A small opaque ball is held in the path of a parallel beam of light and casts a shadow on a screen. When the ball is replaced by one that is much smaller (comparable in size to the wavelength of light), a bright spot is observed at the centre of the shadow instead of a dark region. (i) What does the formation of a sharp shadow by the larger ball tell us about the nature of light? (ii) How does the appearance of a bright spot in the centre of the shadow of the very small ball challenge this model?
Generated by claude-sonnet-4-6 · 2026-06-26 01:11 · grounding rag
Model Answer
(i) The sharp shadow cast by the larger ball tells us that light travels in straight lines (rectilinear propagation). A small source of light casting a sharp shadow of an opaque object indicates that light does not bend around the object and follows a straight-line path (ray model of light).
(ii) When the ball is very small (comparable to the wavelength of light), light bends around it — a phenomenon called diffraction. The bright spot at the centre of the shadow cannot be explained by the straight-line (ray) model. This shows that light behaves as a wave, challenging the ray model of light.
Source: Chapter 9, Introduction
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Explanation
- Part (i) tests rectilinear propagation — the key phrase is "sharp shadow → straight-line travel."
- Part (ii) tests diffraction — examiners expect the word diffraction, a mention of wave nature, and why the ray model fails.
- Don't over-explain; 2 crisp sentences per sub-part is enough for 3 marks (1+2 split likely).
- The passage explicitly states both ideas, so quote/paraphrase it closely.