Experiment 1: Reflection and Refraction
Part one:
You will experimentally test the law of reflection, and also measure the index of refraction of a
piece of plastic.
CAUTION: LASER LIGHT CAN DAMAGE YOUR EYES. The beam's intensity is similar to
direct sunlight. If the beam shines in your eyes, it's like looking at the sun.
PROCEDURE:
You have a laser, and a stand holding a plastic semicircle with protractors attached. You will shoot
the laser beam at the flat side of the semicircle. Leave the clamp on base of the semicircle loose
enough to turn it to different angles, but tight enough that it will stay when you let go. Before you
begin, align the laser with the protractor:
Mount the center of the semicircle at about the same height as the laser. Then adjust the screw or
screws under the laser to tilt it until the beam lines up with the 90 mark when perpendicular to the
plastic surface. (It's perpendicular when the reflected beam goes back into the laser's opening.) If
the law of reflection does not seem to work when you start the experiment, adjust this further.
When the beam hits the plastic, part of it will reflect back into the
air, and part will be refracted into the plastic. Record
the angle of incidence, θi, the angle of reflection, θa, (a for "air"),
and the angle of refraction θp (p for "plastic") for four different
values of θi of over 10 . (Small angles give large percent
uncertainties.) To observe these, you can move the stand holding
the plastic toward or away from you to make the beam hit the
numbers on the protractor. Do not change its distance from the
laser, or it may affect the aim of the beam.
Remember that these are all acute angles measured from a normal (perpendicular) to the air-plastic
boundary, so you must subtract the protractor's reading from 90 . It's also a good idea to try the
beam on both sides, as shown, and average the results to compensate for any misalignment of the
protractor.
Comment on whether your results agree with the law of reflection. (Consider the angles to be good
to ±1 )
Use Snell's Law to compute np, the plastic's index of refraction (assume n= 1.00 for air), and
average to obtain a best value. Include in your report the details of these calculations for at least one
trial. (Note that you are experimentally determining np: do not look it up in a table.)
Part two:
- 2 You work with a lens and then with a mirror mounted in front of a light with an arrow painted on it,
which you use as the object.
Convex Lens:
1. Set things up: Mount the meter stick on its feet, then place the lens on it. Clamp the light you
will use for an object to the ring stand from part one, about the same height above the counter as the
lens. (Be sure you don't want to double check anything before you disassemble part one.)
2. Determine the lens's focal length: Place the object far from the lens (40 to 80 cm away). On a
card, locate the image it forms. Measure the object distance and the image distance. Use them to
find the focal length of the lens.
3. Change things around and predict what the image will now be like:
a. Move the object closer to the lens, so that it is now about 5 or 10 cm beyond the focal
point. Measure so and ho (note that ho is the length of the arrow, not its height above the
counter.)
b. Calculate the distance of the image from the lens si, and calculate the height of the image
hi. Also, decide whether the image should be erect or inverted.
c. Solve the problem again, this time by measurement on a ray diagram drawn to scale,
instead of using equations.
4. Move the card along the meterstick to locate the actual image. Record its position, size, and
character.
In your conclusion,
a. State whether the position, size and character you calculated for the image agrees with
what you observed.
b. State whether the position, size and character you predicted with the ray diagram agrees
with what you observed.
Consider all numbers to have 10% uncertainties.
Concave Mirror:
Place the object 10 to 20 cm from the mirror, and measure the object and image distances. The
screen goes in front of the mirror, so you will have to be careful not to block all of the light. A
small screen is a good idea.
Compute the focal length of the mirror, and its radius of curvature. (No comparison in this part.)
- 3 Phy 133
Report on Experiment 1: Reflection and Refraction
PART ONE:
θi
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θa
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Sample calculation of np:
PART TWO:
Converging Lens:
so = _______________
si = _______________
compute f:
3a)
so = _______________
b)
compute si:
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(np)ave =
Comments on law of reflection:
2)
θp
ho = _______________
np
_
_
_
_
_
_________
_
- 4 compute hi:
Should it be erect or inverted?
c) from ray diagram si = _______________ hi = ________________
(attach diagram):
Should it be erect or inverted?
4) experimental: si = _______________
hi = ________________
Is it erect or inverted?
Converging mirror:
so = _______________
compute f:
R=
si = ________________