The normal line is a line at a right angle (90 degrees) to the surface of an object

To draw a normal line to the mirror, you must draw a line at 90 degrees to the surface of the mirror.

Refraction Snell Tir

What is the minimum value of refractive index required for a right angle isosceles prism which is used to deviate beam through 90° by TIR? - Quora

homework and exercises - Why do electric field lines start and end at 90 degree at the surface of a conductor? - Physics Stack Exchange

Normal line - Tilted flat mirrors

Area Of A Non-Right Angle Triangle

Physics WebAssign Ch22 #12

Physics Tutorial: Right Angle Mirrors

If the incident ray makes an angle of with the normal, then what is the angle of reflection? - Quora

5 Ways to Find the Length of the Hypotenuse - wikiHow

Angle of Incidence - Definition, Formula, Diagram, Examples