Dispersion of Light

Section 1 of 8Dispersion

Dispersion

Dispersion the splitting of white light into its separate colours by refraction.

White light

• White light it contains all the wavelengths of the visible spectrum mixed together.

Cause

• Cause each colour travels at a different speed in glass, so each colour is refracted by a different amount.

Natural example

• Natural example a rainbow forms by a similar process.

Section 2 of 8Dispersion by a prism

Prism

• Prism a triangular glass prism refracts white light at both faces and spreads the colours into a spectrum.

Most refracted

• Most refracted violet light slows down the most and bends the most.

Least refracted

• Least refracted red light bends the least.

Rectangular block

• Rectangular block it barely disperses light because the colours emerge parallel and recombine.

Colour

• Colour the frequency, and so the colour of each ray, does not change on refraction.

Section 3 of 8Visible spectrum

Order

• Order red, orange, yellow, green, blue, indigo, violet.

Red end

• Red end it has the longest wavelength and lowest frequency.

Violet end

• Violet end it has the shortest wavelength and highest frequency.

Wavelength range

• Wavelength range about 4 × 10⁻⁷ m for violet to 7 × 10⁻⁷ m for red.

Visible light

• Visible light it is the only part of the electromagnetic spectrum detected by the human eye.

Section 4 of 8Wave speed of light

Wave equation

• Wave equation c = f × λ, with c = 3.0 × 10⁸ m/s in a vacuum.
  What the symbols mean: v = wave speed (m s⁻¹) · f = frequency (Hz) · λ = wavelength (m)

Frequency and wavelength

• Frequency and wavelength for light in a vacuum, a higher frequency means a shorter wavelength.

Section 5 of 8Monochromatic light

Monochromatic light

Monochromatic light light of a single frequency, and so a single colour.

Example

• Example a laser beam is monochromatic.

Dispersion

• Dispersion monochromatic light is refracted by a prism but is not split into other colours.

Section 6 of 8More from the specification

1Virtual images in plane mirrors

• Virtual images in plane mirrors reflected light rays diverging from flat reflective surfaces appear to emanate from erect virtual images positioned behind mirrors.

2Total internal reflection in optical fibres

• Total internal reflection in optical fibres light signals transmit efficiently through flexible glass cores of optical fibres via repeated total internal reflection.

3Dispersion

• Dispersion white light splits into colours when refracted in a prism because each wavelength has a different refractive index

4Refractive index

• Refractive index the ratio of the speed of light in air to that in the material, and violet is bent most

5Total internal reflection

• Total internal reflection occurs at a boundary when the angle of incidence exceeds the critical angle going into a less dense medium

6Reflection

• Reflection the angle of incidence equals the angle of reflection, and a plane mirror forms a virtual image the same distance behind it

7Dispersion

• Dispersion white light splits into colours when refracted in a prism because each wavelength has a different refractive index

8Refractive index

• Refractive index the ratio of the speed of light in air to that in the material, and violet is bent most

9Total internal reflection

• Total internal reflection occurs at a boundary when the angle of incidence exceeds the critical angle going into a less dense medium

10Reflection

• Reflection the angle of incidence equals the angle of reflection, and a plane mirror forms a virtual image the same distance behind it

11Refractive index

• The ratio of the speed of light in a vacuum to its speed in a material, n = c ÷ v.
  What the symbols mean: n = refractive index (no unit) · c = speed of light in a vacuum (3.00 × 10⁸ m s⁻¹) · v = speed of light in the material (m s⁻¹)

12Critical angle

• The angle of incidence in the denser medium at which the refracted ray travels along the boundary.

13Virtual image

• An image formed where rays only appear to come from, which cannot be projected.

14Optical fibre

• A thin glass fibre that carries light by total internal reflection.

15Principal focus

• The point where rays parallel to the principal axis meet or appear to come from.

16Focal length

• The distance from the centre of the lens to the principal focus.

17Plane mirror

• A flat mirror.

18Reflection

• Bouncing of light off a surface.

19Reflected

• Bounced off a surface.

20Normal

• A line at right angles to a surface.

Section 7 of 8Key terms

Term	Meaning
Dispersion	The splitting of white light into its colours by refraction.
Spectrum	The band of colours produced when white light is dispersed.
Prism	A triangular block of glass that disperses white light.
Monochromatic	Light of a single frequency or colour.
Visible light	The part of the electromagnetic spectrum detected by the eye.

Section 8 of 8Exam technique

1

Saying the prism adds colour to white light — it separates the colours already present.

2

Saying red is refracted the most — violet is refracted the most.

3

Saying frequency changes on refraction — it stays the same, so the colour is unchanged.

4

Saying monochromatic light has several wavelengths — it has a single frequency.