Lab 23: Where does reflected light go?
- Reflection from different surfaces
- The Law of Reflection
- Drawing reflected beams with light ray diagrams
7) With 2 mirrors you can make a periscope (for looking around things, left) or a
retroscope (for looking behind you, right).
b) For both scopes, draw lines showing how light from distant objects, through the
scope, and to the eye of someone looking through the bottom of the device.
c) For either scope, does the person see images right-side up or upside down? Explain.
Follow the dotted (red) light ray that starts on the top of the incoming light
rays: it remains on top for the periscope, but is flipped to the bottom for the
retroscope. So, the periscopeʼs image remains upright, while the
retroscopeʼs image is flipped upside-down.
8) The 4 following cases each show a small, point-like light bulb (circle) backed by
different patterns of mirrors (solid lines).
a) In each case, draw the reflections and final directions of several light beams
originating from the bulb.
One possible set of answers is shown.
b) Which mirror(s) do the best job at reflecting light directly forward? Which do the worst
job? What practical application could there be for one of these situations?
The bottom right mirror reflects rays directly forward. The top 2 both do
bad jobs, while the bottom left is somewhere in-between. The bottom right
mirror is called a parabolic mirror; this design is used in headlights (for
example) to project as much light forward in a straight beam as possible.
Lab 24: Masks and Mirrors II ; What you see in a mirror
- Drawing illuminated regions with light ray diagrams
- Pinhole cameras
- Visibility range of a mirror
9) The three diagrams below show a long-filament bulb and a mask with a thin
horseshoe-shaped curved slit cut into it. In each case, to the right, sketch the pattern
that will be displayed on the viewing screen. (Hint: start simple - consider what light
passing through the straight line parts of the
“horseshoe” will do first...)
10) Jeff works in a cubicle in an office, mapped out below, and is scared his boss will
find him goofing off at his desk. So, he devises a system of 2 mirrors to let him see his
boss upon entry through the door.
a) Based on what you know about reflection, will Jeffʼs system work? Draw light beams
on the diagram that prove your conclusion. (You can consider the people to be “small
points” on the diagram.)
Jeffʼs system will work at his desk - it is possible to trace lines from mirror
to mirror according to the Law of Reflection that connect Jeff to his boss
(solid lines).
b) Jeff is also hoping he can waste time at the water cooler and use his system of
mirrors there, too. Is this possible? (Again, use light beams to prove it.)
This is not possible at the water cooler, because light rays reflecting off the
top mirror from Jeff connect with the wall, not the 2nd mirror (dotted lines).
c) Even if it does work, what is a major drawback to spying on his boss through mirrors
that Jeff overlooked, that might actually make things worse for him?
Light beams travel the same in reverse as they do forward - if Jeff can his
boss, his boss can see him goofing off at his desk, too!
Lab 25:
- Images
- Locating and drawing a mirror image
- Propagation of images through multiple mirrors
11) The following diagram shows a letter “L” flat on a table with a mirror, which is rotated
at a 45° angle with respect to the “L”. Find and draw the mirror image of the “L”.
Donʼt let the angle fool you - all flat mirrors work the same! For each point
defining the “L”, draw a line perpendicular to the mirror through the same
distance on the opposite side; the mirror image of each point will be at the
end of these lines, as shown.
12) The next diagram shows a curved mirror. Using the Law of Reflection, and a similar
method you used for a flat mirror, predict where the image of the round object shown
will be located. How is this image location different from a flat mirror? What could be a
use for a mirror like this?
The image formed is where the dotted lines (determined by extending the
reflected lines) intersect. This image is much closer to the surface of the
mirror than the actual object is, unlike for a flat mirror. Note the rays reflect
to a very wide range. Convex mirrors like these are used in place you want
a wide field of view (such as when you want to see around corners in
hallways. Car side rear view mirrors are also slightly convex.)
Lab 26: What does refraction look like?
- Behavior of light in transparent materials
- The Law of Refraction
- Approximate location of refracted images
13) The following four diagrams all show a light beam in air contacting water, producing
both a reflected beam and a refracted beam of light. But, each of these diagrams has
something wrong with it. In the space below, identify what is wrong with each diagram:
A: The refracted beam bends toward the water surface, where it should be
bending toward a perpendicular direction to the surface.
B: The refracted beam doesnʼt bend until itʼs far underwater; light can
ONLY bend at the boundary between two materials.
C: The refracted beam is fine, but the reflected beamʼs angle is wrong.
D: The refracted beam crosses over the line perpendicular to the water - the
light beam cannot bend this much.
Draw the CORRECT diagram for this situation here:
14) A fish underwater sees a fly buzzing above the surface. Where would the image of
the fly appear to be to an underwater observer? If the fish wanted to jump up and eat
the fly, how would it have to adjust its jump? (Hint: You did this for the opposite situation
already, where a person sees an object underwater.....)
The refracted light beams under the water, if extended, converge at a point
above the fly. This means the fish would have to aim lower than the image it
sees in order to catch the fly.