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PHYSICS · Stage 8 · Chapter 7: Light

Physics 7.7 Dispersion — Prisms, White Light and the Visible Spectrum

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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

Section 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.

Section 2 of 7Dispersion

  • Spectrum = all the colours of white light together.
  • 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.

Section 3 of 7The colours of the spectrum

ColourRefractionSpeed in glass
RedLeastFastest
OrangeLessFast
YellowMediumMedium
GreenMediumMedium
BlueMoreSlow
IndigoMoreSlower
VioletMostSlowest
  • Order
    • The colours are red, orange, yellow, green, blue, indigo and violet.
  • Continuous spectrum
    • The colours blend smoothly with no gaps.

Section 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.

Section 5 of 7Refraction and dispersion

  • Dispersion needs refraction
    • Different colours must be refracted by different amounts, so refraction must happen first.
  • Refraction without dispersion
    • A single-colour ray, such as from a laser, refracts in glass without splitting.

Section 6 of 7Rainbows

  • Raindrops
    • 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.
  • Position
    • You see a rainbow when facing falling rain with the Sun behind you, not when the Sun is in front.
  • History
    • About 700 years ago the Iranian scientist Al-Farisi correctly explained how raindrops disperse white light.

Section 7 of 7Observing a spectrum

  1. Place a triangular prism on paper in front of a ray box with a narrow slit.
  2. Direct the beam at one face of the prism at an angle.
  3. Look for a spectrum on a white screen beyond the other face.
  4. Note that red is nearest the original direction and violet furthest.
  5. Label the colours in order on a sketch.

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