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Follow the instructions, use file 1 to finish questions in file 2.
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Reflection and Refraction PhET Lab
Name
Topics:
• Reflection
• Index of refraction
• Refraction & Snell’s law
• Dispersion
1. Go to the PHET physics simulations, and open the “Bending Light” simulation.
https://phet.colorado.edu/en/simulation/bending-light
2. Click “More Tools”.
3. Click the red button on the laser.
4. Keep the laser set to Ray and red (700 nm wavelength) light.
5. For the material on top, choose water from the drop down list.
6. For the material on bottom, choose air from the drop down list.
7. Drag the protractor to be aligned as shown below.
8. Click the Angles checkbox, and make sure the Normal checkbox is also checked.
9. Notice that there is an incident beam coming from the laser, a reflected beam coming from the
water/air interface, and a transmitted beam continuing on through the air.
10. Drag out the green intensity meter to measure the intensity of the transmitted and reflected beams.
11. Change the angle of the incident beam to the following values, and complete the table below.
N1: Water, N2: Air
Angle of Incidence Angle of
Intensity of
Angle of
Intensity of
(degrees)
Reflection
Reflected Beam
Refraction
Transmitted Beam
(degrees)
(%)
(transmitted beam, (%)
degrees)
10
20
30
40
50
60
70
12. What is the relationship between the angle of incidence and the angle of reflection? Write an
equation that shows this.
13. As the angle of incidence increases, the intensity of reflected light:
a. Increases
b. Stays the same
c. Decreases
14. As the angle of incidence increases, the intensity of transmitted light:
a. Increases
b. Stays the same
c. Decreases
15. As the angle of incidence increases, the intensity of reflected light + the intensity of transmitted
light:
a. Increases
b. Stays the same
c. Decreases
16. Was there a point at which all of the light reflected and none transmitted? This phenomenon is
called Total Internal Reflection. The smallest angle at which total internal reflection occurs is
called the Critical Angle. For our settings, what is the critical angle?
17. Right before the incident angle reaches the Critical Angle, what value does the angle of refraction
approach?
18. When the incident angle is between the Critical Angle and 90 degrees to the Normal, what is the
intensity of the reflected beam?
19. Right after total internal reflection occurs, decrease the index of refraction of the top medium.
What happens? What does this indicate about how the critical angle relates to the relative indices
of refraction of the two media?
20. Settings: n1 = water, Angle of incidence = 30.1 degrees. Make the brown speedometer measure
the speed of the incident beam in water, as shown below.
21. Change the medium for the incident light, and fill out the table below. Record your speeds as
percentages of the speed of light in a vacuum, e.g. 0.75c on the meter -> 0.75 for v/c in the table.
Index of Refraction
1.600
Speed (v/c)
1.499 (glass)
1.332 (water)
1.211
1.170
1.075
1 (Air)
22. Make a scatterplot of the index of refraction vs v/c. Then add a power trendline, and have the
trendline equation show. Insert below a picture of your scatterplot with trendline & trendline
equation.
23. Given the equation for the trendline, what is the simplified equation relating the index of
refraction (n) and v/c?
24. Put all meters back in the container. Change the settings to: 1 = Air, 2 = Water. What is the
maximum angle of refraction that is possible?
25. What does this indicate about what you can see if you are underwater and looking upwards along
the normal out of the water into the air? Refer to the photo below for a hint.
26. What does this indicate that you see from underwater if you look upwards at the surface of the
water at an angle of 60 degrees from the normal?
27. Set the laser to wave, red, and the angle of incidence to 0 degrees. Move the Time meter out of
the container, and make it measure both the incident wave and the transmitted wave. Do they
have the same frequency? Try different combinations of 1 , 2 , and different angles of
incidence. What trend do you see for the frequency?
28. From what you observe of the wave pattern, do the incident and transmitted wave have the same
wavelength? If they’re different, which one is shorter? Try different combinations of 1 , 2 , and
different angles of incidence. What trend do you see for the wavelength?
