The sky appears blue instead of red because of a phenomenon called Rayleigh scattering. When sunlight enters the Earth's atmosphere, it is made up of different colors of light, which we see as white light. This white light is a combination of all the colors of the rainbow.
Blue light has a shorter wavelength and is scattered more easily by the molecules in the atmosphere compared to red light, which has a longer wavelength. The molecules in the atmosphere, like nitrogen and oxygen, are much smaller than the wavelength of light.
When sunlight enters the atmosphere, the blue light is scattered in all directions by the molecules. This scattered blue light reaches our eyes from all parts of the sky, giving it a blue color. On the other hand, red light is less scattered and passes through the atmosphere without much change in direction.
The scattering of blue light is why we see a blue sky during the day. However, during sunrise or sunset, when the sun is lower in the sky, the light has to pass through a larger portion of the Earth's atmosphere. This results in a longer path for the light to travel, and more of the blue light is scattered away. As a result, the sunlight that reaches our eyes is mostly red and orange, giving the sky a reddish hue.
In summary, the sky appears blue during the day because blue light is scattered more easily than red light by the molecules in the Earth's atmosphere. The scattering of blue light in all directions is what gives the sky its blue color.