Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Scattering of light is the phenomenon in which light rays are deviated from their straight path on striking an obstacle like dust or gas molecules.
The Tyndall Effect is the scattering of a beam of light by medium particles (colloids or fine suspensions), making the path of light visible.
The color of scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light (shorter wavelengths), while larger particles scatter light of longer wavelengths like red.
If the size of the scattering particles is large enough (like water droplets in clouds), the scattered light may even appear white because all wavelengths are scattered equally.
The molecules of air and other fine particles in the atmosphere have sizes smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end.
At sunrise and sunset, the Sun is near the horizon. Light has to travel a longer distance through the atmosphere. Most of the blue light is scattered away, leaving only the longer wavelengths (red/orange) to reach our eyes.
In the absence of an atmosphere (e.g., for an astronaut in outer space), there is no scattering, and the sky appears dark.
📐Formulae
💡Examples
Problem 1:
Why is the color of the clear sky blue?
Solution:
The molecules of air (nitrogen and oxygen) are smaller than the wavelength of visible light. They scatter the shorter wavelengths (blue/violet) more strongly than the longer wavelengths (red).
Explanation:
According to Rayleigh scattering, intensity . Since blue light has a shorter wavelength than red light, it is scattered much more. Although violet is scattered even more, our eyes are more sensitive to blue.
Problem 2:
Why are 'danger' signal lights red in color?
Solution:
Red light has the longest wavelength among the visible colors, and according to the relation , it is scattered the least by fog, dust, or smoke.
Explanation:
Because it is scattered the least, red light can travel longer distances through atmospheric impediments and remains visible even from far away.
Problem 3:
Explain why the Sun appears reddish early in the morning.
Solution:
During sunrise, the light from the Sun travels through a thicker layer of the atmosphere. The shorter wavelengths (blue) are scattered out of the line of sight, and only the less-scattered longer wavelengths (red) reach the observer.
Explanation:
The distance traveled through the atmosphere is maximum at the horizon. This long path allows for the complete scattering of the blue end of the spectrum, leaving the red light to dominate the observer's vision.