29. Using the fact that = / , find a mathematical expression relating 1 , 2 , 1 , 2 .
30. Using the expression you just found, and the fact that the velocity of a wave is = = , and
that 1 = 2 (as you saw in question 24), find a mathematical expression relating,
1 , 2 , 1 , 2.
31. Does increasing the index of refraction (increase, decrease, or not effect) the frequency of light in
that medium?
32. Does increasing the index of refraction (increase, decrease, or not effect) the wavelength of light
in that medium?
33. Does increasing the angle of incidence (increase, decrease, or not effect) the frequency of the
transmitted beam?
34. Does increasing the angle of incidence (increase, decrease, or not effect) the wavelength of the
transmitted beam?
35. Try different combinations of n1, n2, and angle of incidence. How do the settings affect the width
of the transmitted beam?
36. For the first table you made, copy the relevant data into the following table below:
Angle of
Incidence, 1
(degrees)
Index of
Refraction for
medium of
incident beam,
1
Angle of
Refraction
(transmitted
beam), 2
(degrees)
Index of
Refraction for
medium of
transmitted
beam, 2
1 ∗ sin 1
2 ∗ sin 2
10
20
30
40
50
60
70
37. From the table above, what equation relates the angle of incidence, the angle of refraction, and
the indices of refraction of their two respective media? This equation is called Snell’s Law.
38. True / False: When light enters (at a non-zero angle to the normal) a medium with a higher index
of refraction than the medium it is leaving, it bends toward the normal.
39. True / False: When light enters (at a non-zero angle to the normal) a medium with a lower index
of refraction than the medium it is leaving, it bends away from the normal.
40. Term: The bending of light when it changes mediums is called refraction. For air, the index of
refraction is almost equal to 1, because the speed of light in air is nearly equal to the speed of
light in a vacuum.
41. In the table above, for the incident angle 1 = 60 degrees, is there any value of the refracted angle
2 that could make 2 sin 2 equal to the value you found for 1 sin 1? Why not?
42. Therefore, given Snell’s law that you derived above, what is an equation that would give an
expression for the Critical Angle (the smallest angle at which total internal refection occurs)?
43. Set the laser to red (700 nm) at 30 degrees, and n1 > n2. Change wavelength of the laser and
observe the angle of refraction. Now set n2 > n1, and change the wavelength of the laser. Try
various combinations of n1 & n2 and change the wavelength of the laser. What trends do you
observe?
44. What does this indicate about the index of refraction relative to the wavelength of the light
passing through the medium? Is it constant? If not, how does it change with changing
wavelength?
45. Given the change in the index of refraction with wavelength of light, according to the simulation,
which travels faster in water, 700 nm light or 380 nm light?
46. Place all meters back into their original container. Set the laser to red, the top medium to air, and
the bottom medium to “Mystery A”. Complete the chart below.
Angle of
Incidence, 1
Index of
Refraction for
medium of
incident beam,
1
Angle of
Refraction
(transmitted
beam), 2
1 sin 1
sin 2
Compute 2
using Snell’s
law
10°
30°
50°
70°
Average value for 2
47. Plot 1 ∗ sin 1 vs. sin 2 in a scatter plot, and then find a linear trendline. Make sure the
trendline equation shows. Paste an image of your chart below.
48. Write the equation for the trendline below, but using sin 2 and 1 ∗ sin 1 in place of x & y
appropriately. Ignore any quantities that are approximately zero.
49. What is the slope of the line? How does the slope of the line relate to 2 (as an equation, i.e.
slope = some expression involving 2 )? What does the value for the slope of the line indicate
that the value of 2 should be? How does this compare to your average 2 that you computed in
your table?
50. Using the chart below of various indices of refraction for various media, identify your mystery
material A.
51. Find the percent error of your observed value (slope) using the identified index of refraction as
your accepted value.
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