum.

For instance, if you calculate the frequency of red light with wavelength 700 nm, you get approximately 4.29 × 10¹⁴ Hz, which lies at the lower frequency end of the visible spectrum.

Properties of Visible Light

Visible light is a transverse wave. This means the oscillations of the electric and magnetic fields are perpendicular to the direction the wave travels.

In air (or vacuum), visible light travels at approximately 3.0 × 10⁸ m/s, which is the speed of light.

When visible light encounters different materials, it can be:

• Reflected – bouncing off surfaces like mirrors
• Refracted – bending as it passes through materials like glass or water
• Absorbed – taken in by materials, often converting to heat
• Transmitted – passing through transparent materials

These interactions explain many everyday phenomena such as seeing colours, rainbows, and shadows.

Colour and Wavelength

Each colour in the visible spectrum corresponds to a specific range of wavelengths:

• Violet: ~400–450 nm
• Indigo: ~445–450 nm
• Blue: ~450–495 nm
• Green: ~495–570 nm
• Yellow: ~570–590 nm
• Orange: ~590–620 nm
• Red: ~620–700 nm

The frequency $f$ of visible light relates to its wavelength $\lambda$ by the equation:

$$f = \frac{c}{\lambda}$$

where $c = 3.0 \times 10^8 \text{ m/s}$ is the speed of light.

Visible light frequencies range approximately from $4.3 \times 10^{14} \text{ Hz}$ (red) to $7.5 \times 10^{14} \text{ Hz}$ (violet).

Light can also be described as photons, particles of light, each carrying energy $E$ given by:

$$E = hf$$

where $h = 6.63 \times 10^{-34} \text{ Js}$ is Planck’s constant.

Higher frequency (shorter wavelength) light has more energetic photons. For example, violet light photons have more energy than red light photons.

For instance, the energy of a red photon with wavelength 700 nm is:

$$f = \frac{3.0 \times 10^8}{700 \times 10^{-9}} = 4.29 \times 10^{14} \text{ Hz}$$

$$E = 6.63 \times 10^{-34} \times 4.29 \times 10^{14} = 2.84 \times 10^{-19} \text{ J}$$

Applications and Effects

Visible light is essential in optics and imaging. Cameras, microscopes, and the human eye all rely on visible light to form images.

Our perception of colour depends on how objects absorb and reflect different wavelengths of visible light. For example, a red apple appears red because it reflects red light and absorbs other colours.

Prisms demonstrate the dispersion of white light into its component colours. This happens because different wavelengths refract by different amounts when passing through glass, spreading out the colours.

Rainbows are natural examples of dispersion, where sunlight is refracted and reflected inside water droplets, separating the light into the visible spectrum.

For example, when white light passes through a prism, violet light bends more than red light because violet has a shorter wavelength and is refracted more strongly.