Spectrum — Part 1: Dispersion and the electromagnetic spectrum: visual lesson, ICSE Class 10 Physics (Science Paper 1)

Lesson

Where do the colours come from?

Part 1 — sunlight looks white, yet a glass prism, a cut-glass chandelier or a soap bubble shows it is a mixture of colours. And visible light is only a tiny part of a much bigger family of waves.

In this lesson you will

Explain why a prism splits white light and why violet deviates most and red least
List the colours of the visible spectrum in order of wavelength
Arrange the regions of the electromagnetic spectrum in order of wavelength
State properties common to all electromagnetic waves and use c = f λ

Table of four observations and what each shows about light and radiation
You see	What it shows
Colours from a chandelier drop in sunlight	White light is a mixture of colours
A prism spreads a beam into a band	Each colour bends by a different amount
A TV remote works without visible light	There is radiation our eyes cannot see
Doctors photograph bones	Some radiation passes through flesh

Diagram

Dispersion of white light by a prism

When a narrow beam of white light passes through a glass prism, it splits into a band of colours called a spectrum: V I B G Y O R — violet, indigo, blue, green, yellow, orange, red. This splitting is dispersion. Red is deviated least (it appears nearest the apex side) and violet is deviated most (nearest the base). A prism does not create the colours; it only separates colours already present in white light.

Concept

Why the colours separate

In vacuum (or air) all colours travel at the same speed, $3 \times 10^{8}$ m s⁻¹. In glass each colour has a different speed, so the refractive index of glass depends on the colour (wavelength): it is largest for violet (shortest wavelength, slowest) and smallest for red (longest wavelength, fastest). A larger refractive index means more bending, so violet deviates most and red least.

Table of wavelength, refractive index of crown glass and deviation for four colours
Colour	Wavelength in air (about)	Refractive index of crown glass (about)	Deviation
Red	700 nm	1.513	Least
Yellow	580 nm	1.517	More
Blue	470 nm	1.524	More still
Violet	410 nm	1.532	Most

Cookies