REFLECTION, REFRACTION AND TOTAL INTERNAL
REFLECTION OF LIGHT
Part A: Reflection of Light on a Plane Mirror
Purpose
To study how rays are reflected on a plane mirror and to determine the law of reflection
Introduction
The shape and location of the image created by reflection from a mirror of any shape is
determined by a few simple principles. You already know one of these principles: light
propagates in a straight line. You will have an opportunity to learn the remaining principles in
this experiment.
Angle of Angle of
Incidence θi Reflection θr
Incident Ray
Reflected Ray
Figure 1: Reflection of Light on a Plane Mirror
Equipment
– Light Source – Optics Bench
– Plane Mirror
Procedure
Mount the Light Source and the Ray Table Base on the Optics Bench as in the figure 2. Put the
Ray Table on the base with the DEGREE SCALE facing up. Position the Light Source near the
edge of the Ray Table. Adjust the slit mask on the Light Source so one light ray shines across
the middle of the top of the Ray Table. Rotate the table so the light ray shines along the
NORMAL line on the table.
Figure 2: Setup for Reflection of Light
Place the plane mirror on the COMPONENT line on the Ray Table with the plane surface
facing the light source. Rotate the Ray Table a few degrees. Measure the angle of incidence (θi)
and the angle of reflection (θr). Both these angles should be measured from the NORMAL line.
Change the angle of incidence and measure the incident and reflected angles again. Repeat
with the incident ray striking from the other side of the normal to the surface of the mirror.
Fill in the following table:
Table 1: Incident ray strikes from the left of the normal to the surface
BULB: 12V 1Table G-4
Angle of incidence
Angle of reflection
0
10
20
30
40
50
60
70
80
Repeat with the incident ray on the other side of the normal to the mirror surface:
Table 2: Incident ray strikes from the right of the normal to the surface
Angle of incidence
0
10
20
30
40
50
60
70
80
Angle of reflection
Data Analysis
1. Are the results for the two sets of trials the same? If not, to what do you attribute the
differences?
2. What relationship holds between the angle of incidence and the angle of reflection?
3. What can you tell about the direction of the two rays? Do these rays appear to be lying
in the same plane or not?
Part B: The Law of Refraction
Purpose
To explore the refraction of light and to determine the law of refraction.
Introduction
The direction of light propagation changes abruptly when light encounters a reflective surface.
The direction also changes abruptly when light passes across a boundary between two different
media of propagation, such as between air and acrylic, or between glass and water. In this case,
the change of direction is called Refraction.
As for reflection, a simple law characterizes the behavior of a refracted ray of light.
According to the Law of Refraction, also known as Snell's Law:
The quantities n1 and n2 are constants, called indices of refraction; they depend on the two
media through which the light is passing. The angles θi and θr are the angles that the ray of
light makes with the normal to the boundary between the two media as in Figure 3.
θi
n1
n2
θr
Figure 3: Refraction of Light
In this experiment, you will test the validity of Snell's Law, and measure the relative index of
refraction for acrylic.
Equipment
– Light Source – Optics Bench
– Cylindrical lens
Figure 4: Setup for Refraction of Light
Procedure:
Set up the equipment as shown in Figure 4 above. Adjust the components so a single ray of
light passes directly through the center of the Ray Table Degree Scale. Align the flat surface of
the Cylindrical Lens with the line labeled "Component". With the lens properly aligned, the
radial lines extending from the center of the Degree Scale will all be perpendicular to the
circular surface of the lens.
Without disturbing the alignment of the Lens, rotate the Ray Table and observe the refracted
ray for various angles of incidence.
1. Is the ray bent when it passes into the lens perpendicular to the flat surface of the lens?
2. Is the ray bent when it passes out of the lens perpendicular to the curved surface of the lens?
By rotating the Ray Table, set the angle of incidence to each of the values shown in Tables 3
and 4 below. For each angle of incidence, measure the angle of refraction. Repeat the
measurement with the incident ray striking from the opposite side of the normal and fill in the
two tables below.
Table 3: Incident ray strikes from the left of the normal to the surface
Angle of incidence
Angle of Refraction
0
10
20
30
Table 4: Incident ray strikes from the right of the normal to the surface
Angle of incidence
Angle of Refraction
0
10
20
30
Data Analysis
1. Are your values for the angles of refraction for the two sets of measurements the same? If
not, to what do you attribute the differences?
2. Solve for the index of refraction, n, for the acrylic material. (Assume that the index of
refraction for air is equal to n1 = 1.0).
Part C: Total Internal Reflection
Equipment Needed
– Light Source – Optics Bench
– Ray Table and Base – Cylindrical Lens
Purpose
To determine the critical angle at which total internal reflection occurs.
Theory
Snell’s Law states that the angle of an incident light ray relative 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 (θC), there is no refracted ray
and total internal reflection occurs.
Figure 5: Total Internal Reflection
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:
Assuming the medium of lesser index of refraction is air with
n2 = 1
and the medium of greater index of refraction is the acrylic cylindrical lens with
n1 = n = 1.5.
Solving for the critical angle gives
Therefore:
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 top of the Ray
Table. Turn the Ray Table so the light ray shines along the NORMAL (zero degrees) line of
the table.
Figure 6. Setup for Total Internal Reflection
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.
Rotate the Ray Table until the light ray emerging from the Cylindrical Lens just barely
disappears. Hold a piece of white paper next to the edge of the Ray Table so you can see the
light ray. Just as it disappears, the ray separates into colors. The table is rotated far enough if
the red color has just disappeared.
Figure 7. Angle Between Incident and Reflected Rays at Total Internal Reflection
When the incident ray makes an angle equal to θc with the normal, all the incident light reflects
off the inside flat surface of the Cylindrical Lens. Measure the total angle between the incident
and the reflected ray on the degree scale. Note that this total angle is twice the critical angle
because the angle of incidence equals the angle of reflection. Compare this measured value
with the one calculated above.