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:

  • Absorption
  • Reflection
  • Scattering
  • Refraction

Absorption

Light energy goes into the object itself. Because the light goes into the object rather than leaving the surface of the object - and then some of that light entering the eye - the object is not "seen" as very bright. Instead, it is perceived to be dark (meaning that little light is traveling from that object into the eye). However, the object might still be obvious to a viewer, e.g. a dull matt-black object would still be seen if observed on a clean white surface. In that case the contrast makes the presence of the dark object obvious.

In general, dark objects are move likely to absorb light energy, while objects that are light (in colour) are more likely to reflect or scatter the light energy they receive.

Reflection

Light reaches the surface of a very shiny object and 'bounces' off the object in the same way as a hard ball would bounce off an even flat surface (e.g. as in the game of snooker).

That is, reflected light leaves the surface of an object at one particular angle relative to the angle from which it reached that surface.

Law of Reflection:

Angle of incidence = Angle of reflection (i.e. the same angle)

Scatter

On reaching the surface of an object, light leaves that surface not in any one particular direction, but in many directions spread over a wide range of angles. This applies particularly to non highly-polished surfaces, such as paper, or walls painted matt white.

Scatter is the most common of these possibilities when visible light is incident on ordinary everyday solid/opaque objects.

Refraction

This is another case of light entering the object instead of leaving the surface of the object.

Refraction only applies to objects that light can pass through, such as blocks of glass or plastic, windows, water, and spectacles. It is mentioned here for completeness.

In the context of explaining how light reaches a person's eye from objects in the real world in front of him or her, refraction is less important that the other possibilities described above. However, refraction plays an important role in the eye and visual system for other reasons, such as focusing images onto the retina. It is therefore explained later in this section.

So

  • 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 ?

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