You're likely thinking of the case where a light ray enters a medium at a perpendicular angle, not "pependicular." In this case, the light ray does not deviate, meaning it continues in a straight line without bending. Here's why:

* Snell's Law: This law governs the refraction of light as it passes from one medium to another. It states:

* n1 * sin(θ1) = n2 * sin(θ2)

* where:

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

* θ1 is the angle of incidence (angle between the incoming ray and the normal)

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

* Perpendicular Incidence: When the light ray hits the surface perpendicularly, the angle of incidence (θ1) is 0°. Since sin(0°) = 0, Snell's Law becomes:

* 0 = n2 * sin(θ2)

* Result: For this equation to hold true, sin(θ2) must also be 0. This means θ2 = 0°, which means the refracted ray also travels at a 0° angle to the normal, continuing in a straight line.

In summary: When a light ray enters a medium perpendicularly, there's no change in direction because the angles involved in Snell's Law are both 0°.