Refraction and Lenses Lesson Plan with Simulations
Updated 2026-10-02
This refraction and lenses lesson plan uses two free simulations so students can measure how light bends, find the refractive index of an unknown material, trap light by total internal reflection, and then see how a lens uses the same bending to form an image. No darkened room, no ray boxes and no protractors that slip. Everything is ready to use: learning goals, setup, predictions, a step-by-step activity with the readings students should get, a five-question set for the class link, an extension and ideas for differentiation. Every number comes from the simulations' own models.
Lesson at a glance
- Level: grades 9–11 (ages 14–17), physics or physical science.
- Time: one 60-minute period, or two shorter ones (refraction, then lenses), plus an optional 15-minute extension.
- Prior knowledge: angles measured from the normal, the sine button on a calculator, light travels in straight lines.
- Format: pairs on laptops or tablets, or the whole class with one projector.
- Simulations: Light rays – refraction, total internal reflection and prisms and Lenses and curved mirrors – ray diagrams.
Learning goals
By the end of the lesson, students can:
- Describe how a ray bends toward the normal when it enters a medium with a higher refractive index, and away from it when it leaves.
- Use Snell's law, n₁·sin i = n₂·sin r, to calculate a refractive index from measured angles.
- Explain total internal reflection and find a critical angle.
- Use a ray diagram and 1/f = 1/d + 1/d′ to describe the image formed by a converging lens.
- Explain why a lens made of a denser material has a shorter focal length.
What the simulations do
Light rays (refraction). The Intro screen shows a laser hitting the boundary between a top medium (n₁) and a bottom medium (n₂). Each can be Air (1.00), Water (1.33), Glass (1.50), Mystery A or Mystery B. Drag the laser, or use the slider and the − and + buttons, to set the angle of incidence from 0° to 89° in 1° steps. The readout lists n₁ and n₂, the angles i and r to 0.1°, the critical angle, and the percentage of light reflected and transmitted. For a mystery medium, n and the critical angle show "?" until someone presses Check mystery. The Prisms and Tools screens add dispersion, a light-intensity probe and a wave view.
Lenses and curved mirrors. The Lens screen sets a converging or diverging lens from its radius of curvature R (20–100 cm) and refractive index n (1.3–1.9), using f = R / (2(n − 1)). Drag the red object arrow, or use the d slider, to set the object distance from 5 to 150 cm. The status line gives f, d, the image distance d′, the magnification k and the image type. Tick boxes add the three principal rays (on by default), marginal rays, a second point, a ruler and Point source + screen.
Two details worth knowing before class:
- The angle of refraction appears even for mystery media. Only n and the critical angle are hidden.
- Magnification has a sign. In the lens simulation, a negative k means an inverted image and a positive k an upright one.
Materials and setup before class
Materials: one device per pair (or a projector), the tables below on paper, and a calculator with sine and inverse sine.
Setup (10 minutes, once):
- Open the light rays simulation. The defaults are what you need: Intro screen, air on top, water below, 45°, protractor on.
- Click Share and create a link for each class, for example "Physics · Period 6". Do the same for the lens simulation.
- Optional: on the Questions tab of the light rays simulation, enter the question set below and attach it to the link. The two predictions lock the simulation until each student commits.
- Post both links, or open the first one in present mode and show the QR code.
The mystery indices are not in the Starting values panel, so step 3 works on an ordinary link.
Lesson sequence
1. Hook and predictions (6 minutes)
Ask: "Why does a straw look bent in a glass of water?" Collect ideas, then have students commit to two predictions, on paper or on the link:
- P1. "A laser goes from air into water at 45°. In the water, what does the ray do?"
- P2. "Light goes from glass into air. You slowly increase the angle of incidence. What happens to the ray that comes out into the air?"
Many students think light bends away from the normal going into water, and few expect the outgoing ray to vanish. Don't correct anyone yet. The Predict–Observe–Explain guide explains why the commitment matters.
2. Snell's law from data (12 minutes)
Pairs keep air on top and water below, set each angle with the slider or the + and − buttons, and record r from the readout. The last column is for you.
| i (°) | r (°) | Reflected | sin i ÷ sin r |
|---|---|---|---|
| 10 | 7.5 | 2% | 1.33 |
| 20 | 14.9 | 2% | 1.33 |
| 30 | 22.1 | 2% | 1.33 |
| 45 | 32.1 | 3% | 1.33 |
| 60 | 40.6 | 6% | 1.33 |
| 80 | 47.8 | 35% | 1.33 |
The ray always bends toward the normal, and the ratio sin i ÷ sin r stays at 1.33, the refractive index of water. That is Snell's law with n₁ = 1. Check P1 now.
