Question

When would the sky appear red instead of blue?

Answer 1

Hint: The sky has no particular colour in general. It is the colour detected by the retina of our eyes that we see. It is the wavelength of the light received by our eye. When the sunlight passes through the atmosphere, it undergoes elastic scattering due to the molecules present in the atmosphere.

Complete answer:
The colour of the sky that we see is what our eyes perceive which depends on the scattering of the different wavelengths present in the sunlight.
Now let us first understand the concept of scattering of light.
As the sunlight passes through the atmosphere during the day, molecules present in it absorb the sunlight and reemit it. The molecules scatter the light in all directions. This is called elastic scattering as there is no change in energy due to absorption. It obeys Rayleigh’s law of scattering which is defined as
 $I \propto \dfrac{1}{{{\lambda ^4}}}$
Where $I$ is the intensity of the scattered light and $\lambda $ is the wavelength of the light scattered.
Now as we know that blue light has the shortest wavelength therefore the scattering intensity of the blue light will be the most and hence the sky appears blue at daytime. But during sunrise or sunset, the sun is away from us as it reaches the horizon, therefore the colours of shorter wavelength like blue are scattered away completely in the atmosphere before reaching our eyes whereas the red light which has the longest wavelength reaches our eyes and the sky appears red instead of blue.

Note: The range of wavelengths visible to human eyes is the spectrum of light which consists of the seven colours that we see around us with our naked eyes. The sunlight along with the visible spectrum of wavelengths carries some other wavelengths which are not visible to us.