To understand the nature of diffused light, humans must first answer the question, "What is light?" Physicists define light as electromagnetic radiation. Traditional theory holds that light moves as a wave. Its amplitude gives the brightness, and the differing wavelengths make the different colors. Modern quantum theory says that particles of energy called photons make up light. The number of photons gives the brightness, and the energy in the photons creates its color. Both theories are correct. Light acts as both particle and wave. Simply put, light is that which enables humans and other animals to see.
TL;DR (Too Long; Didn't Read)
Light diffuses when it bounces off the many angles of a rough surface, or when it travels through a substance that changes its angles.
How Do Humans See?
We see an object because it emits light (for example, the sun, fire, a light bulb), or we see objects that reflect light.
What Is Diffused Light?
Diffused light is a soft light with neither the intensity nor the glare of direct light. It is scattered and comes from all directions. Thus, it seems to wrap around objects. It is softer and does not cast harsh shadows.
What Causes Light Diffusion?
When a light beam strikes a smooth surface, most of it reflects back in the same concentration. This is specular reflection, which gives us direct, bright light. A mirror is a common example of a smooth surface that causes specular reflection. On rough surfaces, even microscopic irregularities create roughness. This does not break the law of reflection. Each ray reflects back at the same angle at which it struck the object but in a different direction. So diffuse light is scattered light. This scattering is what causes the diffusion and softness of the light beam.
Application of Diffuse Light
Photographers use the principle of diffuse light to create pictures with vivid details because there are no sharp shadows to distract attention. On a sunny day, they use light diffusers to create soft shadows. Horticulturists are now discovering that diffused light creates a better growing environment in greenhouses. It allows for a greater horizontal spread of light and exposes the middle leaf layers to light. Drivers find that wet roads have a greater glare than dry roads, because the cracks and crevices on the road's surface fill up with water, creating a smooth surface. This results in specular reflection that creates the annoying glare. Fog lamps try to make use of the principle of diffused light to provide a safer beam.
The human eye cannot see all the rays on the light spectrum. Infrared rays, for example, are too long for the human eye to perceive, and ultraviolet rays are too short. So, direct (spectral) light seems stronger than diffuse light. However, the total light transmission is the same.