Put together, Reflection literally translates to "the act of bending back." In physics, it is the process where a wave (like light) hits a surface and bounces back into the medium it came from.
When light travels and hits a smooth surface like a mirror, it doesn't stop. It "bends back." The rule it follows when bouncing is perfectly predictable.
The Angle of Incidence (i) ALWAYS equals the Angle of Reflection (r).
The incoming beam of light that is travelling toward the surface. ("Incidence" comes from a Latin word meaning "to fall upon").
The beam of light that has bounced off the surface and is travelling away from it.
An imaginary dashed line drawn exactly perpendicular (at a 90-degree angle) to the surface where the light hits. We measure our angles from this line!
We measure the angle between the rays and the Normal line. No matter how you tilt a mirror, angle i and angle r will be identical.
If light bounces off almost everything we see, why can you see your face in a mirror, but not in a painted wall?
The answer lies in how the light "bends back." A mirror is incredibly smooth at a microscopic level. When perfectly parallel rays of light from your face hit the mirror, they all hit at the exact same angle. Because of the Law of Reflection, they all bounce back totally parallel to one another. This organized bouncing is called Specular Reflection, and it creates a clear image.
A painted wall, on the other hand, is microscopically bumpy and rough. When parallel rays of light hit those tiny bumps, each individual ray hits a slightly different angle. They all still follow the Law of Reflection, but because the "surface" is angled differently at every microscopic point, the light rays scatter in thousands of different directions! This scattered bouncing is called Diffuse Reflection. The light reaches your eyes, allowing you to see the wall, but the organized image of your face is completely scrambled.