Revised 8/03
Experiment 16: Reflection, Refraction and
Dispersion of Light
Purpose
To observe the interactions of light with transparent matter: reflection, refraction, the index of
refraction, and dispersion of light during refraction.
Apparatus
A ray box; a rectangular Lucite slab and two equilateral Lucite prisms; a plane mirror. Drafting
equipment: two triangles; a protractor; a sharpened pencil; (8½ x 11 in.) white paper; tape; a desk
lamp.
Theory
(A) The Law of Reflection: The angle of reflection is equal to the angle of incidence. Both angles
are measured between the light ray and the normal. (See Fig. 1.)
law of reflection:

 =  



The normal to the surface
Incident light ray
Ref lected light ray
Incident
angle

Ref lected
angle

Ref lecting surface
Fig. 1 The law of reflection 
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Experiment 16
(B) The Law of Refraction (Snell’s law): The product of the sine of the angle of incidence times
the indices of refraction of the first media equals the product of the sine of the angle of refraction
times the indices of refraction of the second media. Again, both angles are measured between the
normal to the refracting surface and the corresponding rays.
Snell's Law:
n1 sin θ1 = n2 sin θ2
(2)
(C) Total Internal Reflection:
From Equation 2 we find
n1 · sin θ1 = sin θ2
n2
If it happens that:
n1 · sin θ1 > 1
n2
Snell’s law breaks down since the sine of an angle is always less than 1. In this case, rather than
refraction, a reflection occurs inside the media n2. This reflection obeys the law of reflection,
according to formula (1) and is called total internal reflection.
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Experiment 16
(D) Dispersion:
The index of refraction is not the same for different wavelengths (colors) of light. This results in
separation of a “white” ray into individual many colored rays. This is called “dispersion by
refraction.”
Procedure Part I: Reflection
Note: The quality of your results in this experiment will depend much upon the accuracy
of your drawings.
(a) Tape a regular 8½” by 11” sheet of white paper to the table and place the mirror vertically
approximately 10 cm from the top of the paper. (See Fig.3.) Using a sharp pencil, lightly outline
the mirror's position on the paper.
(b) Remove the mirror; mark a point K on the outline of the mirror and draw the normal through
K, marking it by a centerline using a protractor.
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Experiment 16
(c) Replace the mirror back over its outline. Now you have the normal drawn to the mirror's
surface and can start doing the experiment. Take the ray box and direct the light at an angle of
~20º at the point K. This is your first angle of incidence θ1. Mark a point M1 on this ray (close to
the ray box), as well as a point N1 on the reflected ray a similar distance. When you have
completed these steps, your drawing should look similar to that of Fig. 4 below.
Points M1, K and N1 will allow you to trace the incident and reflected rays, and to measure
angles θ1 and 1.
ray-box
normal
M
1
incident ray

1


1

N
1
reflected ray
K
mirror
Fig. 4 Reflection of Light
(d) Now carefully move the ray box (do not move the mirror) and repeat step (c) for four other
angles of incidence evenly spaced between 20º and 70º. Label the new points M2, N2, M3, N3,
etc. until all five pairs are selected. Upon completing this, remove the mirror, turn on the desk
lamp and draw the lines: incident M1-K; reflected K-N1, etc. tracing all five incident rays and the
corresponding five reflected rays.
NOTE: Before proceeding further, check your drawings with your instructor.
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Experiment 16
Procedure Part II: Refraction
(e) Turn the sheet over and tape it, as before, to the table but now in the vertical position. Place
the Lucite slab flat in a horizontal position as shown. Outline the slab. Remove it and mark a
point K about one-third from the left edge at the top of the slab. This is the mark for the normal.
Now construct the normal through K, marking it by a centerline. (See Fig. 5.)
(f) Replace the slab. Direct a ray at point K at an angle of incidence ~20º; mark a point M (near
the ray box). Note: the refracted ray inside the slab will not be visible, but an emerging ray will
be visible. Now you need to mark two points, P and Q, as far apart as practical, on the emerging
ray. From this you will be able to locate the point of emergence B and reconstruct the path KB of
the ray inside the slab. (See Fig. 5.)
ray-box
the normal
M
1
~ 18 cm
incident ray
K
2
B
Lucite slab
P
emerging ray
Q
Fig. 5 Refraction of light
(g) By moving the ray box (do not move the slab) repeat (f) for four other angles of incidence,
evenly spaced between 20º and 70º. Label all relevant points with subscripts from “2” to “5”,
namely: M1, P1, Q1, etc.
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Experiment 16
Procedure Part III: Total Internal Reflection
(h) Direct a ray to one of your equilateral Lucite prisms as in Fig. 6. The angle  should be
between 75º and 85º. Arrange for the total internal reflection at point B, near the middle of the
second side of the prism. Outline the prism and mark the incident and emerging rays by dots.
Also mark the point B, which can be detected by a glow.
Procedure Part IV: Dispersion by Refraction
(j) Set the first prism on the path of a light ray to be tilted ~30º. You should observe that the
intermediate ray already shows some color dispersion. Intercept this ray by a second prism, even
more tilted, and observe that the emerging ray is dispersed even further. Mark the edges with a
label for their color (e.g. "red” or “violet”). Complete your own diagram as in Fig. 7.
ray box
dispersion angle
red ray
violet ray
Fig. 7 Dispersion of Light
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
Experiment 16
Lab Report
Part I: Reflection
(1) Complete your drawings; carefully
measure all angles; display your
results in Table One (as shown).
TABLE 1. REFLECTION.
θ
ψ
Run #
(2) Answer Question #1: How accurately
can you measure angles with your
protractor to the best of your ability?
1
2
3
......
|θ–ψ|
(IN DEGREES)
Part II: Refraction
(3) Complete your drawings; carefully
measure all angles; display your
results in Table 2 (as shown).
The values of n must be quoted to
the accuracy of three significant
digits.
(4) Answer Question #2: How accurate
do you expect your measured
value of n (n average) to be?
Explain your reasoning and state
your estimate in %.
TABLE 2. REFRACTION.
Run #
θ1
θ2
1
2
3
4
5
n
MAXIMUM %
DEVIATION
IN n
AVERAGE
Part III: Internal Reflection
(5) Measure the angles and , as per Fig. 6, and display them in your report.
(6) Answer Question #3: Should the angles  and ß be the same or not?
Explain your reasoning (Hint: use geometrical reasoning and Snell’s Law.
No calculations are needed).
Part IV: Dispersion by Refraction
(7) Measure and quote the angle . (See Fig. 7.)
(8) Answer Question #4: Why are the red rays, in the Fig. 7, “above” those of the violet rays?
(Hint: See your textbook on Dispersion).
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