Colours in light

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Lesson notes

White Light and the Spectrum

  • White light is made up of seven different colours.
  • Passing white light through a prism splits it into a spectrum of colours.
  • This splitting is called dispersion and is what causes rainbows.
  • The colours in order from longest to shortest wavelength are: red, orange, yellow, green, blue, indigo, violet (ROY G. BIV).

White light is dispersed into the visible spectrum by a glass prism.

White light is dispersed into the visible spectrum by a glass prism.

Dispersion and Refraction

  • Dispersion happens because different colours of light have different refractive indices in a medium.
  • Each colour refracts by a different amount when entering a prism.
  • Violet refracts most because it experiences the highest refractive index ("violet bends most violently").
  • Red refracts least because it experiences the lowest refractive index.

White light is dispersed into the visible spectrum by a glass prism.

White light is dispersed into the visible spectrum by a glass prism.

Colour Filters

  • A colour filter transmits only one colour of light and absorbs all other colours.
  • For example, a red filter transmits only red light; a green filter transmits only green light.

Colour of Objects

  • An object appears a certain colour because it reflects that colour of light into our eyes.
  • A green object reflects only green light and absorbs all other colours.
  • A red object reflects only red light.
  • A white object reflects all seven colours of the spectrum.

Combining Filters and Object Colour

  • If white light passes through a yellow filter, only yellow light is transmitted.
  • If that yellow light shines on a blue object, the object appears black because it can only reflect blue light, but no blue light is present.

Refraction of light entering a prism: different colours refract by different amounts (violet most, red least).

RefractionAirGlassnormalincident40°refracted25°

Reflection of light from a surface: angle of incidence equals angle of reflection.

Reflectionnormalincident45°reflected45°

Slides

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Practice questions

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  1. 1.What is the splitting of white light into a spectrum called?

    Easy
    • ADispersion
    • BReflection
    • CRefraction
    • DAbsorption
  2. 2.How many colours are in the spectrum of white light?

    Easy
    • A7
    • B5
    • C6
    • D3
  3. 3.Which colour of light refracts the most in a prism?

    Easy
    • AViolet
    • BRed
    • CBlue
    • DGreen
  4. 4.Which colour of light has the longest wavelength in the visible spectrum?

    Easy
    • ARed
    • BViolet
    • CBlue
    • DGreen
  5. 5.A red filter is placed in front of a white light source. What colour light is transmitted?

    Easy
    • ARed
    • BWhite
    • CBlue
    • DNo light
  6. 6.Why does a green object appear green?

    Medium
    • AIt reflects green light into our eyes.
    • BIt transmits green light.
    • CIt absorbs green light.
    • DIt emits green light.
  7. 7.White light is passed through a yellow filter and then shines onto a blue object. What colour does the object appear?

    Medium
    • ABlack
    • BBlue
    • CYellow
    • DGreen
  8. 8.Which of the following statements about dispersion is true?

    Medium
    • ADispersion is caused by different colours refracting by different amounts.
    • BDispersion is caused by reflection.
    • CAll colours refract by the same amount in a prism.
    • DDispersion only occurs in rainbows.

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