reflectivity
/ ree-flek-TIV-ih-tee /
Shine a flashlight at a mirror and almost all of it comes straight back; shine it at black velvet and almost nothing returns. Reflectivity is just the score for how much light a surface throws back rather than swallowing or letting through — a mirror scores near the top, soot near the bottom.
Formally, reflectivity is the fraction of incoming light energy that a surface bounces back, often written as a number between zero and one, or a percentage. Why a surface reflects comes down to a sudden change in how fast light travels: light bends and partly bounces wherever it meets an abrupt edge between two materials. Metals are spectacular reflectors because their loose sea of electrons can instantly slosh to cancel the incoming wave and fling it back. The value generally depends on the light's color and on the angle at which it strikes.
Reflectivity matters everywhere from mirrors and mirrored sunglasses to the silvery coatings that keep buildings and spacecraft from overheating, and the anti-reflection layers on camera lenses and solar panels that do the opposite. The common misconception is that a shiny look means high reflectivity at all colors; in truth gold reflects red and yellow strongly but rejects blue, which is exactly why polished gold looks warm rather than white like silver.
The blue-purple sheen you see on camera lenses is an anti-reflection coating: a film so thin that its two reflections cancel each other for most colors, dropping the lens's reflectivity from a glary 4% to a fraction of a percent so that more light reaches the sensor.
A coating thinner than a wavelength of light makes glass reflect less and transmit more.
Reflectivity is about how much light bounces back; refractive index is about how strongly the material bends and slows light. They are tightly linked — a big mismatch in refractive index across a surface is what makes that surface reflect more.