When light reaches the surface of an object
This explanation of the basic physics of light continues from the page introduction to light.
Light reaches objects from many different sources e.g. from large and powerful sources of illumination such as the sun or the main lights in a room, and also by reflection or scattering from surrounding objects e.g. reflection from mirrors, windows, buildings, the sea, lakes or ponds.
In general, when light reaches an object it can do one or a combination of:
- Light reflects from some surfaces, scatters from other surfaces, and is absorbed by some (dark non-shiny) surfaces.
- In the cases of objects that light can pass through, e.g. glass or water, light may be refracted at the surface of the object. Not necessarily all of the light is refracted though - a proportion of the light may be reflected, and some may be scattered or absorbed, especially if the surface is dirty or textured e.g. patterned or "frosted" glass.
What determines which of these possibilities will apply in any particular situation ?
A solid opaque object absorbs, scatters, or reflects light (or some combination of these) depending on its physical properties, including the properties of the material the object is made or formed from.
Aspects of objects that influence the onward path taken by light reaching their surfaces include:
- the physical state of the object (solid, liquid, gas),
- the substance it is formed from (wood, rock, glass etc.),
- the texture of its surface (rough, polished, carved, etc.),
- the thickness of the object / material (thin sheet of ice, or huge iceberg), and even
- its colour.
Other factors that affect how much light is absorbed, reflected, and scattered concern the light itself and how it arrived at the surface of the object, such as:
- the wavelength (colour) of the light, and
- the angle at which it reaches the surface.
Light also has other properties (e.g. polarization states) that are more complicated to explain but are also relevant to some aspects of vision (e.g. explaining polarizing sunglasses). These are omitted from these introductory pages.
In simple terms, the same object, e.g. a cube, is more like to:
- Absorb light - if it has an opaque matt (non-shiny) black surface
- Scatter light - if it has an opaque matt (non-shiny) white surface
- Reflect light - if it has a shiny finish e.g. mirror surface
- Refract light - if it is transparent to the wavelength of light that reaches its surface (e.g. if it consists of colourless glass and is illuminated by visible light e.g. a green laser beam) and the light reaches the surface of the object within a certain range of angles.
Diagrams to show the difference between light being reflected from an object, and light being scattered from / by an object ?
Remember the diagrams used to show reflection, compared with scattering:
Reflection vs. Scattering:
When light is reflected at a surface, it leaves that surface in a specific direction (according to the Law of Reflection).
When light is scattered at a surface, it leaves that surface in very many different directions.
An example of a reflecting surface is a high-quality mirror.
An example of a scattering surface is a sheet of good quality matt white paper.
In the real world (as opposed to in scientific theory), most objects are mostly-reflective, mostly-scattering, or mostly-absorbing - but some proportion of incident light may behave in the other ways.
Most light-coloured objects around us at home, at school, and in offices are mostly-scattering.
So, during daylight or in an illuminated area, light bounces off most objects - predominately by scattering, but in some cases also by reflection.
It is necessary to know the above in order to understand the eye and vision because eyesight, which is also called sight, seeing, vision and visual perception, is the perception by humans (and other animals) of light received by the organism's eye(s) from objects in a scene, sensed via the eyes, then eventually processed by the brain.
NEXT : Read about What is eyesight ?