Question: do you know why the grass is green and the sky is blue ?

Simon Holyoake answered on 21 Jun 2013:

The grass is green because it contains chlorophyll, this is a green pigment found inside the chloroplasts. The reason is appears green is because the pigment absorbs all of the red, yellow and blue light from the sunlight, but does not absorb the green light, which is reflected into our eyes

The sky is blue due to a phenomenon known as Rayleigh scattering, it happens due to particles much smaller than the wavelength of light, the particles in the atmosphere scatter blue light (shorter wavelength) more readily than red light (longer wavelength) and due to this, the sky appears blue due to the light being scattered and diffused throughout it more, some of the light is scattered away and we don’t percieve it, this is also why the atmosphere helps to protect us from UV light (very short wavelength) as it scatters it very effectively and diffuses its intensity

however in the evening, the sunlight has to pass through more atmosphere as it is relatively lower in the sky, this causes most of the blue and green light to scatter out, which means only the red light reaches us, hence the sky appears redder