Question

The Sun appears red or yellow or orange during sunrise and sunset. But normally the sky appears blue in colour. Describe an experiment that would be helpful in explaining the cause of these phenomena. OR In winter, path of light through the gaps of the branches of trees can be seen clearly due to scattering.

• [(a)] By what name is the phenomenon known where the path of light becomes visible in this manner? Through what type of medium does light travel when this occurs?
• [(b)] What does the intensity of scattering depend on?

Hint:

Blue light scatters more → sky is blue
Red light least scattered → Sun appears red at sunrise/sunset

Answer 1

(A) Experiment to explain scattering of light
Concept: Scattering of light (Tyndall effect)
The color of the sky and the Sun is explained by scattering of light by small particles in the atmosphere. Experiment:

• Take a glass tank filled with clean water.
• Add a few drops of milk (to create a colloidal solution).
• Pass a beam of white light (torch or laser) through the solution.

Observation:

• The path of light becomes visible due to scattering.
• Light coming out from the side appears bluish.
• Light observed at the far end appears reddish.

Explanation:

• Shorter wavelengths (blue light) scatter more → sky appears blue.
• At sunrise/sunset, light travels a longer path → blue light scatters away → longer wavelengths (red/orange) dominate.

Conclusion: \[ \text{Scattering of light explains both blue sky and red/orange Sun.} \] (B) Scattering of light in atmosphere
(a) Name of the phenomenon
Step 1: Identify phenomenon}
The visible path of light is called: \[ \text{Tyndall effect} \] Step 2: Type of medium}
This occurs when light passes through a: \[ \text{colloidal medium (dust, fog, smoke)} \] Conclusion (a): \[ \text{Tyndall effect in a colloidal medium} \] (b) Intensity of scattering
Step 1: Dependence on wavelength}
Scattering depends on wavelength: \[ \text{Scattering} \propto \frac{1}{\lambda^4} \] Step 2: Other factors}

• Size of particles
• Number of particles
• Nature of medium

Conclusion (b): \[ \text{Intensity increases for smaller wavelengths and more particles} \]