Light bends when it passes from one material into another. Today you’ll measure how much, turn your measurements into a number that describes the material, and use that number to explain the bent straw and to find out when light gets trapped.
🎯 Goals
Measure how light bends entering acrylic, and calculate the block’s index of refraction.
Use n = c ÷ v and Snell’s law to find speeds and angles.
Explain why a straw looks bent and a pool looks shallow.
Find a critical angle and explain total internal reflection.
🧰 Materials
🧊A clear acrylic block, rectangular for Sections 1–3, and semicircular for Section 4 if you have one
🔦A laser pointer or ray box. Pins and cardboard also work
📐A protractor and a ruler
📄Plain paper, taped to a flat board
🥤A clear glass or cup, water, and a straw or pencil
🪙A coin and an opaque cup (for the coin trick)
🧮A scientific calculator with a sin and sin⁻¹ button
📓Physics notebook
Section 1 of 4
Bending Light Through Acrylic
1
Trace the block and draw the normal.
Place the rectangular block on a sheet of paper and trace its outline. Pick a spot on one long side and draw the normal there, a line at 90° to the side. Use a protractor to mark rays at 20°, 30°, 45°, 60°, and 70° from the normal.
Index of refraction: n = c ÷ v, where c = 3.00 × 10⁸ m/s Snell’s law: n₁ sin θ₁ = n₂ sin θ₂ Going into a material with a bigger n, light bends toward the normal. Both angles are measured from the normal.
2
Trace the beam through the block for five angles.
For each angle, send a narrow beam along your marked ray so it strikes the block at the normal. Mark two dots on the incoming beam, and mark where the beam leaves the far side, plus one dot along the emerging beam. Remove the block, and join the entry point to the exit point. That line is the path inside the block. Measure θ₂, the angle between that line and the normal, at the entry point.
⚠️ Never point a laser at anyone’s eyes, and keep the beam flat on the paper. No laser? Use pins: stand two pins in line with the incoming ray, look through the block, and line up two more pins with the pins’ images.
3
Calculate n for every row, then average.
Use n = sin θ₁ ÷ sin θ₂ (air has n = 1.00). The Refraction Checker does one row at a time. Average your five values for your best estimate of the block’s n.
Enter one row of your data. The checker finds n for that row. Add a reference value, such as 1.49 for acrylic, to see how close you got.
What good data looks like
Your θ₂ should always be smaller than θ₁, and your n values should cluster around 1.5 for acrylic, give or take 0.05. Rows that are far off usually have a misplaced entry or exit point. Also check that your emerging ray is parallel to your incoming ray. In a block with parallel sides it always is, just shifted sideways.
📓 Physics Notebook
Does the light bend toward or away from the normal entering the acrylic? What is your best estimate of n, and how close is it to the accepted 1.49? What does a bigger n tell you about the speed of light in the material?
Section 2 of 4
Speed, Index, and Snell’s Law
4
Practice with four calculations.
The index of refraction is a speed ratio, and Snell’s law turns it into an angle. Use your calculator in degree mode. Answers within 2% count as correct.
Snell’s Law Practice
Use c = 3.00 × 10⁸ m/s, with n = 1.00 for air.
What is the speed of light in water, with n = 1.33? Use c = 3.00 × 10⁸ m/s.v = c ÷ n. Type it like 2.26e8
Light travels at 2.0 × 10⁸ m/s in a certain glass. What is its index of refraction?n = c ÷ v
Light in air hits glass (n = 1.50) at 30° from the normal. What is the angle of refraction?1.00 sin 30° = 1.50 sin θ₂
Light in water (n = 1.33) hits air (n = 1.00) at 30° from the normal. What is the angle of refraction?1.33 sin 30° = 1.00 sin θ₂
Section 3 of 4
The Bent Straw and the Shallow Pool
5
Look at a straw in water.
Stand a straw or pencil in a clear glass of water. Look at it from the side, then from above, then from different angles. Sketch what you see at the water’s surface.
6
Try the coin trick.
Put a coin at the bottom of an empty opaque cup. Move back until the rim of the cup just hides the coin. Now have a partner pour water into the cup without moving the coin or you. The coin comes back into view.
7
Explain what happened with the diagram.
Use the Shallow Pool below. Light from the coin bends away from the normal as it leaves the water, but your brain assumes it traveled in a straight line, so it places the coin higher than it is.
The Shallow Pool
Two rays leave a coin at the bottom of the water. One goes straight up. The other bends away from the normal as it leaves the water. Your eye follows the rays back in straight lines (the dashed line), so the coin seems to be higher than it really is.
Looking straight down: apparent depth ≈ actual depth ÷ n = 8 ÷ 1.33 = 6.0 cm The slanted ray in the diagram meets a little off this value, since the formula is for a straight-down view.
📓 Physics Notebook
Explain in your own words why a straw looks bent at the surface of the water, and why the coin appeared when water was poured in. A 2.0 m deep pool seems how deep when you look straight down? (Use apparent depth ≈ actual depth ÷ n.)
Section 4 of 4
Total Internal Reflection
8
Find the critical angle.
Use the semicircular block with its flat side facing away from you. Aim the beam at the middle of the curved side, so it enters without bending and strikes the flat face from inside. Slowly increase the angle of incidence inside the block. At first, part of the beam escapes into the air and bends away from the normal. At one angle, the escaping beam runs along the flat face and then vanishes. That inside angle is the critical angle. Measure it from the normal.
9
Compare your measurement with the prediction.
Light leaving a denser material is bent away from the normal. At the critical angle, it would bend a full 90°, so it skims the surface. Beyond that angle, all of the light reflects. The prediction is θ_c = sin⁻¹(n₂ ÷ n₁), which for acrylic to air is sin⁻¹(1 ÷ n).
Sample data (use only if you can’t run this part)
Measured critical angle 41°, with n for the block = 1.49.
Predicted θ_c = sin⁻¹(1 ÷ n) (°)
Measured θ_c (°)
% difference
Critical Angle Checker
Enter your block’s n from Section 1, and the critical angle you measured. The checker predicts it and compares.
Critical Angles
Light going from each material into air.
What is the critical angle for light going from glass (n = 1.50) into air?θ_c = sin⁻¹(n₂ ÷ n₁)
What is the critical angle for water (n = 1.33) into air?
What is the critical angle for diamond (n = 2.42) into air?
📓 Physics Notebook
Total internal reflection needs two things. What are they? Name one device that uses it (fiber optics, a prism in binoculars, a bicycle reflector), and explain why it needs a smooth, clear material with a high n.
Question 1
Using Table A, how do θ₂ and θ₁ compare when light goes from air into acrylic? What is your block’s n, and how close is it to 1.49?
Question 2
Explain why light bends when it enters glass, using the idea of its speed.
Question 3
Explain why a straw looks bent in water and a pool looks shallower than it is.
Question 4
What conditions are needed for total internal reflection? Describe one use, and why it works.
📤 Turn In
Your traced ray diagram and completed Table A, with your average n
Your Snell’s Law Practice results and your notebook note on the bend direction
Your straw sketch, your coin-trick explanation, and the shallow-pool answer
Completed Table B (critical angle) and your Critical Angles results