How does light bend in water?

Aim a light ray at water. Measure its direction before and after the surface, then use your evidence to explain refraction.

Starting notebook…

Simulation · Light & Optics

How does the incoming angle affect the refracted angle when light travels from air into water?

Refraction is a change in the direction of light as it crosses between materials where its speed differs. The normal is an imaginary line at right angles to the surface. Measure both angles from this line, not from the surface. Light entering water from air bends toward the normal unless it enters straight along the normal. Some light is also reflected at the surface.

Review the related lesson →

Science words you will use

A hypothesis is a possible explanation. A prediction is what you expect to observe. The independent variable is what you change or compare; the dependent variable is what you measure. Controlled variables stay the same in a fair test. Qualitative observations describe what you notice; quantitative observations use numbers and units.

Accuracy means closeness to a trusted value. Precision describes how closely repeated readings agree. Repeating a result does not prove it is accurate. A pattern between two quantities does not, by itself, show that one caused the other.

What good work includes

Measure both angles from the normal, with the protractor centred at the entry point.
Collect at least six angle pairs for air to water and check two readings.
Use at least three angle pairs to explain the pattern and describe one possible reading error.

Plan your investigation

My scientific notebook

Plan your investigation

Use your own words. You can return and change your ideas as you collect evidence.

My research questionWhat can you investigate using this equipment or record?My hypothesisGive a possible explanation: I think… because…My predictionIf… then I predict… because…What I change or compareName the independent variable. In an observation study, name what you compare.What I measureName the dependent variable and its units.What stays the sameHow will you keep the material pair, light colour, entry point and protractor alignment the same while changing the incoming angle?My procedure and changesUse repeatable steps. Record changes to the plan and why you made them.Reading my equipmentWhat is the smallest marked division? What units and rounding will you use?

Your apparatus

Investigate with PhET Bending Light

Open Intro in PhET Bending Light. Choose Ray, keep Air above and Water below, and turn on the laser. Show the Normal. Drag the protractor so its centre sits where the ray meets the surface and its 0° line follows the normal. Measure the ray inside the water, not the reflected ray above it.

Open PhET in a new tab

On a phone, turn sideways and open the model in a new tab for larger controls. Your notebook stays on this page.

Choose Intro, turn on the laser and use the protractor. Record the angles you read yourself. If the tools are too small, open PhET in a new tab.

PhET Interactive Simulations, University of Colorado Boulder.

Materials & safety

PhET Bending Light simulation
On-screen protractor
Digital notebook

This is a computer model. Do not use a real laser for this activity. Never aim a real laser at eyes or shiny surfaces.

Suggested procedure — adapt and record changes

Write your prediction before testing. Choose Intro, Ray, Air above and Water below. Keep these materials and the light colour the same for the first comparison.
Turn on the laser and show the Normal. Centre the protractor at the point where the light meets the surface. Check which protractor scale measures from the normal.
Move the laser to an incoming angle near 10°. Read the actual angle from the normal. Measure the refracted angle in water from the same normal. Enter both readings, in degrees, with the group Air to water.
Repeat for incoming angles near 20°, 30°, 40°, 50° and 60°. Record your actual readings rather than copying the target angles. Keep the materials and protractor alignment fixed.
Check at least two readings by moving the laser away, setting it again and remeasuring. Re-reading one unchanged model value checks your reading; it is not an independent physical experiment.
Plot refracted angle against incoming angle. Compare the two angles in each pair. Describe the pattern using your readings; a straight-line pattern is not guaranteed.
Optional extension: after saving your first comparison, keep Air above and choose Glass below. Repeat as a separate group Air to glass. Compare refracted angles at matching incoming angles. Keep the two material pairs under different group names.

Collect · check · compare

Your data and graph

Both angles are measured from the normal in degrees. Zero is valid for straight-on entry. Use a separate group for each pair of materials. Record your measurements, not calculated answers.

Graph titleGraph view

The table keeps your exact readings. On a small screen, scroll inside the table or graph to see the rest. With a keyboard, focus it and use the arrow keys.

Add readings to enable data and graph downloads.

Explain your results and reflect

Think like a scientist

Explain, question and reflect

Patterns in my resultsUse at least three pairs of measured angles. As the incoming angle grows, what happens to the refracted angle? Which angle is smaller for air into water?Limits and possible errorsWhat is the smallest protractor division you can read? Could measuring from the surface or choosing the reflected ray change your result? Explain how a computer model differs from a real beam and water surface.My conclusionAnswer your research question. Your result does not have to match your prediction.The evidenceUse measurements, units and trial numbers to support your conclusion.Why the evidence supports my conclusionExplain how your measured angles support your claim about bending. Use this frame if helpful: When the incoming angle was ___°, the refracted angle was ___°. This shows ___ because ___. One limit of my evidence is ___.What I would improve or test nextWhat would you change? Why? Revisit your hypothesis and prediction.

A new question to explore

Send light straight along the normal. Does its direction change when it enters water? Explain why a ray can change speed without changing direction. Then compare air-to-water and air-to-glass readings at the same incoming angle.

Keep your work

Your lab report

Preview your notes, measurements, calculations and graph. Choose Print / Save as PDF in the report, or download an HTML copy. Submit that PDF in Google Classroom using your teacher’s instructions. This website does not submit your work.

Student name (optional)Class (optional)Investigation date (optional)

Restore notebook backup

A backup is a JSON file for restoring this notebook. It is different from your finished report. Keep your own copy before clearing browser data or changing devices.