LAB 12
Refraction & Lens
OBJECTIVES
1. Use Snell’s Law to determine the index of refraction of the Acrylic trapezoid.
2. Determine the focal length and radius of curvature of a converging lens.
3. Use the thin lens equation to determine the image characteristic and location of an object.
EQUIPMENT
Ray Box kit, protractor, compass, converging lens (f = 20 cm) and optical system.
THEORY
When a light ray encounters a boundary between two media, a reflected ray and a refracted ray
generally appear. The angle of angle of refraction is related to the angle of incidence by Snell’s
Law: n1 sinθ1 = n2 sinθ2, where 1 is the angle of incidence, 2 is the angle of refraction, and n1
and n2 are the respective indices of refraction of the materials.
A converging lens will focus parallel rays of light at the focal point. When a light source is shined
through a converging lens, it produces images given by the thin-lens equation:
1 1 1
 
f s s
where s is the object location and s′ is the image location.
PROCEDURE
Part 1: Refraction and Snell's Law
a. Setup a ray box on a white sheet of paper so that a single ray
passes through the trapezoid as shown.
b. After tracing out the trapezoid and marking where the ray enters
and leaves the trapezoid, carefully draw the best normal to the
surface for the incident and transmitted rays.
c. Measure the incident and refracted angles of the ray (at both
surfaces) with a protractor relative to the normal. Change the angle of incidence and repeat
this procedure for a total of three incident angles.
d. Use Snell’s Law to predict the index of refraction nthy for the Acrylic trapezoid and compare it
to the accepted value naccepted = 1.5 using a percent difference. How do they compare?
e. Questions: explain your reasoning using short concise sentences.
 Does the refracted ray bend towards or away from the normal at the first surface?
 Does the refracted ray bend towards or away from the normal at the second surface?
Part 2: Focal Point of a Lens
Estimate the focal length (with f = 20 cm = 200 mm) of the converging lens using two methods:
a. Hold the converging lens and focus the overheads lights onto a sheet of paper by focusing
the image. Measure the focal length fa from the center of the concave mirror surface to the
focal point.
b. Mount the same converging lens (with f = 20 cm) and a half screen on an optical bench.
Using a very distance object, accurately measure the focal length fb of the converging lens.
Repeat this as many times as there people in your group and obtain an average value.
c. Compare the focal length measures from part (2a) and (2b) using a percent difference to
make your comparison. How do they compare? Explain any discrepancies.
Part 3: Image Characteristics of a Converging Lens
a. Mount a light source, the lens from part (2) and a half screen onto an optical bench. On the
optical bench, center the lens and tape off the two focal points (f-point) and the two twicethe-focal distances (2f-points).
b. Without doing any calculations, move the light source to the values indicated in the table
below and describe the image characteristics using the table below.
Focal length f
Twice-focal length (2f)
Object s
Real/Virtual
Inverted/Upright
Size
s > 2f
s = 2f
2f > s > f
s<f
Thin-Lens Equation
c. Now mount the light source at one end of the optical bench and place the converging lens a
distance of 60 cm from the light source.
d. Using the thin-lens equations, predict the image distance s´thy and the image height h´thy.
e. Use a screen to find the image location s´exp and image height h´exp. Compare s´thy & s´exp and
h´thy & h´exp using a percent difference. How do they compare?
d. Keeping the light source at one end of the optical bench and adjusting the lens distance
from the light source, repeat steps (3c – 3e) for object distances of 40 cm & 30 cm.
f(cm)
s(cm)
sthy (cm)
sexp (cm)
%
hthy (cm)
hexp (cm)
%
60
40
30
Part 4: Type of Lens
There is an assortment of lens at this station. Determine the (i) type of lens (converging,
diverging or something else) and (ii) its focal length (if possible).
Part 5: Microscope and Telescope
a. Telescope
 Use a 200 mm and 100 mm lens to form a telescope. Which lens should be the objective
lens and the eyepiece lens? Explain your reasoning. Draw a ray diagram showing the
distances between the lens system.
 Now use the two lenses and setup a telescope to read the “eye chart” across the parking
lot.
b. Microscope
 Using the same two lenses for the telescope above, create a microscope and magnify
the hair strand. Which lens should be the objective lens and the eyepiece lens? Explain
your reasoning. Draw a ray diagram showing the distances between the lens system.