Visible Rays

What Makes Visible Light Special

Visible light is the only part of the electromagnetic spectrum that the human eye can detect. It is the most familiar form of EM waves because it directly carries information about the world around us — the colours, shapes, and brightness we see come from visible light emitted or reflected by objects.

Frequency and Wavelength Range

Visible light occupies a narrow band within the EM spectrum. Its boundaries are defined by two equivalent ranges:

• Frequency range: from about $4 \times 10^{14} \, \text{Hz}$ to about $7 \times 10^{14} \, \text{Hz}$.
• Wavelength range: from about $700 \, \text{nm}$ to $400 \, \text{nm}$.

The relationship between frequency ($f$) and wavelength ($\lambda$) for any EM wave is:

where $c = 3 \times 10^8 \, \text{m/s}$ is the speed of light in vacuum. This means the product of frequency and wavelength is constant for all EM waves in vacuum.

Why These Numbers Matter

• The lower end ($4 \times 10^{14} \, \text{Hz}$ or $700 \, \text{nm}$) corresponds to red light.
• The upper end ($7 \times 10^{14} \, \text{Hz}$ or $400 \, \text{nm}$) corresponds to violet light.
• All other colours (orange, yellow, green, blue, indigo) lie between these extremes.

Biological Variation

Different animals have different "visible" ranges. For example: …