PASCO OS-8459 Beginning Optics System - Manual (Page 16)

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B e g i n n i n g O p t i c s S y s t e m E x p e r i m e n t 5 : T o t a l I n t e r n a l R e f l e c t i o n
16
Procedure
1. Place the light source in ray-box mode on a sheet of white paper. Turn the
wheel to select a single ray.
2. Position the rhombus as shown in Figure 5.3, with the ray entering the
rhombus at least 2 cm from the tip.
3. Rotate the rhombus until the emerging ray just barely disappears. Just as
it disappears, the ray separates into colors. The rhombus is correctly posi-
tioned if the red has just disappeared.
4. Mark the surfaces of the rhombus. Mark exactly the point on the surface
where the ray is internally reflected. Also mark the entrance point of the
incident ray and the exit point of the reflected ray.
5. Remove the rhombus and draw the rays that are incident upon and
reflected from the inside surface of the rhombus. See Figure 5.4. Measure
the angle between these rays using a protractor. (Extend these rays to
make the protractor easier to use.) Note that this angle is twice the critical
angle because the angle of incidence equals the angle of reflection.
Record the critical angle here:
θ
c
= _______ (experimental)
6. Calculate the critical angle using Snell’s Law and the given index of
refraction for Acrylic (n = 1.5). Record the theoretical value here:
θ
c
= _______ (theoretical)
7. Calculate the percent difference between the measured and theoretical values:
% difference = _______
Questions
1. How does the brightness of the internally reflected ray change when the incident
angle changes from less than
θ
c
to greater than
θ
c
?
2. Is the critical angle greater for red light or violet light? What does this tell you
about the index of refraction?
Incident
ray
Reflected
ray
Refracted
Ray
Figure 5.3
2q
c
Exit point
Entrance
point
Reflection
point
2q
c
point
ce
Re
po
Figure 5.4