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3.2.4Waves

Dispersion OF Light — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 15 of 35 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

3.2.4 Dispersion of Light — IGCSE PHYSICS notes: Explain dispersion of white light by a prism and the order of the visible spectrum.Dispersion — the separation of white light into its colours by refraction in a glass prismWhite light — contains the wavelengths of all the colours …

  • Memory aids
  • Seven colours
  • Definition
  • Monochromatic
  • Visible light
  • White light — contains the wavelengths of all the colours of the spectrum

Part 1 of 7Dispersion

Dispersion
  • Dispersion the separation of white light into its colours by refraction in a glass prism
White light
  • White light contains the wavelengths of all the colours of the spectrum
Colours
  • Colours each has a different wavelength and frequency, a very narrow part of the electromagnetic spectrum
Rainbow
  • Rainbow formed in a similar way

Part 2 of 7Why it happens

Refraction
  • Refraction each colour is refracted by a different amount
Violet
  • Violet refracted the most
Red
  • Red refracted the least
Result
  • Result the colours spread out into a spectrum

Part 3 of 7Order of colours

Order
  • Order red, orange, yellow, green, blue, indigo, violet
Red
  • Red longest wavelength and lowest frequency
Violet
  • Violet shortest wavelength and highest frequency
Memory aids
  • Roy G.
  • Biv or Richard Of York Gave Battle In Vain

Part 4 of 7Visible light in context

Human eye
  • Human eye visible light is the only part of the spectrum the eye can detect
Animals
  • Animals birds, bees and some fish can also see infrared and ultraviolet
Seven colours
  • Seven colours correspond to different wavelengths

Part 5 of 7Monochromatic light

Definition
  • Definition light of a single frequency, a single colour
Example
  • Example a laser beam, for example green laser light

Part 6 of 7Demonstration steps

1
Shine a narrow beam of white light from a ray box at a triangular glass prism.
2
Let the beam refract at the first face and again at the second face.
3
Catch the emerging light on a white screen.
4
Observe the spectrum from red, the least refracted, to violet, the most refracted.

Part 7 of 7More from the specification

1Chromatic dispersion and optical fibres
  • Chromatic dispersion and optical fibres wavelength-dependent refraction splits white light, requiring single-mode optical fibres to prevent chromatic pulse spreading.
2Dispersion
  • Dispersion white light splits into a spectrum because each colour has a different refractive index and bends by a different angle.
3Refraction
  • Refraction light changes direction at a boundary between media because its speed changes;
    • red bends least, violet most.
4Refractive index
  • Refractive index n equals sin of angle of incidence divided by sin of angle of refraction.
5Total internal reflection
  • Total internal reflection occurs when light in a dense medium meets a boundary at more than the critical angle, so all of it is reflected.
6Reflection images
  • Reflection images a plane mirror forms a virtual image, with angle of incidence equal to angle of reflection.
7Dispersion
  • Dispersion white light splits into a spectrum because each colour has a different refractive index and bends by a different angle.
8Refraction
  • Refraction light changes direction at a boundary between media because its speed changes;
    • red bends least, violet most.
9Refractive index
  • Refractive index n equals sin of angle of incidence divided by sin of angle of refraction.
10Total internal reflection
  • Total internal reflection occurs when light in a dense medium meets a boundary at more than the critical angle, so all of it is reflected.
11Reflection images
  • Reflection images a plane mirror forms a virtual image, with angle of incidence equal to angle of reflection.
12Distance
  • Total length of path travelled, a scalar.
13Refractive index
  • Ratio of the speed of light in a vacuum to its speed in a material
14Critical angle
  • Incline angle at which sliding just begins.
15Virtual image
  • Image formed where rays only appear to meet, not projectable on a screen
16Optical fibre
  • Thin transparent strand that guides light by TIR
17Principal focus
  • Point where parallel rays meet or appear to come from
18Focal length
  • Distance from the lens centre to the principal focus
19Plane mirror
  • Flat mirror.
20Reflection
  • Light bouncing off a surface.
21Boundary
  • Surface between two media at which waves are partially reflected
22Normal
  • Line at 90° to a surface.

Key terms

TermMeaning
DispersionSplitting of white light into colours by refraction
SpectrumBand of colours from red to violet
MonochromaticLight of a single frequency
Visible lightPart of the electromagnetic spectrum detectable by the human eye
PrismTriangular glass block used to disperse light

Avoid these mistakes

1
Saying the prism adds colour to the light — the colours are already in white light.
2
Saying red is refracted most — violet is refracted most.
3
Saying all colours have the same wavelength — each colour has its own wavelength and frequency.
4
Mixing up the order of the spectrum — red has the longest wavelength and violet the shortest.

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