What will happen to sun if the concentration of nitrogen, oxygen and carbon increases to 97 % and hydrogen and helium decreases by 3 %? Can we still call it a star?

No.

Explanation:

If the concentration of nitrogen, oxygen and carbon increases to 97 % and the concentration of hydrogen and helium decreases by 3 %, then it is not considered as a star, it will be called a planet just like earth. The higher concentration of hydrogen and helium are responsible for the luminous characteristics of stars such as sun so if the concentration changes, the luminous intensity decreases that leads to formation of planet.

second mirror gathers light from a faraway object and reflects it to another mirror. - a.

scattering of light by tiny particles in air to produce the colors we see on sky, is dependent on the following factors:

(i) wavelength of light

(ii) size of the scattering particle  

(iii) distance traveled by light through the atmosphere.

according to rayleigh's law of scattering, the amount of scattering is inversely proportional to the fourth power of its wavelength. shorter the wavelength, greater is its scattering. hence violet/blue light is scattered more than red.

sun's light is a composite light ( visible light) with wavelengths in the range 400 nm (violet) to 700 nm (red). as sunlight passes through our atmosphere, the molecules of air scatter light according to their wavelengths. during sunrise and sunset, the sun's position is below the horizon. hence light from the sun travels a greater distance in the atmosphere when compared to its position a noon.

as sunlight enters the earth's atmosphere, blue light is scattered away and at the end of its journey in the sky, only red and orange wavelengths remain in the beam.

hence at sunrise and sunset, the sky appears reddish orange.