Angle of Incidence
Angle of Incidence is the tilt between a light ray and the flat surface it hits.
Think of it this way
A flashlight beam hits a flat tray of water. Stand a pencil straight up from the tray — that upright line is the normal. Shine the light straight down the pencil, and the angle between them is zero. Tilt the light to the side, and the angle grows — that is the angle of incidence.
Explaining angle of incidence by grade level
When you hold a book flat under a lamp, the light hits it straight on. Tilt the book and less light lands on it. A solar panel works the same way. The straighter the light hits, the more power you get.