Why does the moon become red during a Lunar eclipse?

Because Earth’s atmosphere is really good at scattering blue light. During the day, sky appears blue because blue light is scattered more (its called Rayleigh Scattering). During evening , sunrays travel a longer path in the atmosphere so blue light is scattered so much that we primarily see red. Dring a Lunar eclipse, most of the light reaching the moon is due to refraction from Earth’s atmosphere, which scatters away the blue light, so light reaching the moon is mainly red.