Top 10 light facts

Blue-eyed people. The iris plays no role in the effect, but the blue-eyed people have relatively low melanin in the fundus as well. Therefore, they show a much stronger red-eye effect, as the light passing fundus is not absorbed by melanin. The role of melanin in red-eye effect is nicely demonstrated in case of heterochromia (one eye blue, one brown): typically, only the blue eye displays the effect.

A speed of light is always constant. 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.

Quasars. A quasar is an extremely energetic source of radiation powered by accretion disc around galaxy's central supermassive black hole. Quasars were much more common in the early universe, as all known quasars are extremely distant.

Light painting. The photo was taken with long exposure. For the first 15 seconds, the author hold still with lights on, to achieve the light in the background. Later, he switched off the light and wrote his name with light in his hand. He used his nose as a reference point so he could close the "O" letter - which explains the flare at this position.

Tonic water is transparent in daylight. Under ultraviolet light, the quinine makes it fluorescent (pictured), even though it is present in a negligible concentration.

Rayleigh scattering, named after the British physicist Lord Rayleigh (John William Strutt), is an elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the radiation.

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.

Reflection, refraction and dispersion . 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.

One hundred billion stars. Supernovae are extremely luminous and often briefly outshine an entire galaxy. No supernova has been observed in the Milky Way since 1604, but they are often observed in distant galaxies.

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.