Question

Name and explain the phenomenon of light due to which the path of a beam of light becomes visible when it enters a smoke filled room through a small hole. Also state the dependence of colour of the light we receive on the size of the particle of the medium through which the beam of light passes.

Answer 1

Step 1: Visibility of Light Beam in Smoke:
The phenomenon that makes a light beam visible in a smoke-filled room, when entering through a small opening, is known as light scattering. Smoke particles in the air diffuse the light in multiple directions, allowing the path of the beam to be perceived as scattered light reaches the observer's eyes.
- Scattering occurs when light interacts with minuscule particles within a medium, such as smoke. These particles redirect the light from its original trajectory, dispersing it across different directions and rendering the beam discernible.

Step 2: Particle Size and Light Colour Dependence:
The perceived colour of light is influenced by the size of particles in the medium it traverses, a principle explained by Rayleigh scattering.
- When particles are significantly smaller than the light's wavelength, shorter wavelengths (blue and violet) are scattered more intensely than longer wavelengths (red). This effect accounts for the blue appearance of the sky and the sun's yellow or red hues depending on scattering conditions.
- As particle size increases, such as with dust or smoke, wavelength-dependent scattering diminishes. Larger particles tend to scatter all light wavelengths uniformly, potentially producing a whitish or hazy appearance.

Step 3: Summary:
The visibility of a light beam in a smoke-filled room is a consequence of light scattering by smoke particles. The colour of the scattered light is contingent on particle size; smaller particles scatter shorter wavelengths (blue/violet) more, while larger particles scatter light more evenly, often resulting in a whitish hue.