Physics Critical Angle and Total Internal Reflection Lab
To be completed in your lab notebook
Pre Lab:
• On a fresh piece of paper in your lab notebook write today’s date and the title of the lab.
• Don’t forget to continue the page numbers in your lab notebook.
• Don’t forget to add this lab and its page number to your Table of Contents.
1. Define Critical Angle and include a labeled drawing.
2. Define Total Internal Reflection and include a labeled drawing.
Link: http://phet.colorado.edu/en/simulation/bending-light
Or search for “PhET light simulation” and choose the one about refraction. The simulation is free to be
downloaded or used on their website. Choose the “Prism Break” tab at the top. Turn on the Normal line
and the Protractor.
Part 1: Critical Angle of Glass Prism
1. Draw a semicircular prism outline into your lab notebook. You should make it at least three
inches on the flat side.
2. Drag the semicircular prism in front of the laser in the simulation.
3. Semicircular prisms are convenient because if you aim at the exact center on the flat side the
light ray will only bend once because as it enters the prism on the rounded side it will strike the
boundary between the prism and the air at 90º. You can double check that you are indeed at the
center when the first normal line is lined up with the ray as it enters at 90º.
4. Carefully move the prism so that the angle of incidence increases, always keeping the ray aimed
at the center of the flat side! Carefully watch that the incident ray lines up with the normal to
insure that it is hitting the round portion perpendicularly and thus hitting the flat side right at the
center.
5. Continue increasing the incident angle until the refracted ray disappears and the ray just begins
total internal reflection.
6. Decrease the angle very slightly so that refraction is at its maximum (essentially 90°). This angle
of incidence is the critical angle.
7. Sketch the path of the light onto your outline in your lab notebook. If possible use a protractor to
draw it correctly to scale.
8. Place an arrowhead on the incident ray.
9. Label the critical angle θc on the drawing. Measure it using the protractor in the simulation and
record its value your lab notebook as “Experimental value of Critical Angle of glass prism.”
10. Draw a refracted ray at 90°, and label the angle of refraction 90°.
11. Place an arrowhead on the refracted ray.
Part 2: Mystery Prism A
1. Notice that at the bottom of the simulation where you selected your semi-circular prism there is a
drop down menu to select “Object.” Leaving your semi-circular prism out select “Mystery A.”
2. Complete the same steps as in your first experiment but determine the “Experimental value of
Critical Angle of Mystery prism A.”
3. Don’t forget to sketch the prism and the light ray as well as label all rays and angles.
Part 3: Mystery Prism B
4. Notice that at the bottom of the simulation where you selected your semi-circular prism there is a
drop down menu to select “Object.” Leaving your semi-circular prism out select “Mystery B.”
5. Complete the same steps as in your first experiment but determine the “Experimental value of
Critical Angle of Mystery prism B.”
6. Don’t forget to sketch the prism and the light ray as well as label all rays and angles.
Part 4: Total Internal Reflection
1. Select the trapezoidal prism and place it in front of the laser in the simulation. Choose “glass”
again under “Objects.”
2. Adjust the incident angle until you achieve total internal reflection.
3. Sketch the position of the laser, prism and the light rays for this total internal reflection.
4. Label each point of total internal reflection with “TIR.”
Analyzing:
Underneath your prisms drawings, write “Analyzing” and you will answer the questions below in
complete sentences. Don’t forget to number your answers!
3. You measure the critical angle for your glass prism. Assuming an index of refraction for air of
1.0003, calculate the index of refraction of the glass prism. Show a GUESS calculation in your
lab notebook.
4. You measure the critical angle for your Mystery A prism. Assuming an index of refraction for air
of 1.0003, calculate the index of refraction of the Mystery A prism. Show a GUESS calculation
in your lab notebook.
5. You measure the critical angle for your Mystery B prism. Assuming an index of refraction for air
of 1.0003, calculate the index of refraction of the Mystery B prism. Show a GUESS calculation
in your lab notebook.
6. What is the angle of incidence for your trapezoid prism at the point where total internal reflection
takes place? Is the angle of incidence of the trapezoid prism larger or smaller than the critical
angle for glass you measured in Part 1? Why does that make sense?
Applyng:
Underneath your Analyzing section, write “Applying” and you will answer the questions below in
complete sentences. Don’t forget to number your answers!
7. Copy the following and fill in the blanks:
dense to a
a. Total internal reflection can occur only when light goes from a
dense material.
b. Total internal reflection only occurs when the angle of incidence is
than the
critical angle.
n.
c. The critical angle occurs only in the material with the
d. The value of the angle of refraction which always accompanies the critical angle is
degrees.
8. Another student completes this lab after you and wants to compare answers. They measured the
angle of incidence for their trapezoidal prism to be 35° and said that it totally internally reflected.
Describe what you would say to them about the accuracy of their lab and justify your response
with your own data.