As the sun sets, its light has to travel through more of the Earth's atmosphere to reach our eyes. This means that more of the sun's light is scattered away by molecules in the atmosphere, so the sun appears dimmer.

Here's a more detailed explanation of what happens:

1. Scattering of sunlight: When sunlight passes through the Earth's atmosphere, it interacts with molecules of gases and particles in the air. These molecules and particles can scatter the sunlight in all directions.

2. Blue light is scattered more: The shorter wavelengths of light, such as blue light, are scattered more by molecules and particles in the atmosphere. This is why the sky appears blue during the day.

3. Sunlight travels through more atmosphere at sunset: As the sun sets, its light has to travel through more of the Earth's atmosphere to reach our eyes. This means that there are more molecules and particles in the atmosphere that can scatter the light.

4. More scattering means less sunlight: Because more of the sunlight is scattered away by the atmosphere at sunset, less of it reaches our eyes. This is why the sun appears dimmer at sunset.

In addition to scattering, the sun's light is also absorbed by molecules and particles in the atmosphere. This absorption also contributes to the sun's appearance as being dimmed at sunset.

The combination of scattering and absorption by the atmosphere is what causes the sun to appear red, orange, and yellow at sunset.