contribute to this mind boggling capability.
You do not need a telescope to understand what is meant by resolving power. You can
experience it simply by looking at a car’s headlights at night. At a large distance, the
car’s lights look as if they come from a single source, in one blur of light. As the car
comes closer, the blur resolves into two separate sources of light.
Diffraction causes the stars and the headlights to “blur” together. Concept 2 shows the
diffraction pattern created by two sources that are far enough apart that their diffraction
patterns do not significantly overlap. Concept 3 shows what occurs when the diffraction
patterns overlap. One “blurry” central image is the result.Two resolved images
Diffraction patterns remain separate
Blurry image
Significant overlap of diffraction patterns
34.9 - Resolving power of the eye
Human eyes, and those of other animals, resolve objects. From ten meters away, a
person with good eyesight can resolve two objects separated by only three millimeters.
Some animals can do far better. At the same distance, an eagle can resolve two objects
that are only 0.8 mm apart. This ability allows eagles in flight to pick out potential prey
on the ground with amazing accuracy. For example, an eagle can distinguish a rabbit
from its surroundings from as far as a mile away.
Painters and other artists sometimes exploit the limitations of the human eye to create
their effects. The style of painting known as Pointillism was famous for this. To the right,
we have created our own Pointillist art. The “zoom-in” shows that the picture is
composed of many small “dabs” of color. As you step back, your eye can no longer
resolve the individual dabs, and you see the collective image.
If your school lacks a first rate collection of Post-Impressionistic paintings, the same
effect is demonstrated in your daily newspaper. Look closely enough at a picture and
you will see the dots that compose it.Lack of resolution leads to art
34.10 - Physics at play: CDs and DVDs
Most computers today contain a CD-ROM drive and many contain a DVD drive. A CD
or DVD can hold a great quantity of information. The drive relies on the principles of
interference to “read” the disc.
A CD or DVD contains a long spiral track with pits in it. These pits are formed in a disc
by an injection molding process, and represent some of the smallest mechanically
manufactured objects. A thin layer of metal such as silver or aluminum covers the pits.
This layer in turn is covered by a thin layer of plastic.
The pits are created on the top of the CD or DVD but the disc is read using a laser that
is projected up from the bottom. From the bottom of the disc, the pits appear to be
raised areas. Non-pitted areas of the disc are called land. (An incidental fact: The pits
are nearer the top of the disc than the bottom, so scratches on its top are more likely to
damage the CD than scratches the bottom, or “reading” side!)
CDs or DVDs created by burners do not create pits in the fashion described above, but
rather change the color of a layer within the disc.
The CD or DVD reader contains a laser diode that emits a beam of light that reflects off
the disc. The intensity of the reflected light varies as the disc rotates and the lightCDs and DVDs
Laser shines light onto track of CD, DVD
Discs have “pits” in surface
Sensor reads signal from laser(^634) Copyright 2000-2007 Kinetic Books Co. Chapter 34