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LUZ - Tutorial a detalle.

PARTE 1: Básico
A traves de este artículo utilizaré un diagrama con una bola blanca que esta sobre un plano blanco para demostrar el comportamiento de la luz en diversas situaciones:

Here it illustrates a sunny afternoon. The main source of light is the sun, whilst the blue sky supplies a second source of light with very different characteristics. Somelight is also bouncing between the white base and the ball and supplies a third source of light.
The brightest light is coming from the sun and is white light emanating from a small source, which causes it to cast sharp edged shadows. The second source, the blue sky, is a very large light source and as a result has very soft shadows (which in any case are completely masked by the direct light comingfrom the sun). I will go into more detail later about light sources and shadow size, but for now just remember that the smaller the source of the light, the harder the shadows.
The light coming from the blue sky has a very strong colour cast which affects everything in this scene. The shadow cast by the ball is blue because it is illuminated by blue skylight, since the ball is shielding it fromthe white light of the sun. The parts of the ball which aren't in direct sunlight also take on a blue hue because they are lit by the blue sky.
Finally the light that is reflected between the card and the ball is also predominantly blue (even though the card and ball are white) since it is blue skylight that is being reflected by the white objects. The surfaces which are closer together receivemore of this reflected light than areas which are further apart, therefore the bottom part of the ball is lighter than the centre because it is nearer the white card.

The darkest areas in the image are the base of the cast shadow and the border between the areas in sunlight and shade on the ball: this zone is called the terminator. The base of the cast shadow is very dark because it receives nosunlight and the ball is also masking it from most of the skylight and bounced light. The other end of the cast shadow is lighter because it is receiving more light from the sky and also bounced light from the ball.

Why is the terminator the darkest area on the ball?
Partly because of the effect of contrast, being so near to the very bright side of the ball in sunlight makes it appear to bedarker, but also it is receiving less of the bounced light which is being reflected by the white card. So unlike the rest of the ball, which is receiving either full sunlight or light reflected from the white card, its main source of illumination is the blue sky. It is the area in between the main light (the sun) and the fill light (the reflected light from the card).

Why is the light from thesky blue?
Visible light is made up of tiny particles called photons, these particles have different wavelengths depending on their colour: blue light comprises of particles with shorter wavelengths whereas red light is made of particles with longer wavelengths.

White light from the sun is made up of a continuous spectrum of colours which, conventionally, is divided into the colours of therainbow (with progressively longer wavelengths: violet, indigo, blue, green, yellow, orange and red). It is the mixture of these colours that produces white.
However what happens to light when it travels through the atmosphere of the earth is that the shorter wavelengths of light become scattered. Our atmosphere is made from various gases and the atoms and molecules that these are formed from aresuspended within it. Photons travelling through the atmosphere physically collide with these atomic particles and a collision will deflect the photons and make them bounce in another direction. Shorter wavelengths are more likely to be deflected than longer ones, so that the photons which are scattered in all directions by these collisions are predominantly blue.

On a cloudless day blue light...
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