When a ray of light passes from one medium to another normally, meaning it hits the interface at a 90-degree angle, it does not deviate. This is because the angle of incidence (the angle between the incoming ray and the normal to the surface) and the angle of refraction (the angle between the refracted ray and the normal) are both zero.

Here's why:

* Snell's Law: This law governs the refraction of light. It states:

* n1 * sin(θ1) = n2 * sin(θ2)

* Where:

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

* θ1 is the angle of incidence.

* θ2 is the angle of refraction.

* Normal Incidence: When θ1 = 0 (normal incidence), sin(θ1) = 0. Therefore, for the equation to hold true, sin(θ2) must also be 0, meaning θ2 = 0.

In other words, if the light enters the second medium perpendicularly, it will continue traveling in a straight line without changing direction.