Each colour within the visible light spectrum has its own narrow band of…

The idea

Visible light is the narrow slice of the electromagnetic spectrum the eye detects, running from about 400 nm at the violet end to about 700 nm at the red end. Each colour occupies its own band of wavelengths rather than a single value, and the bands run continuously into one another, which is why a prism gives a smooth spectrum and not seven stripes. Every colour travels at the same speed in a vacuum, about 3 × 10⁸ m/s, so wave speed = frequency × wavelength fixes the frequency band once the wavelength band is known: red has the longest wavelength and therefore the lowest frequency, violet the shortest wavelength and the highest frequency. White light is the whole set of bands arriving together.

What earns the marks

• Stating that each colour occupies a narrow band of wavelengths rather than one single wavelength.
• Ordering the colours by wavelength, red longest through to violet shortest.
• Stating that frequency increases as wavelength decreases because every colour travels at the same speed in a vacuum.
• Using wave speed = frequency × wavelength with the wavelength converted to metres.

Common mistakes and how to put them right

• Giving each colour one exact wavelength, so that 700 nm is treated as the only red.

• Claiming red light travels more slowly than violet light in a vacuum.

• Substituting a wavelength in nanometres straight into v = f λ without converting to metres.

• Reversing the order and saying red light has the highest frequency because it looks the brightest.

Exam tips

• Quote the visible range as roughly 400 nm to 700 nm and name which end is which colour.
• Convert nanometres to metres before any calculation: 500 nm is 5 × 10⁻⁷ m.
• If a question compares two colours in a vacuum, say explicitly that the speed is the same for both.