Dispersion of LightCambridge IGCSE Physics: Revision notes

Dispersion is the separation of white light into its component colours when it travels through a medium such as glass. This occurs because different colours of light have different wavelengths and frequencies, causing them to refract (bend) by different amounts when entering or leaving a material.

When white light enters a glass prism:

1. Light refracts as it enters the glass (bends towards the normal)
2. Each colour refracts by a slightly different amount due to its different wavelength
3. Red light (longest wavelength) bends the least
4. Violet light (shortest wavelength) bends the most
5. The colours separate and emerge from the prism in order, producing a spectrum

The greater the change in refractive index between air and the glass, the more pronounced the dispersion effect.

The traditional seven colours of the visible spectrum, in order from longest wavelength to shortest wavelength (or lowest frequency to highest frequency), are:

A useful mnemonic is ROY G. BIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet).

Important relationship: As wavelength increases, frequency decreases, and vice versa. Red light has the longest wavelength and lowest frequency; violet light has the shortest wavelength and highest frequency. This is why red refracts the least and violet refracts the most when light enters glass.

The amount of refraction depends on the wavelength of the light:

• Longer wavelengths (e.g. red light) refract less when entering a denser medium like glass
• Shorter wavelengths (e.g. violet light) refract more when entering a denser medium

This relationship explains why a prism separates white light into its component colours. When light travels from air into glass:

1. The refractive index of glass is slightly different for each wavelength (a property called dispersion)
2. Glass has a higher refractive index for violet light than for red light
3. Using Snell's law, $n_1 \sin(\theta_1) = n_2 \sin(\theta_2)$, a higher refractive index causes a greater change in angle
4. Therefore, violet light bends more than red light

This is why the spectrum always appears in the same order: red on the outside (least bent) and violet on the inside (most bent).

Monochromatic light is light of a single frequency (or equivalently, a single wavelength). Unlike white light, which contains all visible frequencies mixed together, monochromatic light contains only one specific colour.

Key properties of monochromatic light:

• It consists of only one frequency and one wavelength
• It appears as a single colour to the human eye
• Common examples include:
  • Red laser light (λ ≈ 650 nm)
  • Green laser light (λ ≈ 532 nm)
  • Yellow sodium lamp light (λ ≈ 589 nm)
  • Any single colour from the visible spectrum

How monochromatic light behaves with a prism:

Unlike white light, monochromatic light will not disperse into multiple colours when passing through a prism. Instead, it will refract as a single ray, bending by an amount determined solely by its wavelength and the refractive index of the glass. This is how we can identify monochromatic sources — no spectrum is produced.

A glass prism is the standard apparatus used to demonstrate dispersion of white light in a school physics laboratory.

What happens in the dispersion experiment:

1. White light (from a ray box or sunlight) is directed at an angle onto one face of a triangular glass prism
2. The light refracts as it enters the glass, bending towards the normal
3. Inside the prism, the different colours travel at slightly different speeds due to their different wavelengths
4. Each colour refracts by a different amount at the air–glass boundary:
   • Red (longest wavelength) refracts the least → emerges at the smallest angle
   • Violet (shortest wavelength) refracts the most → emerges at the largest angle
5. The light refracts again as it exits the prism, bending away from the normal
6. The separated colours emerge from the prism and form a spectrum on a screen

Why the colours separate:

The refractive index of glass varies with wavelength. Glass slows down violet light more than red light, causing violet to refract more. This variation in refractive index is the root cause of dispersion and is unique to each material.