Some of our common terms are based on wave theory, others on photon theory.  There are several terms and concepts which we will use regularly:

DIVERGENT lines spread apart as if originating from a point.  Divergent rays can be projected back to form a VIRTUAL  (imaginary) point or image.  A virtual image cannot be projected onto a screen,  it is as if an image were there.  This is also called NEGATIVE VERGENCE.

CONVERGENT rays are rays that come together to meet at a point.  Convergent rays of light will form a REAL image.  A real image can be projected onto a screen.  This is also called POSITIVE VERGENCE.

 Diagram 1.3:  Real image formation.

PARALLEL light rays are rays that never meet.  They have ZERO VERGENCE.

 Diagram 1.4:  No image formation.

The further light travels from its source the less vergence it has.  The rays may eventually be considered parallel.  Light from a distance greater than 20 FEET or 6 METERS is considered to be parallel.  This is called OPTICAL INFINITY.

 WAVES:         CONVERGENT           DIVERGENT                         PARALLEL
Diagram 1.5:  Vergence of waves.

These definitions work just as well with waves as with rays.  If the waves are approaching the "source" they are converging and have positive vergence, if they are leaving the source they are diverging and have negative vergence, and if they travel far enough from the source (optical infinity) then they are parallel to each other.

The further the lens is from the source, the smaller the biggest angle between rays reaching the lens.  In diagram 1.6 the rays with the maximum negative vergence describe a much wider angle for lens 1 than for lens 4.    Lens 1 has the most negative vergence, since is is closest to the source.  Lens 4 has the least vergence, since it is farthest from the source.

Vergence is the reciprocal of distance, measured in meters.  If the distance of lens 1 from the source is 0.25 meters, then the vergence of the rays at lens 1 is 1/0.25 = 4.  If the distance of lens 3 from the source is 50 cm = 0.5 meters, then the vergence of the rays at lens 3 is 1/0.5 = 2.

Vergence is noted with a capital letter.  If the distance for lens 2 from the source is f, then the vergence of the rays at lens 2 is F = 1/f.  If the distance for lens 4 is q, then the vergence of the rays at lens 4 is Q = 1/q.

Particles of light are called PHOTONS.  A photon is the smallest amount of light possible.

A RAY is the path of a single photon of light from a single point on a light source.  A ray would pass through the tiniest hole imaginable in each of two screens.

Diagram 1.7:  One photon.

A PENCIL of light is a group of rays divergent from a single point on a light source.  A pencil of light would pass through one small hole and one large hole in each of two screens.

Diagram 1.8:  A pencil of light.

A BEAM of light is composed of the group of pencils originating from all of the points on a light source.

Light travels at

186,276  miles  per  second  in  AIR
122,100  miles  per  second  in  CROWN  GLASS
139,800  miles  per  second  in  WATER

Light ABSORBED by an object may be absorbed as heat.  Dark colors absorb more than light colors do.  We tend to wear light colors in the summer to help stay cool,  and we tend to wear dark colors in the winter to help us become warm.  Light can also be converted into electrical energy, as happens with silicone solar energy cells.  Or, light can be absorbed as chemical energy, such as happens with photosynthesis in green plants, or as happens when our photochromatic lenses turn dark in the presence of light.

REFLECTION  is the turning of the light ray back into the incident medium.

The light that is reflected by objects is what gives the objects their color.  An object appears to be black if it absorbs all of the wavelengths of the visible spectrum.  An object appears to be white if it reflects all of the wavelengths of the visible spectrum.  An object appears to be red if it reflects light from the red end of the spectrum, and absorbs the rest.

REFRACTION is the bending or change in direction of light when it passes from one transparent medium to another of different optical density.

Three factors determine how much a ray of light will be refracted:

1. The medium itself:  each medium has a different optical density and therefore a different bending ability.
2. The obliquity of the incident light:  light incident normal to a surface is not bent,  but is slowed down.  Light incident at any other angle changes direction.  The greater the angle the more the light ray is bent.
3. The wavelength of the incident light.  This is related to the medium -- because different wavelengths will be slowed different amounts.

Copyright 2001, Ellen Stoner, MALS, ABOM