Point out the reflected column too. At a steep 80°, 35% of the light bounces off, which is why a lake glares at sunset.
3. Identify the mystery materials (10 minutes)
Pairs choose Mystery A as the bottom medium, measure r at three angles and calculate n = sin i ÷ sin r. Then they repeat for Mystery B and press Check mystery.
| i (°) | r for Mystery A (°) | r for Mystery B (°) |
|---|---|---|
| 30 | 24.6 | 18.2 |
| 45 | 36.1 | 26.2 |
| 60 | 46.2 | 32.8 |
| n | 1.20 | 1.60 |
Each angle gives n within 0.01 of the true value. Ask: "Which material slows light down more?" Mystery B, because a higher n means a slower speed, v = c/n. The Tools screen shows these speeds, for example 2.26 × 10⁸ m/s in water and 2.00 × 10⁸ m/s in glass.
4. Total internal reflection (10 minutes)
Pairs put Glass on top and Air below, so the light now leaves the denser medium. They raise i slowly.
| i (°) | 20 | 30 | 40 | 41 | 42 |
|---|---|---|---|---|---|
| r (°) | 30.9 | 48.6 | 74.6 | 79.8 | none |
| Reflected | 4% | 6% | 25% | 38% | 100% |
The ray bends away from the normal, the reflected ray gets stronger, and at 42° the refracted ray disappears: total internal reflection. The readout gives the critical angle, 41.8°. Check P2 now.
Then compare other pairs. Water to air has a critical angle of 48.8° and glass to water 62.5°. The smaller the ratio n₂/n₁, the smaller the critical angle, which is why diamonds and optical fibers trap light so well.
5. From refraction to lenses (17 minutes)
Open the lens link. The defaults are a converging lens with R = 40 cm and n = 1.5, so f = 40.0 cm, and the object at d = 60 cm. Pairs move the object and record the status line:
| d (cm) | d′ (cm) | k | Image |
|---|---|---|---|
| 120 | 60.00 | −0.50 | real, inverted, diminished |
| 80 | 80.00 | −1.00 | real, inverted, same size |
| 60 | 120.00 | −2.00 | real, inverted, magnified |
| 50 | 200.00 | −4.00 | real, inverted, magnified |
| 40 | at infinity | none | no image |
| 30 | 120.00 | 4.00 | virtual, upright, magnified |
| 20 | 40.00 | 2.00 | virtual, upright, magnified |
Students check one row with 1/f = 1/d + 1/d′. For d = 60 cm: 1/d′ = 1/40 − 1/60 = 1/120, so d′ = 120 cm. At d = 40 cm the object sits at the focal point and the rays leave parallel.
Then tick Point source + screen. With d = 60 cm, the screen starts at 120 cm and the light lands as a sharp point. Drag the screen nearer or further and the patch grows. Move the object to d = 30 cm: no screen position gives a point, because a virtual image can't be caught on a screen.
Finally, link the two simulations. Set the lens's n from 1.5 to 1.8: f drops from 40.0 cm to 25.0 cm. Ask: "Why does denser glass give a stronger lens?" It bends each ray more at each surface, exactly as the higher-index mystery material did in step 3.
6. Exit check (5 minutes)
Use questions 3–5 of the set below. Then open View answers and show the Prediction and After columns for P1 and P2 side by side.
Question set for this lesson
Enter these on the light rays simulation's Questions tab. Question 5 uses the lens link, so post both links together. Suggested Instructions for students: "Use the Intro screen. Change the angle with the + and − buttons. Read i and r from the readout."
1. Multiple choice · Before, as a prediction · Ask again after the simulation
- Question: "A laser goes from air into water at 45°. In the water, what does the ray do?"
- Options: Bends toward the normal (correct) / Bends away from the normal / Goes straight on / Reflects back completely
- Explanation: "Light slows down in water, which has a higher refractive index (1.33) than air (1.00), so the ray bends toward the normal. Snell's law gives sin r = sin 45° ÷ 1.33, so r = 32.1°, smaller than i."
2. Multiple choice · Before, as a prediction · Ask again after the simulation
- Question: "Light goes from glass into air. You slowly increase the angle of incidence. What happens to the ray that comes out into the air?"
