Ambient light is all around us and is the result of the sun and lamps scattering photons every which way, reflecting off of and wavelengths being absorbed by objects. Rather than try to calculate the exact intensity of each and every photon, use the Ambient light settings to generally light the scene.
You can specify the amount of Ambient light (the color of which is set in the world settings); use an off-white to simulate the color of the ambient lighting inside closed rooms or on different planets based on its sun’s visible spectrum. Generally, about half of RGB gives a nice soft lighting to a scene.
Setting Ambient Light
You set the color of ambient light by clicking on the color swatch and using the color picker to choose your color, or by manually adjusting the RGB sliders, or LMB clicking on the slider value (number) and entering a number using your keyboard. These fields are outlined in red in the example above (World panel, World sub-context).
This simple example of ambient light settings mimics Earth on a sunny day. The color of the sky at the horizon is white with a touch of haze and a tinge of blue, and the zenith overhead is a dark blue. The sun is out and bright, with a yellow tinge, as on a cloudless day. Think Caribbean, and you’ll get the idea. London in the fog is grey; night-time is black, etc.
Material Ambient Effect
Each object’s material settings (Shaders panel) has an Ambient slider that lets you choose how much ambient light that object receives. Generally, about 0.1 to half is good, depending on the color of the ambient light. A setting of 0.0 means that the object does not receive any ambient light; it is only lit by light that actually gets to it. A setting of 1.0 means that it is lit by all of the world’s ambient light. Increasing the Ambient setting has the effect of flattening the shading, since the ambient light washes out the diffuse shading.
You should set the material slider as shown above in the three cube example, based on the amount of ambient light you think the object will receive. Something deep in the cave will not get any ambient light, whereas something close to the entrance will get more. Note that you can animate this effect, to change it as the object comes out of the shadows and into the light.
World Ambient Occlusion
A trick for games is to illuminate surfaces based on how far they are away from occluding geometry, called ambient occlusion. If looks like ambient light is lighting the objects in the scene, and is the process of Ambient Occlusion. The color of the “light” applied to surfaces is affected by the world ambient color. See wiki page on Ambient Occlusion.
Radiosity is the process of applying radiated light from objects in the scene as they add to the color of nearby ambient light, which in turn affects the color of the ambient light. See wiki page on Radiosity.