Overview
This lecture explains the phenomenon of scattering of light and how it causes the color of the sky, clouds, sunsets, and the use of red in danger signal lights.
Introduction to Scattering of Light
- Scattering of light is not the same as reflection, refraction, or dispersion.
- Light becomes visible when scattered by particles like dust or colloids in the air.
- Tyndall effect refers to the visible path of a light beam due to scattering by colloidal particles.
Scattering Mechanisms
- Light can be considered as a wave (reflected randomly) or as photons (absorbed and re-emitted in random directions).
- Scattering varies depending on the size of the particles involved.
Particle Size and Scattering
- Particles larger than the wavelength of visible light (water droplets, dust) scatter all colors of light equally.
- Particles smaller than the wavelength (air molecules like nitrogen and oxygen) scatter different colors unequally—shorter wavelengths are scattered more.
Color of the Sky and Clouds
- The sky appears blue because air molecules scatter blue light (shortest wavelength) more than red or green.
- Without an atmosphere, the sky would appear black (no scattering).
- Clouds appear white because larger particles in clouds scatter all colors of light equally.
Sun’s Color at Different Times
- At sunrise and sunset, sunlight travels a longer path through the atmosphere; blue and green light are scattered out, leaving mainly red light, making the sun appear red.
- At noon, sunlight travels a shorter path; little scattering occurs, so all colors reach us almost equally, making the sun appear white.
Danger Signal Lights
- Red has the longest wavelength and is scattered the least by air molecules, which is why red lights are used as danger signals—they are visible from further distances.
Key Terms & Definitions
- Scattering of Light — The process by which light is redirected in various directions by particles in its path.
- Tyndall Effect — Visibility of a light beam in a colloidal medium due to scattering.
- Wavelength — The distance between successive crests of a wave; for visible light, 400–700 nm.
- Preferential Scattering — When particles scatter shorter wavelengths of light more than longer wavelengths.
Action Items / Next Steps
- Review the definitions of scattering and Tyndall effect.
- Understand why the sky, clouds, and sun appear as they do at different times.
- Prepare to explain the reason for the use of red in danger signals.
- No specific homework assigned.