PHYSICS 124 EXPERIMENT NO. 8
REFLECTION, REFRACTION, LENSE AND
OPTICAL INSTRUMENTS
Goal: The purpose of this laboratory is to study the Laws of Reflection and Refraction,
to measure the index of refraction of glass and observe dispersion. Then you investigate
images of lenses.
Introduction:
This laboratory is to show that the very simple principles of reflection and refraction can
lead to sophisticated ideas about optical instrument. We begin with a ray box that has a
slotted mask in front of a light bulb to produce a set of thin beams (or "rays"). The rays
lie along a plane surface (a sheet of paper). Your measurements will consist of pencil
mark on a piece of paper lying on this surface, indicating the direction of these rays
before and after striking a mirror or glass prism. Then you will progress to the study of
lenses, which are nothing more than refractor with curved surfaces.
Part I-Light Wave Refection:
Equipment:
1
1
“Ray” table
Triangular prism
Slits create light beams
(all but one covered)
Lamp
(sliding)
on rails)
Triangular prism
Table with 8.5x11” paper
for “ray” tracing
Fig 1.
The slit that the light comes through creates multiple rays. You should find a way to
block all but the central one.
A. Use the ray box, a mirror, and protractor to verify that θincident = θreflected (or θinc
= θrefl). Do this for at least three different incident angles.
B. Show that the (virtua1) image produced by a mirror is located as far behind the mirror
as the object is in front of the mirror. Do this by marking the point that the rays start at,
the point that the ray hits the mirror and a point out along the reflected beam. Do this for
3 beam angles. Measure the distance between the mirror and the point that the rays start
at, then trace back the reflected rays until they meet and measure the distance between
that point and the mirror and record those two distances in your lab book.. The two
distances should be about the same.
Part II- Light Wave Refraction
A. Snell's Law
Use the ray box, the glass triangle and protractor to verify Snell's Law about refraction:
ninc sin θinc = nrefl sin θrefl
Where ninc and nrefl are the refractive indices of the incident and refracting media
respectively. Take nair = 1 and nglass = 1.5. Verify the law by finding the ratio of the sines
for at least two different angles θinc. There is a picture below of what you should draw in
your lab book. The yellow line is the light beam and the black line is perpendicular to the
side of the prism. Theta1 is θinc and theta2 is θrefl.
B. Critical angles
Snell's law also tells us that if we reverse things, i.e., let light hit a glass-air boundary
then
Now sinθrefl can never be greater than 1, and is maximal for θrefl = 90o the angle θinc (this
is now on the inside of the glass, not the angle the outside of the glass is hit at) where this
happens is given the name critical angle since for any θinc > θcritical, the sine would have to
greater than 1. Since this can not be, light must be trapped inside the glass, it must be
totally reflected. Use your prism to find θcritical. Compare your measured angle with what
you expect on the basis of the equation above.
C. Dispersion
Use the prism to observe light dispersion. Wavelength has a small effect on the index of
refraction and so different colors will bend at different angles. To see this get the light to
reflect internally once before it leaves and have the beam leave close to parallel to the
side of the prism. The further the beam travels out of the prism the more you will be able
to see the spread. You should mark both edges of the spread in the beam marking which
side is red and which is blue.
Q1. Which colors are bent the most?
From measured angles, deduce nblue and nred for your prism.
Part III ---Lenses
Equipment:
1
1
2
1
1
“Optical Bench: a rail to mount the holders of lenses/screen on
Box with lenses (Use #3 and # 4 only – marked at bottom of shaft)
Holders for lenses/screen
Screen
Lamp with arrow (our object to be imaged)
Lens frame
Screen
(Image)
Rail
Arrow
(Object)
Lamp
Box with lenses: use
#4: f =5 cm, converging
#3: f =10 cm, diverging
Holders
Fig. 3
The apparatus consists of an optical bench which serves as a convenient holder for
objects, lenses and a ground glass screen for locating images. The object is an arrow
painted on a piece of ground glass illuminated from behind by a collimated light bulb.
Measure the focal point of the 5cm converging lens (the one marked with a 4 on
the bottom) and compare your number to the marked focal length. To do this set put the
painted arrow close to the lamp, fix the screen some distance away from the arrow and
then move the lens until the image on the screen is most focused. At this point lock the
lens into place and measure the distance from the arrow to the lens (x0) and the distance
from the lens to the image (xi). The focal length is found by the formula
1/f = 1/x0 +1/xi.
Be careful here because the lens is not in the center of the holder, so you should measure
the distance from the center of the lens-holder to the center of the lens and then either add
or subtract to the distance to get the correct value of xo or xi.
Write-Up:
Include the following in your lab report:
a) Answer question 1.
b) Verify that θincident = θreflected
c) Verify that the image is the same distance
d) Verify Snell’s Law
e) Calculate critical angle and compare to what is expected from the
formula
f) Find nblue and nred.
g) Find the focal length of the two lenses and compare to their labeled
value.