1. The light ray must strike the boundary at an angle: If the light ray hits the boundary perpendicularly (at 90 degrees), it will pass through without changing direction. Refraction only happens when the light ray hits the boundary at an angle, causing it to bend.
2. The two mediums must have different refractive indices: The refractive index of a medium is a measure of how much light slows down when it travels through that medium. When light moves from a medium with a higher refractive index to a medium with a lower refractive index (e.g., from water to air), it bends away from the normal (the line perpendicular to the surface). Conversely, when light moves from a lower to a higher refractive index (e.g., from air to water), it bends towards the normal.
