Why is the index of refraction for air smaller than the index of refraction for a solid like glass?

The index of refraction is a measure of how much light slows down when passing through a medium. In solids like glass, the atoms are closely packed together, causing the light to interact more with the atoms and slow down more than in air where the atoms are more spread out. This difference in atomic arrangement leads to a higher index of refraction in glass compared to air.