Question

One cannot see through fog because ___________
A. Fog absorbs light
B. Refractive index of fog is unity
C. Light suffers total internal reflection at the droplets in fog
D. Light is scattered by the droplets in fog

Answer 1

Hint: Fog is an aftereffect of nonselective scattering or geometric scattering. When the light incident on the water droplet or dust particles inside the fog, then the light gets deflected from the straight path.

Complete Step-by-Step solution:
Fog is a collection of water droplets or ice crystals and that will be suspended in the air near the Earth surface. An alternative definition for the fog is the cloud formation near the earth surface. The only difference clouds will present in the sky and fog will present near to the earth. The fog consists of water droplets, dust, smoke etc. They can scatter the light when the light falls on these substances. So the viewer can’t see anything effectively due to the light scattering. That’s why we are not advised to drive if the fog is present in the air. We can see only the obstacles or other vehicles when it approaches very near to us. This is also known as Tyndal effect. So the correct answer is D.
Additional information: There are two types of scattering; elastic and inelastic.
Elastic scattering; Here the incident photon energy is conserved. So the light scattered by the particle will emit at the same frequency of incident light. Rayleigh scattering, non selective scattering and Mie scattering are examples of elastic scattering.
Rayleigh occurs only if the particle size is less than the wavelength of the incident light. It is a very weak scattering and depends on the wavelength of the incident light. The nitrogen and oxygen present in the atmosphere are smaller than the wavelength of UV and Visible regions. Therefore sunlight undergoes the Rayleigh scattering. That’s why the sky appears in blue. Here the light of shorter wavelength undergoes the scattering. During sunset, the light has to travel a longer distance than meantime. So the red colour of the visible spectrum undergoes the scattering. So the sky appears reddish during sunset.
Mie scattering will happen only if the particle has a size equal to or greater than the wavelength of the incident light. Clouds appear as white due to Mie scattering. Cloud droplets can scatter all the wavelengths of visible light. That’s why it appears as white.
The non-selective scattering will happen only if the particle has a size larger than the wavelength of incident radiation. This scattering is also known as the geometrical scattering. Larger dust particles, water droplets scatter the light and decrease the contrast of the images that we are seeing. Fog is an aftereffect of nonselective scattering.
Inelastic scattering; If the energy of the incident photon is not conserved, then the scattering is inelastic scattering. Raman scattering and Compton scattering are examples of inelastic scattering.

Note: The particle size determines the scattering. Normally the water droplets and dust particles contained in the fog have a larger size than the wavelength of incident radiation. If the size is reduced, it will become Rayleigh or Mie scattering. If the fog absorbs all the light then it will appear as black.