Question

A narrow beam, PQ of white light is passing through a glass prism ABC as shown in the diagram.

Draw a ray diagram to show the emergent beam as it falls on the screen DE. Also write the phenomenon involved and its cause. Using the second law of refraction state which colour of light must have the highest value of refractive index amongst seven visible colours of light. Justify your answer.

Answer 1

1. Passing a white light beam through a solid prism results in a seven-colour spectrum. The breaking of white light into its constituent colours is known as the dispersion of light.
2. The second law of refraction, or Snell's Law, states that the refractive index (n) is inversely proportional to a medium's light speed.
3. Mathematically, Snell's Law is given by:
   n₁ sin (θ₁) = n₂ sin (θ₂)
   Where:
   
   • n₁ and n₂ are the refractive indices of the two media,
   • θ₁ and θ₂ are the angles of incidence and refraction, respectively.
4. Light speed is inversely related to a medium's refractive index. When light enters a denser medium, the hue with the highest refractive index slows down the most.
5. Among the seven visible light colours, violet light has the shortest wavelength.
6. Shorter wavelengths have a higher refractive index compared to longer ones. Violet light must have the highest refractive index among the seven visible colours.
7. In summary, violet light has the highest refractive index due to its shorter wavelength and the link between the two.