Teach lesson
Snell's Law: measure refraction with a real protractor
Students use the Snell's Law (II) remote lab to read incident and refracted angles, calculate refractive index, compare media, and observe total internal reflection.
Learning Outcomes
Measure incident and refracted angles from the normal using the Snell's Law remote lab.
Calculate refractive index from n = sin(i) / sin(r) when light passes from air into a medium.
Compare water, oil, and acrylic using controlled incidence angles and evidence.
Explain why reading from the surface instead of the normal changes the result.
Describe total internal reflection qualitatively and bound the critical angle with the lab's 10-degree steps.
Write a claim that uses data, calculation, and uncertainty rather than a screenshot alone.
Student activity preview
Activity Content
Preview only. In a class session, students can fill in responses and submit their work to the teacher.
Frame the optics question
8 min
When light crosses from air into another transparent medium, it usually changes direction. Snell's law relates the angle of incidence i, the angle of refraction r, and the refractive index n. The most common measurement mistake is using the surface as the reference line. Snell's law uses angles measured from the normal, the line perpendicular to the surface at the point where the ray enters.
Angle reference
Use angles measured from the normal. A complementary angle measured from the surface changes the sine value and gives the wrong refractive index.
Model for air into a transparent medium
n = sin(i) / sin(r)
For Snell's law, i and r must be measured from what reference line?
Predict what will happen when a ray enters water from air, and separately when a ray enters acrylic from air, at the same incidence angle. Use this clue: acrylic has a higher refractive index than water. Which ray should bend more toward the normal, and why? Use 3-4 sentences.
Use the remote lab deliberately
10 min
The Snell's Law (II) lab uses real video views of a laser, a transparent cell or acrylic block, and a printed protractor. The no-verification version does not give the refraction angle as a number. You read the refraction angle from the video, record it, and calculate outside the lab.
Lab workflow
The lab supplies the real visual evidence. Your job is to read the angles consistently and calculate the refractive index.
Water setup
Water in normal orientation. The incident and refracted rays are read against the printed protractor.
Open Snell's Law (II)
Open the Snell's Law (II) lab.
On the Configuración tab, choose Water, orientation Normal, and one iteration.
Go to the Observación tab and use the ← → arrows to set incidence to 20 degrees. Record
i(shown on screen), then readroff the printed protractor using both video views (Vista cenital and Vista lateral); the lab does not showras a number, so if the line is hard to read, recordrwith an uncertainty of about 1-2 degrees.Repeat at 40, 60, and 80 degrees.
Back on the Configuración tab, choose Acrylic or Oil as a second medium and repeat the same incidence angles.
If time allows, measure the third medium or a second iteration.
The observation screen
On the Observación tab you read both video views. This screenshot is at 0 degrees only to show the controls; your measurement rows begin at 20 degrees. The incidence angle is shown as text and changed with the ← → arrows; the refraction angle is not shown as a number — you read it off the printed protractor.
Which method choice makes the comparison fair?
Record and process evidence
22 min
Complete a table row for each incidence angle and medium. Record r as a reading with uncertainty, not as a perfect textbook value. If the protractor is hard to read, use both views and state a reasonable uncertainty such as about 1-2 degrees. For normal-orientation rows, check that r < i and n > 1; if not, recheck whether you used the wrong protractor scale or measured from the surface.
Oil setup
In this lab, oil is expected to have a larger refractive index than water. Use the same incidence angles when comparing.
Snell's Law measurements
Fill at least 8 rows: four incidence angles for water and four for a second medium. Use calculator degree mode to calculate n = sin(i)/sin(r). In the note column, record both your uncertainty and whether the row came from your lab session, a classmate replicate, or a teacher reference row.
| Medium | Orientation | i from normal deg | r from normal deg | n = sin(i)/sin(r) | Uncertainty / source |
|---|---|---|---|---|---|
Average refractive index by medium
Add one summary row for each measured medium before answering the comparison questions. Average only rows from normal orientation.
| Medium | n values used | Average n | Main uncertainty |
|---|---|---|---|
Inspect your table. What evidence shows that you measured from the normal and used the same planned angle set for both media? Mention at least two rows from each medium and give their incidence angles.
Calculate and compare refractive index
12 min
For each normal-orientation row, calculate n = sin(i) / sin(r). Use calculator degree mode. Then average the n values for each medium. Do not expect every row to be identical; protractor reading and video visibility introduce uncertainty.
Show one complete calculation from your table: write i, r, sin(i), sin(r), and n. Then report the average n for water and for your second medium.
Which medium bent the ray more toward the normal? Use your average n values and one pair of matching incidence rows as evidence. Explain whether the result matches your prediction.
Evidence artifact
Attach a screenshot, hand sketch, or a small table you built that shows one measured ray and your calculation table. The artifact must include a normal line, one marked angle, and the related calculation row; a screenshot alone is not enough.
Observe total internal reflection
8 min
In normal orientation, light goes from air into the medium. In inverted orientation, the ray can travel from the medium toward air. For this short observation, use the same second medium you measured above; if your group has not chosen one, use Acrylic. You do not need to repeat all three media unless your teacher assigns the extension. Near the critical angle, the outgoing refracted ray may become weak or grazing; above it, no clear outgoing refracted ray emerges and the light reflects inside the medium instead. For water, oil, and acrylic this change happens between the 40 and 50 degree settings, so sweep carefully through that range. Because the lab changes angle in 10-degree steps, you can bound the critical angle but not measure it exactly.
Inverted orientation
Return to Configuración, keep your chosen medium, set orientation to Inverted, then go back to Observación. Look for the angle interval where the outgoing refracted ray becomes weak, grazing, or disappears.
In inverted orientation, the light travels from the medium toward air. Step the incidence angle up in 10-degree settings from about 30 degrees through 80 degrees and watch the outgoing refracted ray. Between which two 10-degree settings does it become weak, grazing, or disappear (so the light reflects internally instead)? Report your answer as an interval, or say what you observed if it was not clear.
Write the scientific claim
10 min
Your conclusion should be useful to someone who was not watching your screen. Name the evidence origin, calculations, uncertainty, and the limit of the claim. Do not say the lab "proved" the textbook index exactly; say what your readings support.
List two sources of uncertainty in this lab and explain how each could change the calculated value of n.
Write a claim-evidence-reasoning conclusion. Include: (1) whether the ray bends toward or away from the normal when entering a higher-index medium, (2) your average n values, (3) which medium bent light more, (4) what you observed in inverted orientation (if your class did that phase), and (5) one limitation of the evidence.
Optional extension: three media and critical angle
15 min
If your class has time, measure water, oil, and acrylic with the same incidence angles. Then use your average n to estimate a critical angle for each medium with theta_c = arcsin(1/n). Compare the estimate with the inverted-orientation interval you observed. Note: oil and acrylic have nearly equal indices (about 1.47 and 1.49), so their refraction angles differ by less than 1 degree — below what the protractor can resolve. Use the three media to show that both bend light noticeably more than water, not to rank oil against acrylic.