Light questions

UV radiation was discovered in 1801 when the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light itself.

“Camera” means “chamber” and “room” in Latin. Camera obscura was an optical device used in drawing, and an ancestor of modern photography. This is a source of the modern word camera.

The speed at which light propagates through transparent materials, such as glass or air, is less than c. However, this "slowing of the light" is only due to the fact of photos being absorbed and re-emitted after a slight delay between atoms. When a photon is emitted, it's speed is always c. In exotic materials like Bose–Einstein condensates near absolute zero, the effective speed of light may be only a few metres per second.

A terminator or twilight zone is a moving line that divides the daylit side and the dark night side of a planetary body. A terminator is defined as the locus of points on a planet or moon where the line through its parent star is tangent. An observer on the terminator of such an orbiting body with an atmosphere would experience twilight due to light scattering by particles in the gaseous layer.

A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.

Luciferin is a generic term for the light-emitting compound found in organisms that generate bioluminescence. Luciferins typically undergo an enzyme-catalysed oxidation and the resulting excited state intermediate emits light upon decaying to its ground state.

Sound travels through air at 331.2 meters per second, so after 3 seconds it would travel approximately 1 kilometer.

The Sun is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. Hot plasmas (thermal plasmas) have electrons and the heavy particles at the same temperature, i.e. they are in thermal equilibrium with each other. Cold plasmas (nonthermal plasmas) on the other hand have the ions and neutrals at a much lower temperature, whereas electrons are much "hotter".

The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source

Wearing sunglasses without UV filter causes the pupils to dilate and eyes to absorb more UV rays than they normally would without the sunglasses. Hence, it's much better not to wear sunglasses at all, than to wear ones without UV filter.