Why do high-rise buildings have a red light on the top of the building?
A. Red light is the least scattered by fine particles
B. Red light can be spotted by the pilot easily
C. Both A and B
D. None of the above

Hint: To answer this we have to see the properties of red light. Scattering of light occurs when light rays collide with an obstruction such as dust, gases in the atmosphere, or water vapors and divert from their straight course. The scattering of light depends on the wavelength of the light.

Complete answer:
There are red lights on the top of the building so that a pilot flying an airplane can notice them and a collision between airplane and building can be avoided.For this, we use red-colored light. We know in the atmosphere scattering of light occurs because of the collision between particles and rays of light.

The amount of scattering depends on the wavelength of the light. Every color light has a different wavelength. Scattering of light \[ \propto \dfrac{1}{{{\lambda ^4}}}\], where $\lambda $ is the wavelength of the light. Since red light has the longest wavelength, among all the other visible-light colors. Therefore, it is least scattered by fine particles and can be seen from a distance so can be spotted by the pilot easily.

Hence option C is the correct answer.

Note: Very fine particles mainly scatter light of shorter wavelength, large size particle scatters light of longer wavelength. In the atmosphere, there are fine particles and molecules of air which have a smaller size than the wavelength of visible light. Violet light scatters the most due to its shorter wavelength.