NDA (II) 2023 General Ability Test · Question 51 of 146

Correct — B, dust particles in the air scatter light into our eyes. A beam of sunlight travelling through air sends its light forward, away from the observer, so a beam in perfectly clean air would be invisible from the side. What makes the shaft visible is that fine particles suspended along its path — dust, smoke, tiny water droplets — intercept the light and redirect a small part of it in all directions, including towards the eye of anyone standing to one side. NCERT names this redirection of light by fine suspended particles the Tyndall effect, and gives sunlight passing through the canopy of a dense forest as one of its standard examples — exactly the scene described in the stem.

• (a)dust particles in the air reflect light into our eyes — Reflection is a directional bounce off a surface that is large compared with the wavelength, sending the beam off in one particular direction. Airborne dust is far too small and randomly oriented for that; it redirects light in every direction at once, and the physics term for such redirection by fine particles is scattering, which is also the word the syllabus uses for the Tyndall effect.
• (c)dust particles in the air refract light into our eyes — Refraction is the bending of light as it crosses the boundary between two transparent media of different optical density, as when light enters water or glass. It changes the direction of the beam as a whole but does not spray light sideways out of the beam, so it cannot make the beam visible to an observer standing off to the side.
• (d)dust particles in the air polarize light into our eyes — Polarisation restricts the direction in which the light wave vibrates. Scattered sunlight is indeed partly polarised, but polarisation only describes a property of the light that already reaches the eye — it does not deliver any light to the eye. The delivery is done by scattering.

Light travels in straight lines, so a beam is seen only when something along its path sends part of it towards the observer. Fine particles suspended in air — dust, smoke, mist droplets — do exactly that, redirecting light in all directions. That redirection is called scattering, and when it makes the path of a beam visible in a heterogeneous medium it is known as the Tyndall effect.

The tempting near-miss is reflection, and NCERT itself describes the light as reaching us after being reflected diffusely by these particles. But the phenomenon has a name in the syllabus, and that name is the Tyndall effect, defined as the scattering of light by fine suspended particles. When an examiner offers reflection, refraction, scattering and polarisation side by side for a visible sunbeam, scattering is the intended term. The same chapter also notes that the path of a beam through a true solution is not visible, while through a colloidal solution it is, because the particles there are relatively larger.

• NCERT Class 10 Science names the visible path of a light beam through fine suspended particles the Tyndall effect and describes it as the scattering of light.
• The same chapter gives sunlight passing through the canopy of a dense forest, and a sunbeam entering a smoke-filled room, as everyday examples.
• A beam passing through a true solution leaves no visible path; through a colloidal solution it does, because the particles are relatively larger.
• The colour of the scattered light depends on particle size — very fine particles scatter mainly blue light, larger particles scatter longer wavelengths, and large enough particles make the scattered light look white.

• Choosing reflection because the textbook also mentions diffuse reflection — the named phenomenon is still scattering.
• Assuming scattered light is always blue; the colour depends on the size of the scattering particles.
• Confusing dispersion, which splits white light into colours through a prism, with scattering, which redirects light off particles.

NDA asks either for the name of the phenomenon that makes a beam visible, or for the reason behind a blue sky, a red sunset or a visible sunbeam — decide by matching particle size to the wavelength that gets scattered.

• practice — not a real PYQ

  The path of a beam of light is clearly visible when the beam passes through

  • (a)a true solution of common salt in water
  • (b)a colloidal solution such as milk diluted in water
  • (c)distilled water
  • (d)a vacuum

  Answer(b) a colloidal solution such as milk diluted in water — the comparatively larger colloidal particles scatter the beam sideways and make its path visible, while a true solution's particles are far too small to do so.

• practice — not a real PYQ

  Very fine particles in the atmosphere scatter mainly

  • (a)red light
  • (b)blue light
  • (c)infrared radiation
  • (d)all wavelengths equally

  Answer(b) blue light — very fine particles scatter mainly the shorter blue wavelengths, while larger particles scatter light of longer wavelengths.