There are many millions of colours which we can distinguish with our eyes. Each colour of light we see has a different wavelength (and frequency). The longest wavelength light we can see is red, at about 700 nm (700 × 10-9 m). Any longer than this, and we cannot see it.
The shortest wavelength light we can see is violet, at about 400 nm (400 × 10-9 m). Any shorter than this, and we see nothing.
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