Here's why:

* Refractive index: This is a measure of how much a material slows down light.

* Snell's Law: This law describes the relationship between the angles of incidence and refraction, and the refractive indices of the two mediums. It states:

* n1 * sin(θ1) = n2 * sin(θ2)

* Where:

* n1 is the refractive index of the first medium

* θ1 is the angle of incidence

* n2 is the refractive index of the second medium

* θ2 is the angle of refraction

* Equal refractive indices: When n1 = n2, Snell's Law becomes:

* sin(θ1) = sin(θ2)

* Equal angles: For the sine of two angles to be equal, the angles themselves must be equal. Therefore, θ1 = θ2.

In simpler terms: If light enters a material with the same "light-slowing power" as the material it's coming from, it won't change direction.

Examples:

* Light passing from air to another air layer.

* Light passing from water to another body of water.

Note: This situation is often referred to as "no refraction" because the light doesn't bend.