- Options: It bends closer to the normal / It stays at the same angle as the incident ray / It bends further from the normal, then disappears (correct) / It disappears as soon as the angle is above 0°
- Explanation: "Leaving glass for air, the ray bends away from the normal. As i grows, r reaches 90°. Above the critical angle, 41.8° for glass to air, there is no refracted ray at all and all the light reflects back into the glass: total internal reflection."
3. Number · After the simulation
- Question: "Put Air on top and Mystery A below. Measure r for i = 30°, 45° and 60°, and calculate the refractive index of Mystery A."
- Answer: 1.20, tolerance ± 0.03, no unit
- Explanation: "With air on top, n = sin i ÷ sin r. At 30° the readout gives r = 24.6°, so n = 0.500 ÷ 0.416 ≈ 1.20. The other angles give the same value. Mystery A bends light less than water (1.33) and glass (1.50)."
4. Number · After the simulation
- Question: "Put Mystery B on top and Air below. Find the critical angle by changing i in 1° steps."
- Answer: 38.7, tolerance ± 1, unit °
- Explanation: "At 38° a refracted ray still leaves Mystery B, at 39° it disappears, so the critical angle lies between them. From sin c = 1 ÷ 1.60, c = 38.7°. Mystery B has a higher index than glass, so its critical angle is smaller than glass's 41.8°."
5. Number · After the simulation
- Question: "In the lens simulation, keep the converging lens with f = 40 cm and put the object at d = 120 cm. How far from the lens is the image?"
- Answer: 60, tolerance ± 1, unit cm
- Explanation: "1/d′ = 1/f − 1/d = 1/40 − 1/120 = 2/120, so d′ = 60 cm. The image is real, inverted and half the size of the object (k = −0.50), because the object is further than 2f from the lens."
Questions 1 and 2 are the predictions, asked again after the simulation so students can compare. The formative assessment guide explains how to read the results across classes.
Extension: color and diverging lenses
Dispersion (8 minutes). On the light rays simulation's Prisms screen, white light splits into six colored rays. The readout shows why: the prism glass has n = 1.537 for violet light (410 nm) but n = 1.491 for red light (650 nm), so violet bends more. Students drag and rotate the prism to make the spectrum as wide as they can.
Diverging lens (7 minutes). Back on the Lens screen, press Diverging. With R = 40 cm and n = 1.5, f = −40.0 cm. At d = 60 cm the image is virtual, upright and smaller: d′ = 24.00 cm, k = 0.40. At d = 20 cm, d′ = 13.33 cm and k = 0.67. Ask: "Can a diverging lens ever make a real image of a real object?" No: every row gives a virtual, upright, smaller image.
The Mirror screen of the same simulation covers concave and convex mirrors if you want a third lesson.
Differentiation
Support:
- Give the tables with the angles filled in, so students only read and record.
- In step 2, use only 30°, 45° and 60°, and give the sines on the sheet.
- Give sentence starters: "Going into a denser medium, the ray bends…" and "Above the critical angle…"
Stretch:
- Predict every r in the step 2 table from Snell's law before measuring.
- Find the critical angle for Mystery A to air without pressing Check mystery (56.4°), then confirm.
- In the lens simulation, find the R that gives f = 20 cm with n = 1.5 (20 cm), and explain the result with f = R / (2(n − 1)).
English learners: the simulations are available in six languages. Create a second link in the student's language so the labels are familiar while the discussion stays in English.
Standards alignment
This lesson fits the light and optics unit of a high school physics course, and the refraction and lenses topics of GCSE Physics. The v = c/n discussion in step 3 touches NGSS HS-PS4-1 (wave speed in different media), but geometric optics has no dedicated NGSS performance expectation, so we don't claim more. The lesson does exercise Analyzing and Interpreting Data and Using Mathematics and Computational Thinking.
For more ideas, see interactive physics lesson ideas. If you want both labs on one class page, embedding simulations in Google Sites uses these exact simulations as its example. For general routines, see how to use interactive simulations in the classroom.
FAQ
Why do my students' Snell's law ratios come out as 1.32 or 1.34?
The readout rounds r to 0.1°, so at small angles the ratio can drift in the second decimal: 5° gives 1.32. Angles of 10° and more, or the mean of three angles, bring it back to 1.33.
Is there a single simulation for both refraction and lenses?
Yes. Refraction, total internal reflection and lenses puts a Snell's law panel and a lens panel on one screen. It suits a quick review lesson; the two simulations in this plan give more to measure.
Can students do this lesson on phones?
Yes. Students open the links without an account. The + and − buttons make exact angles easy on a touch screen; reading the lens status line is easier on a tablet or laptop.