Read the following and answer the questions that follow:
Rahul, a Class 10 student, notices that when sunlight enters his room through a small gap in the curtains, he can clearly see dust particles floating in the air along the beam. During a science fair, he sets up a glass tank filled with a colloidal suspension of dilute milk in water and shines a torch through it — the beam is clearly visible from the side. He recalls that a similar effect is seen when sunlight filters through the canopy of a dense forest on a misty morning.
(i) Name the phenomenon Rahul observes in all three situations. [1]
(ii) Why is the path of light visible through the colloidal suspension but not through a true (clear) solution? [1]
(iii) How does the size of scattering particles determine the colour of scattered light? [2]
Generated by claude-sonnet-4-6 · 2026-06-26 01:09 · grounding rag
Model Answer
(i) The phenomenon Rahul observes in all three situations is the Tyndall Effect — the scattering of light by colloidal particles, making the path of the beam visible.
(ii) In a true (clear) solution, the particles are too small to scatter light, so the beam's path is invisible. In a colloidal suspension (dilute milk in water), the particles are relatively larger and scatter light, making the beam visible from the side.
(iii) The colour of scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light (shorter wavelengths). Particles of larger size scatter light of longer wavelengths. If particles are large enough, the scattered light may even appear white.
Source: Chapter 10, Section 10.6 — Scattering of Light / Tyndall Effect
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Explanation
- (i) Directly name "Tyndall Effect" — do not just say "scattering."
- (ii) The key contrast is particle size: true solution particles are too small; colloidal particles are large enough to scatter. One mark — keep it to one-two lines.
- (iii) This is 2 marks, so mention all three size ranges: very fine → blue; larger → longer wavelengths (red/orange); very large → white. The passage states this explicitly — quote it closely.