why is the sky blue in simple

The sky appears blue because of how sunlight scatters in the Earth's atmosphere: 

Sunlight is made of many colors

Sunlight appears white, but it's actually made up of many colors, like those in a rainbow. Each color has a different wavelength, or type of wave. 

Air molecules scatter light

When sunlight reaches the Earth's atmosphere, it's scattered by tiny air molecules, mostly nitrogen and oxygen. 

Blue light scatters more

Blue and violet light have shorter wavelengths than other colors, so they scatter more. This means more blue light reaches our eyes, making the sky appear blue. 

Sunrise and sunset

When the sun is low in the sky, sunlight has to travel through more of the atmosphere to reach us. This scatters away the blue light, so the sky appears red and orange. 

This effect is called Rayleigh scattering, named after Lord Rayleigh who first discovered it.