Question

why does light refract when it crosses from air to glass?

Explanation:

Light refracts when moving from air to glass because the speed of light changes as it enters a medium (glass) with a different optical density than air. Glass is optically denser than air, so light slows down upon entering glass. When light hits the glass - air interface at an angle (not perpendicular), the change in speed causes a change in direction, which is refraction. The phenomenon is governed by the laws of refraction (Snell's Law), which relate the angles of incidence and refraction to the refractive indices of the two media. The refractive index of glass ($n_{glass}$) is greater than that of air ($n_{air}\approx1$), and the formula $n_1\sin\theta_1 = n_2\sin\theta_2$ (Snell's Law) describes how the angle changes with the change in speed (since $n=\frac{c}{v}$, where $c$ is the speed of light in vacuum and $v$ is the speed in the medium).

Answer:

Light refracts when moving from air to glass because the speed of light changes (it slows down as glass is optically denser than air) when entering a medium with a different optical density, and if the light hits the interface at a non - perpendicular angle, this speed change causes a change in direction (governed by Snell's Law: $n_1\sin\theta_1 = n_2\sin\theta_2$, where $n_1$ and $n_2$ are refractive indices of air and glass, and $\theta_1$, $\theta_2$ are angles of incidence and refraction).