Question

Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an astronaut on the Moon? Give reason to justify your answer.

Answer 1

Hint: The sunlight from the sun reaches the earth’s surface by travelling through the earth’s atmosphere. This atmosphere acts as an obstruction for the rays of sun causing scattering of sunlight. The absence of the atmosphere will result in no scattering of light.

Complete step-by-step answer:
The Sun appears reddish in the morning because of a phenomenon known as Rayleigh Scattering.
Rayleigh Scattering is the electromagnetic radiation or elastic scattering of light by particles tinier than the wavelength of the radiation. The Rayleigh scattering of sunlight in the atmosphere of earth is the scattering of solar radiation from the direct solar beam by particulates and molecules present in the atmosphere, this is also known as diffuse sky radiation.
This scattering of light occurs when sunlight interacts with air molecules which are located randomly in the atmosphere, which gives the sky its colour and brightness.
The shorter wavelength, blue and violet light will scatter more as compared to the longer wavelength which are yellow and red light because Rayleigh scattering is inversely proportional to the fourth power of wavelength. Also, the oxygen in the atmosphere absorbs light at the edge of the ultra-violet spectrum which is of low wavelength, by the combination of this the sky looks pale blue which is actually the mixture of the scattered colours, dominantly blue and green. The sun has its own spectrum and the colours of a longer wavelength which are directly visible such as red and yellow light give the Sun its yellowish hue.
In the morning, the sun is at the horizon due to which the sunlight must pass through more portions of the atmosphere. Also, the significant portion of the atmosphere near the Earth’s Surface is denser from which this light passes in the morning, this results in the removal of a crucial portion of the shorter and medium wavelength light (mostly blue and green) from the path of the observer. The remaining light is of longer wavelengths and appears red. This intensifies the reddishness of the Sun in the morning.
For the astronauts on the moon, the Rayleigh scattering of light in the atmosphere does not occur and thus the sky looks dark and the Sun looks white.

Note: The scattering of light explains many observed phenomena like the sky appears darker to an astronaut in outer space, the sky looks blue while clouds appear white. Additionally, the Rayleigh scattering also results in a rise to polarization effects.