Refraction, the bending of light as it passes from one medium to another, does occur at 90 degrees, but it might seem like it doesn't because the bending is minimal.

Here's why:

* Snell's Law: The fundamental law governing refraction is Snell's Law: n1 * sin(θ1) = n2 * sin(θ2)

* n1 and n2 are the refractive indices of the two media.

* θ1 is the angle of incidence (angle between the incoming light ray and the normal to the surface).

* θ2 is the angle of refraction (angle between the refracted light ray and the normal).

* At 90 degrees: When the light ray hits the interface at 90 degrees (normal incidence), θ1 = 0. This makes sin(θ1) = 0.

* To satisfy Snell's Law, sin(θ2) must also be 0, which means θ2 = 0.

* This means the light ray continues straight through the interface without bending.

However, there is still a slight change in the speed of light, even at normal incidence. The speed of light changes when it enters a medium with a different refractive index, even if the direction doesn't change. This change in speed is part of the refraction phenomenon.

So, while there's no visible bending of the light ray at 90 degrees, refraction still occurs as a change in the speed of light.