Comparison with Huygens’ wave theory in general terms.

The idea

Newton pictured light as a stream of tiny particles, or corpuscles, travelling in straight lines. Huygens pictured light as a wave spreading out from a source, with every point on a wavefront acting as a source of secondary wavelets whose envelope gives the next wavefront. Both models explain reflection and simple refraction, so a comparison must go beyond those shared successes. The decisive contrast is behaviour at boundaries and around obstacles: the particle model struggles with the partial reflection and refraction that occur together at a surface, and with two beams crossing without disturbing each other, whereas the wave model handles interference and diffraction naturally. A useful method is to list each phenomenon, then state what each theory predicts and whether observation supports it.

What earns the marks

• States that Newton’s model treats light as particles travelling in straight lines, while Huygens’ model treats light as a wave spreading from a source.
• Explains that Huygens’ construction uses secondary wavelets from each point on a wavefront, with their envelope forming the next wavefront.
• Notes that both theories can account for reflection and for simple refraction, so these phenomena alone do not distinguish them.
• Identifies interference and diffraction as phenomena the wave model explains naturally but the particle model does not.
• Explains that in the wave model two beams can cross and continue unchanged, whereas colliding particle streams would be expected to interact.
• Uses a specific observation, such as light passing through a narrow gap and spreading, to support the wave description.

Common mistakes and how to put them right

• Claiming that Newton’s theory cannot explain refraction at all.

  Put it right: It can explain a change of direction at a boundary, but not the full pattern of partial reflection and refraction together.

• Treating Huygens’ wavelets as physical particles emitted by the source.

  Put it right: They are a geometrical construction used to find the next wavefront.

• Assuming the two theories differ only in the speed of light.

  Put it right: The key differences concern the nature of light and the phenomena of interference and diffraction.

Exam tips

• Structure the comparison around named phenomena rather than writing two separate descriptions.
• Use the phrase secondary wavelets when outlining Huygens’ construction, as it shows precise understanding.
• When a question says in general terms, keep the comparison qualitative and avoid unnecessary mathematics.