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Introduction to light
In recent years there has been there has been a movement towards Physically Based Rendering (PBR), where lighting setups mimic real-life light situations. Both realistic and cartoony work require believable lighting, so it is very much in your interests to get an appreciation of how light behaves in the real world.
Before we begin, I want you to remember that although there have been vast improvements in techniques and processing speeds to simulate light and matter in the digital world, you need to be aware that everything in the computer is just that – a simulation.
What is light?
So what is light exactly? Essentially, light is a type of wave; one that is composed of electric and magnetic fields. Hence it is referred to as electromagnetic radiation. From the electromagnetic spectrum, which consists of radio, microwave, infrared, ultraviolet, x- ray and gamma rays, the human eye is only able to see a small portion of this spectrum, which we call light. As light is emitted, small packets of energy called photons are produced, and it is these photons that allow us to view the world around us.
Reflection, refraction and absorption
Our first stop is to take you back to your physics classroom and reacquaint you with the phenomena of reflection, refraction and absorption.
In reflection, when light rays hit a smooth surface such as a mirror, they bounce off at an angle equal to the angle at which the rays hit the surface. The majority of surfaces are not smooth, however, which brings us to the concept of scattering, whereby light rays hit a rough surface and bounce off at multiple angles.
Refraction occurs when light rays pass from one transparent medium to another, for example from air to water. As this happens, the speed of the light changes and the ray bends either towards or away from the normal line, which is an imaginary line running perpendicular to the surface of an object. The degree to which the light bends is known as the angle of refraction, and its size depends on the degree to which the object has slowed the light rays down.
The higher a material’s index of refraction, the more it slows down incoming light. Diamonds have a high index of refraction, and sparkle so very famously by effectively trapping light. Lenses work by refracting light and can serve to improve a person’s vision by making distant objects seem nearer or vice versa.
Staying in the physics classroom, you may remember splitting light using a prism and seeing that white light is made up of the colours of the rainbow. The fact, therefore, that the world around us does not have colour of itself, that objects simply reflect, refract or absorb colour and therefore appear colourful, is pretty mind bending.
The colour of an object is the colour of the light it reflects: your red Ferrari parked outside is not red, it just reflects red wave lengths; the grass of your croquet lawn is green because it reflects green wave lengths.
Objects appear white because they reflect all colour, and conversely, objects appear black because they absorb all colour and in that absorption, heat is created.
This brings us neatly to the topic of colour temperature. Each light source, be it a candle, the sun or a headlamp, has its own individual colour, or colour temperature which varies on a scale from red (warm) to blue (cool). Colour temperature is recorded in Kelvin. Cool colours such as blue will have a Kelvin of over 7000, while warmer colours such as orange or red will measure around 2000K. Daylight, with a Kelvin of around 5000 comes around the middle of this scale. So when you think about the physical world, remember that the hotter an object, the bluer it will be, and the cooler an object, the more red it will be.
Bill Nye the Science Guy – S01E16 Light and Color
This video by Bill Nye is a fantastic primer to get a solid understanding of light and color.
- Light is It a Wave or a Particle? Just what is the true nature of light? Is it a wave or perhaps a flow of extremely small particles? These questions have long puzzled scientists. Let’s travel through history as we study the matter.