I was in the shower watching the water droplets sparkle. Like the cats eyes in the road, the physics of the light bouncing around is a beautiful thing.
The internal refractive index of light in transparent spheres has lead to many practical safety applications, from reflective safety jacket to 3d augmented reality glasses (See CastAR) with retro-reflective materials.
Controlling how light bounces around is truly incredible.
To understand this interesting concept we need to know whats happening in stages.
The water falls from the shower head, while falling the water briefly experiences zero gravity or more accurately, relative neutral gravity.
The water then gets pulled apart into smaller quantities (due to the inconsistencies in size and wind resistance), which then (due to the even surface tension on the water droplet) forms a near perfect sphere (for the sake of argument).
The light from the viewers perspective goes straight into the water droplet, hits the back and gets reflected back to the viewer on the identical path.
If the light reflected back at the viewer is, more than the reflecting from the background, the droplet will seemingly radiate light.
Diamonds are a fascinating example of a well calculated refractive index.
It relies on the light entering the stone, hitting the back surfaces or passing through one path and coming out another. Because the cut diamonds are faceted to
a specific pattern chosen for the size, the light will hit a surface and reflect light to the observer until rotated when the light has passed, causing the diamond to sparkle.
The art is working out how to get the biggest clean diamond from the sample and calculating the best number of facets for its size.
Internal reflections can sometimes be a bad thing. Some early camera lenses had internal reflections. The problem was the light bounced of the elements in a way not intended by the manufacturer. In almost all camera lens designs the light has to be controlled to pass straight through the lens to a focal point without bouncing back of the lens surfaces (mirror lenses among others are an exception to this rule). If light reflects of the elements, it causes stray light, which intern causes a lack of contrast as the blacks are now not true black but washed out by the stray light.
So a big hats off to the diamond cutters and jewelers of the world that make gems sparkle. It really is a art and a science.
Yours, The Enginer’d