13.1. Fundamentals of Light http://www.ck12.org
Color
The colors that we see are blends of some number of individual frequencies of visible light. The surface of materials
have chemical structures that cause particular frequencies of light to be either absorbed or reflected. Our eyes pick
up the frequencies that are reflected (or generated) by the object and the frequencies are blended so that we see one
color. If the surface of an object absorbs all frequencies of light, we see black. If all frequencies are emitted or
reflected, we see white. If you had a book whose cover reflected only blue light and you placed this book in the
presence of white light from an incandescent light bulb, all the colors except blue would be absorbed and the blue
would be reflected, allowing you to see a blue book. If you placed the same book in the presence of only red light,
no light would be reflected from the book and you would see a black book.
Discussions of colors can sometimes become confusing because of the difference between light emitters and light
absorbers. Light absorbers, such as dyes and pigments, absorb frequencies from those that you see, while light
emitters add frequencies to what you see. If I were to shine red light, green light, and blue light into your eye all at
the same time, you would likely see some variation of white. If I were to mix red paint, green paint, and blue paint
together, you would likely see black or some very dark color.
If an object emits light waves, we say the object isluminous(generates light). If an object merely reflects light
that falls on it, we say the object isilluminated(reflects light). Many objects allow light rays to pass through them
without disrupting the organization of the rays. When this happens, you can clearly identify the objects from which
the light came. Objects that allow light to pass through undisrupted are said to betransparent. Air, glass, some
plastics, and some other materials are examples of transparent objects. There are also objects that allow light to pass
through them but they disrupt the rays so that the objects from which the light came cannot be identified. Objects
that allow light to pass through but disrupt images are said to betranslucent. Lampshades, frosted light bulbs, and
fogged glass are examples of translucent materials. Many other objects do not allow any light to pass through. Such
objects are said to beopaque.
Polarization
Remember the illustration of electromagnetic radiation given above? These electric and magnetic waves may
oscillate in any direction. That is, the oscillating fields may be oscillating vertically, horizontally, and in every
other direction. Light typically consists of oscillating fields in all directions, but always travels in a straight line.
Filters can be constructed such that only light with the fields oscillating in a certain direction can pass through the
filter. This is much like having a large number of ropes passing through the slots in a picket fence. The ropes have
transverse waves oscillating in all possible directions moving toward the fence. When the waves encounter the fence,
only the oscillations that are vertical and fit through the slots in the fence will be allowed to pass through. Light that
has all its field oscillations in the same direction is said to bepolarized.