Tyndall effect is a phenomenon of
- (a)scattering of light by the colloidal particles.
- (b)refraction of light by the colloidal particles.
- (c)dispersion of light by dust particles.
- (d)refraction of light by dust particles.
Correct — A, scattering of light by the colloidal particles. The Tyndall effect is the scattering of a beam of light by the tiny suspended particles of a colloid, which makes the path of the beam visible. Because colloidal particles are large enough (roughly 1 to 1000 nm) to scatter visible light, a torch beam becomes visible through fog and sunlight becomes visible streaming through a canopy of trees — that scattering is the Tyndall effect.
- (b)refraction of light by the colloidal particles. — The Tyndall effect is scattering, not refraction. Refraction is the bending of light as it passes between media and is not what makes a colloid's light path visible.
- (c)dispersion of light by dust particles. — It is colloidal particles, not ordinary dust, that define the Tyndall effect, and the process is scattering, not dispersion (the splitting of white light into its colours).
- (d)refraction of light by dust particles. — This is wrong on both counts — the Tyndall effect is scattering (not refraction) and it is caused by colloidal-sized particles.
A colloid is a mixture in which particles of intermediate size (about 1 to 1000 nanometres) are dispersed in a medium. These particles are too small to see but large enough to scatter visible light in all directions. When a beam passes through, the scattered light reveals the beam's path — the Tyndall effect. True solutions (particles below 1 nm) do not show it, so the effect helps tell a colloid from a true solution.
The trap options swap the process (scattering versus refraction versus dispersion) and the particle type (colloidal versus dust). The defining pair for the Tyndall effect is scattering plus colloidal particles, which is option (a).
- The Tyndall effect is the scattering of light by colloidal particles, making the beam's path visible.
- Colloidal particles are about 1 to 1000 nm — large enough to scatter visible light.
- True solutions do not show the Tyndall effect; colloids and fine suspensions do.
- Everyday examples: sunlight through mist or a forest canopy, headlights in fog, a torch beam in milky water.
- Confusing scattering with refraction or dispersion — the Tyndall effect is specifically scattering.
- Thinking any dust does it — the definition ties the Tyndall effect to colloidal-sized particles.
Asked as defining the Tyndall effect — pick the pair 'scattering' plus 'colloidal particles'.
Consider the following statements: 1. Light of longer wavelength is scattered much more than light of shorter wavelength. 2. The speed of visible light in water is 0.95 times its speed in vacuum. 3. Radio waves are produced by rapidly oscillating electrical currents. 4. To detect over-speeding vehicles, police use the Doppler effect of reflected short radio waves. Which of these statements are correct?
- (a) 1 and 2
- (b) 1 and 3
- (c) 2 and 4
- (d) 3 and 4
Answer(d) 3 and 4
Same physics — the scattering of light. Statement 1 there is false precisely because shorter (not longer) wavelengths scatter more; the Tyndall effect is the same scattering process, produced here by colloidal particles.
The Sun appears reddish during sunrise and sunset. The phenomenon in optics which is responsible for this appearance of the Sun is
- (a) Reflection
- (b) Total internal reflection
- (c) Scattering
- (d) Interference
Answer(c) Scattering
Same physics — scattering of light. That 2020 NDA item attributes the reddish Sun at sunrise and sunset to scattering; here scattering by colloidal particles defines the Tyndall effect.
- practice — not a real PYQ
The Tyndall effect can be used to distinguish a colloid from a
- (a)suspension
- (b)true solution
- (c)emulsion
- (d)gel
Answer(b) true solution — a true solution shows no Tyndall effect.
- practice — not a real PYQ
The blue colour of the clear sky is chiefly due to
- (a)reflection of light
- (b)dispersion of light
- (c)scattering of light
- (d)total internal reflection
Answer(c) scattering of light — shorter blue wavelengths scatter most (Rayleigh scattering).