7.7Chapter 7: Light

Dispersion — Alhussan Physics - Stage 8 Revision Notes & Topical Questions

5 min read 9 parts Lesson 7 of 12 2 quizzes

📚 Alhussan Physics - Stage 8 Hub / Topical Revision

0%read

Lesson summary

Physics 7.7 Dispersion — Prisms, White Light and the Visible Spectrum — Alhussan International Academy — Physics (Stages 7–9) notes: Explain how a spectrum of light is produced and why we see rainbows.White light — A normal light bulb looks white, but white light is a mixture of …

Rectangular block
Triangular prism
Inside the prism
Continuous spectrum
Second prism
Conclusion

Part 1 of 7White light is a mixture

White light

A normal light bulb looks white, but white light is a mixture of colours.

Rectangular block

A ray of white light entering and leaving a glass block refracts but stays white.

Triangular prism

At a particular angle of incidence the light leaving the prism is split into colours.

Part 2 of 7Dispersion

Spectrum

Spectrum all the colours of white light together.

Dispersion

Dispersion the splitting of white light into a spectrum.

Cause

Different colours are slowed down by different amounts in glass, so they are refracted by different amounts.
Red light is slowed the least, so it is refracted the least and travels fastest in glass.
Violet light is slowed the most, so it is refracted the most and travels slowest in glass.

Inside the prism

We cannot see the light inside, but the colours leaving show the different paths.

Part 3 of 7The colours of the spectrum

Colour	Refraction	Speed in glass
Red	Least	Fastest
Orange	Less	Fast
Yellow	Medium	Medium
Green	Medium	Medium
Blue	More	Slow
Indigo	More	Slower
Violet	Most	Slowest

Order

The colours are red, orange, yellow, green, blue, indigo and violet.

Continuous spectrum

The colours blend smoothly with no gaps.

Part 4 of 7Recombining the colours

Newton

In 1666 he showed that sunlight can be split into colours by a prism.

Second prism

Placed upside down in the path of the spectrum, it recombines the colours into white light.

Conclusion

The prism does not make the colours; it separates colours that were mixed in white light.

Part 5 of 7Refraction and dispersion

1Dispersion needs refraction

Different colours must be refracted by different amounts, so refraction must happen first.

2Refraction without dispersion

A single-colour ray, such as from a laser, refracts in glass without splitting.

Part 6 of 7Rainbows

1Raindrops

They behave like small prisms.
Sunlight from behind you is refracted, reflected inside the drop and dispersed.
You see the spectrum, which we call a rainbow.

2Position

You see a rainbow when facing falling rain with the Sun behind you, not when the Sun is in front.

3History

About 700 years ago the Iranian scientist Al-Farisi correctly explained how raindrops disperse white light.

Part 7 of 7Observing a spectrum

Place a triangular prism on paper in front of a ray box with a narrow slit.

Direct the beam at one face of the prism at an angle.

Look for a spectrum on a white screen beyond the other face.

Note that red is nearest the original direction and violet furthest.

Label the colours in order on a sketch.

Key terms

Term	Meaning
Dispersion	The splitting of white light into the colours of the spectrum.
Spectrum	The range of colours that make up white light.
Prism	A transparent block, often triangular, that refracts and disperses light.
Monochromatic	Of a single colour.
Continuous spectrum	A spectrum in which colours blend with no gaps.