PASCO OS-8536 OPTICS TABLE - Manual (Page 14)

10
Optics Table 012-06557A
Experiment 6: Total Internal Reflection
EQUIPMENT NEEDED
– Light Source – Optics Bench
– Ray Table and Base – Cylindrical Lens
– protractor – pencil
– white paper
Purpose
To determine the critical angle at which total internal reflec-
tion occurs.
Theory
Snell’s Law states that the angle of an incident light ray rela-
tive to the normal of a boundary between two substances is
related to the angle of the refracted light ray.
If a ray of light traveling from a medium of greater index of
refraction to a medium of lesser index of refraction is incident
with an angle greater than the critical angle (
θ
c
), there is no
refracted ray and total internal reflection occurs. If the angle of incidence is exactly the critical angle, the angle
of the refracted ray is 90 degrees. In this case, using Snell’s Law,
nsin
θ
c
= (1)sin(90
°
)
assuming the medium of lesser index of refraction is air with n
2
= 1 and the medium of greater index of refrac-
tion is the acrylic Rhombus with n
1
= n = 1.5. Solving for the critical angle gives
sin
θ
c
= 1
n
Procedure
Mount the Light Source and Ray Table Base on the Optics
Bench. Put the Ray Table on the base with the DEGREE
SCALE facing up. Position the Light Box near the Ray Table.
Adjust the slit mask on the front of the Light Source so one light
ray shines across the across the top of the Ray Table. Turn the
Ray Table so the light ray shines along the NORMAL (zero
degrees) line of the table.
Place the Cylindrical Lens on the Ray Table so the curved side
of the lens faces the Light Source and the flat side of the lens is
on the COMPONENT line.
θ
c
Air
n
Incident
Ray
Reflected Ray
Figure 6.1
Refracted
Ray
Normal to
surface
Remove the Rhombus and draw a line on the Ray Table connecting the points where the ray entered and left the
Rhombus.
Choose either the point where the ray enters the Rhombus or the point where the ray leaves the Rhombus. At
this point, draw the normal to the surface. Measure the angle of incidence (
θ
i
) and the angle of refraction. Mea-
sure both angles from the normal.
Change the angle of incidence and measure the incident and refracted angles again.
Figure 6.2
P
A
TTERN
SOURCE
COLOR
DISCONNECT
PO
WE
R
BEFORE CHAN
GING BULB
. 5 3 1
RA
Y
SLITS
Light Source
Reflected
Ray
Incident
Ray
Refracted
Ray