Dispersion of White Light by Glass Prism

• Dispersion: Splitting of light into its component colours is called dispersion of light. When light is passed through a glass prism, it undergoes dispersion.
• Spectrum: When white light undergoes dispersion, it splits into seven colours. The band of seven colours obtained is called a spectrum.
  
  The seven colours are VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red.
• Cause of Dispersion: When white light passes through a glass prism, different colours of light bend through different angles. Thus the rays of each colour emerge out distinctly.
  • Red light bends the least.
  • Violet light bends the most.
• Isaac Newton: The scientist who first demonstrated dispersion of white light through a prism.

Recombination of Spectrum

If a second identical prism is placed in an inverted position with respect to the first prism, all the colours recombine to form white light. This proves that sunlight/white light is made up of seven colours.

Rainbow Formation

A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by the dispersion of sunlight by tiny water droplets, present in the atmosphere.

• The water droplets act like small prisms.
• Process: Sunlight enters tiny raindrops and undergoes refraction, dispersion, internal reflection, and once again refraction to reach the observer’s eye.
• Condition: A rainbow is always formed in a direction opposite to that of the Sun. The Sun should be behind the observer.

Atmospheric Refraction

Refraction or bending of light as it passes through different layers of the atmosphere having varying optical density.

1. Twinkling of Stars

Twinkling of stars is due to atmospheric refraction of starlight. Stars appear as point-sized sources of light as they are very far away.

• The starlight undergoes continuous refraction entering the earth’s atmosphere.
• The physical conditions of the earth’s atmosphere are not stationary, so the apparent position of the star fluctuates.
• The amount of starlight entering the eye flickers (brighter/fainter), producing a twinkling effect.

Why Planets Do Not Twinkle: Planets are closer to earth and seen as extended sources. The total variation in light entering our eye from all point-sized sources averages out to zero, nullifying the twinkling effect.

2. Advance Sunrise and Delayed Sunset

The Sun is visible about 2 minutes before actual sunrise and about 2 minutes after actual sunset due to atmospheric refraction.

• Rays from the sun entering the atmosphere undergo refraction and reach the observer even when the sun is slightly below the horizon.
• Time Difference: The total time difference is about 4 minutes (2 mins sunrise + 2 mins sunset).

Scattering of Light

Tyndall Effect

The phenomenon of scattering of light by colloidal particles.

• Example: Visible path of light entering a smoke-filled room or through a canopy of a dense forest (mist particles).
• Dependence on Particle Size:
  • Very fine particles scatter shorter wavelengths (mainly blue).
  • Larger-sized particles scatter longer wavelengths.

Applications of Scattering

• Blue Colour of Sky: Fine particles in the air scatter blue colour (shorter wavelengths) more strongly than red. When this scattered blue light enters our eyes, the sky appears blue.
• Dark Sky at High Altitudes: For passengers flying at high altitudes, the sky appears dark because the atmosphere is thin and scattering is not prominent.
• Space/Moon: The Moon has no atmosphere, so no scattering takes place. The sky appears dark.
• Danger Signal Lights (Red): Red light has the longest wavelength and is the least scattered by fog or smoke. Therefore, it can be seen in the same colour at a maximum distance.

Summary Table of Phenomena