Introduction to Light

Reflection is the bouncing of light in another direction when it hits a surface in its path. Just like a rolling ball bounces off a wall at an equal angle, light follows precise rules whenever it strikes a reflecting surface.

Key terms of reflection

• Incident ray is the incoming beam of light that travels toward a surface like a mirror.
• Point of incidence is the exact spot where the incident ray strikes the surface.
• Reflected ray is the beam of light that bounces off the surface and travels away.
• Normal is a line drawn at ninety degrees to the reflecting surface at the point of incidence.
• Angle of incidence is the angle between the incident ray and the normal, and angle of reflection is the angle between the reflected ray and the normal.

The laws of reflection

The laws of reflection govern how light bounces off a reflecting surface. First, the angle of incidence is always equal to the angle of reflection, meaning that light hitting at thirty degrees bounces off at thirty degrees. Second, the incident ray, the reflected ray, and the normal line at the point of incidence all lie within the exact same plane.

How laser microphones work

A laser microphone uses reflected light to hear conversations inside a closed room. When someone speaks, sound makes the surrounding air molecules vibrate, causing the window glass to vibrate as well. A laser shone at the window reflects off the glass while matching its movements. A sensor catches this reflected light and translates the vibrations back into sound.

Locating a target using reflection

Knowing the angle of incidence allows surveillance teams to calculate where the reflected beam will bounce so they can catch it with a sensor. Teams used this technology to bounce a laser off a window of a compound in Pakistan. The reflected light captured the vibrations of speech inside, allowing voices to be separated and compared to past recordings to confirm who was there.