Optics

Equation

i = r

5.1

Reflection

If you see something, it is reflecting light. We think of light as rays, thin beams of light that travel in straight lines. A beam of light striking a smooth flat surface such as a mirror is strongly reflected off the mirror, with the angle that the light ray bounces off the surface being identical to the angle it hit. The incoming angle is called the angle of incidence, and the angle of the outgoing ray is the angle of reflection. This Law of Reflection can be written as:

The angle of incident, i = the angle of reflection, r
i = r

Rollover the blue area to see the degrees of reflection.This is the easiest equation in physics!  If light strikes a surface at an angle of 30° from the vertical or normal, it bounces off at 30°. The reflection off smooth surfaces is called specular reflection – light bounces off the surface like it does off mirrors (which used to be called specula, singular speculum).

The Law of Reflection works for smooth surfaces, but irregular surfaces are another story. Tiny irregularities in surfaces of paper and other objects will reflect light in many directions. This type of reflection is called diffuse reflection. The reason we see most objects is because light strikes and reflects off their rough surfaces in several directions.

Scientists studying Saturn’s satellite Titan have used specular reflection to prove that liquids exist on that moon. Radar and other sensors had discovered large areas that looked smooth and could be liquid methane. The interpretation that it was liquid, rather than very smooth sands for example, was confirmed when specular reflection of sunlight was imaged by cameras on the Cassini spacecraft. Nearly all natural surfaces other than liquid produce diffuse reflections. Liquids will create diffuse reflections if winds roughen the surface, creating waves. It must have been a windless day on Titan when this image was captured.