When The Sun Goes Red!

The Sun emits a broad spectrum of light, the visible portion of which ranges from violet to red. When observed, this combination appears white.

When light travels through the atmosphere, it can interact with molecules of the air such as $\ce{O2}$ or $\ce{CO2}$ . If the light is of suitable wavelength, it will stimulate the air molecule which then re-radiates the light in random directions, significantly weakening its intensity to an observer on the direct line of the light beam. This interaction is more significant for light of short wavelengths (like blue light) than it is for light of long wavelengths (like red light). In fact, the significance of the scattering depends on light's wavelength $\lambda$ according to $f \sim \frac{1}{\lambda^4}$ .

Fig 1.

At Sunset, the light we see from the Sun has to travel a longer path through the atmosphere to reach us than it must at any other point during the day (see Fig. 1 above), maximizing the number of chances it has to scatter with the atmosphere. Therefore, we are far less likely to see strongly scattered, short wavelength photons than we are to see long wavelength, un-scattered light that arrives to us as a coherent beam. This selective filtering leaves us with rich orange and red hues, and little of anything else.

The sun rays near the horizon pass through thicker layers of air and dust particles and travel larger distances in the earths atmosperebefore reaching our eyes.thus the colours of light having shorter wavelengths scatter easily and only the longer wavelengths are visible to us.