* Refraction: Light bends (refracts) when it passes from one medium to another because the speed of light changes in different materials. The denser the material, the slower light travels.

* Angle of Incidence: The angle at which light strikes a surface is called the angle of incidence.

* Angle of Refraction: The angle at which light bends is called the angle of refraction. This angle depends on the angle of incidence and the difference in the speed of light between the two materials.

* Perpendicular Entry: When light enters a glass square perpendicularly, the angle of incidence is 0 degrees. In this specific case, the angle of refraction is also 0 degrees. This means the light continues traveling straight through the glass without bending.

Think of it like this: Imagine a car driving straight onto a highway. If it enters the highway at a 90-degree angle, it won't change direction. It's the same with light entering glass perpendicularly.

Important Note: If the light enters the glass square at any angle other than 90 degrees, it will bend. This is because the angle of incidence will be greater than 0 degrees, resulting in a non-zero angle of refraction.