PHYSICS AND THE VISUAL ARTS
Notes on Lesson 3
Things to remember from the lesson, from reading the text, and the lab exercise.
You saw in the demonstration with the wave machine in Lesson 2, that reflections occur when the wave encounters
a medium (other part of the wave machine) in which the wave travels with a different speed. For light, even if
the medium is transparent, if the speed of light in the medium is different from that of the incident medium,
there can be reflection. You saw "ghost crystals," that absorbed water to swell many times their
size when dry. The resulting blobs of gel are mostly water and so when
submerged in water they disappear from sight. But when raised up into the air they are easily seen because the
light is reflected at the boundary between the gel and the air.
What do we mean by reflection? When light strikes a smooth surface it is reflected
in a different direction. We can describe its behavior with the
law of reflection: The angle of incidence equals the angle of
reflection.
The angles in optics are measured from the normal to the surface (or
interface). How do you distinguish specular reflection and diffuse
reflection? Specular reflection is the reflection we get from a mirror. In diffuse reflection
the light is scattered in all directions. When you look at a clean mirror, do you see the surface of
the mirror? No. But smudges or dust on the mirror can be seen because it scatters light in all directions.
Metals are good reflectors because they have many "free" electrons that
react to the incident electric field of the light wave and by their motion generate the
reflected wave.
Reflection from a plane mirror reverses the coordinate along the optic axis (the normal).
The image formed by a plane mirror lies behind the mirror. It is a virtual image because no light
actually converges at the image point. For a real object, the image in a plane mirror lies as the
same distance behind the mirror as the distance from the object to the front of the mirror.
You should be able to explain why things appear reversed left to right in a plane mirror
and why things are not reversed in a right angle mirror. (A right angle mirror is made from
two plane mirrors arranged at a right angle.) Can you explain the behavior of a periscope? Can
you tell whether a mirror is a front surface mirror or a back surface mirror?
Spherical mirrors: A convex (diverging) mirror always generates a virtual
image of a real object. A concave (converging) mirror can generate a real
inverted image or a virtual erect image depending on whether the object is beyond or
within the focal distance from the mirror.
Parallel light rays incident on a convex mirror are diverged on reflection as if they came
from a common point. That point is the focal point and its distance from the center of the mirror
surface is the focal length f.
The focal length of the mirror is one half the radius of curvature of the mirror, that is,
f = R/2.
Parallel light rays incident on a concave mirror are converged to a common point in front of
the mirror. That point is the focal point.
Be sure that you know the difference between the focal length and the image distance, the distance
from the mirror to the position of the image.
Be sure to study the ray tracing rules for both mirrors and lenses. I will expect you to be able to
perform ray tracing on the test. It is easy, but you need to practice or you will likely not be able
to do it.
As stated above, a spherical mirror with radius of curvature R has a focal length
f = R/2. We
choose f to be positive for a concave mirror and negative for a convex mirror. The equation
that relates object distance, image distance, and focal length is the same for both mirrors and
lenses. However, there are some subtle distinctions in the sign conventions used. Be sure you know
how to use the equations properly.
In class you saw real images and virtual images. Can you remember what they were and how they were
made? What is a virtual object?
According to the principle of reversibility a light ray traversing any optical system would, if
reflected back along itself, travel back through the system following exactly the same path that it traveled
initially except that it would be traversing it in the reverse direction.
What is special about a "corner-cube reflector?"
What is a "half-silvered mirror?"
Suggested reading. An interesting paper on reflections is A. J. DeWeerd & S. E. Hill, "Reflections on
Handedness," The Physics Teacher Vol. 42,pp. 275-279 (May 2004). You should be able to find TPT in your
library.
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Last Modified: 09/06/04
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