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Physics · Optics

Measure the refractive index of water with a laser pointer

Trace a laser beam entering water at several angles and get the refractive index from Snell's law.

  • DifficultyIntermediate
  • Estimated time1–3 hours
  • BudgetUnder ₹250
  • ClassClass 10–12
  • TeamSolo or up to 2
  • You needHousehold materials

Prerequisites: Refraction and the meaning of refractive index

Original · by TechnifyedExperimentLow costWell documented

My projects

Overview

Shine a low-power laser into a rectangular container of slightly cloudy water. Trace the incoming and refracted beams, measure both angles and plot sin i against sin r.

Why build this?

You get a physical constant from a straight-line graph, which is exactly what examiners look for in an investigatory project.

What you will learn

  • Snell's law
  • How to trace a beam accurately
  • How a graph's slope gives the constant

Skills you will use

Angle measurementGraphing

Safety

  • Use only a low-power (class 2 or below) laser pointer and never point it at eyes, faces, vehicles or aircraft.

Materials and tools

  • Class 2 laser pointer
  • Clear rectangular plastic box
  • Water with two drops of milk
  • Paper, pencil and protractor
  • Pins or tape

Tools and software

Protractor

Expected costUnder ₹250About ₹200 at most, less if you borrow parts
Estimated duration1–3 hours6 build steps, plus the report

Step-by-step roadmap

  1. Start

    You will finish with: ray diagrams for five angles, a graph and a value of the refractive index.

  2. 1

    Prerequisites

    Know this first: refraction and the meaning of refractive index. Then collect the 5 items in the materials list.

  3. 2

    Learn

    • Snell's law
    • How to trace a beam accurately
    • How a graph's slope gives the constant
  4. 3

    Plan

    Write your question, your hypothesis and the one thing you will change. Draw the table you will fill in before you start.

  5. 4

    Build

    1. Put the box on paper and draw its outline and a normal to one side.
    2. Add water and two drops of milk so the beam is visible.
    3. Aim the laser at the normal point at 20° and mark the beam's path in and out.
    4. Remove the box, join the marks and measure the angle of refraction.
    5. Repeat at 30°, 40°, 50° and 60°.
    6. Plot sin i against sin r; the slope is the refractive index.
  6. 5

    Test

    The slope should be close to 1.33.

  7. 6

    Document

    Record every reading with its unit, photograph the setup and draw a graph of the result. Report structure

  8. 7

    Present

    Explain the question, the method and the graph in two minutes; keep the setup ready to demonstrate. Presentation structure · Viva questions

  9. 8

    Publish

    Share the report and photos with your class, a science fair or your school magazine.

Expected outcome

Ray diagrams for five angles, a graph and a value of the refractive index.

Other versions of this project

Beginner version

Measure one angle pair and calculate the index once.

Advanced version

Repeat with sugar solutions of rising concentration and plot index against concentration.

Research version

Use the method to estimate the sugar content of soft drinks and compare with their labels.

Make this project better

  1. BasicMeasure one angle pair and calculate the index once.
  2. This projectMeasure the refractive index of water with a laser pointer
  3. AdvancedRepeat with sugar solutions of rising concentration and plot index against concentration.
  4. ResearchUse the method to estimate the sugar content of soft drinks and compare with their labels.

Ways to do this project

Text marked "You write this" is a prompt for your own work; everything else is specific to this project.

Problem
You write thisWrite your question as: How does [what you change] affect [what you measure]?
Hypothesis
You write thisWrite it as: If [I change this], then [this will happen], because [reason].
Variables
You write thisName the one thing you change (independent), the thing you measure (dependent) and what you keep the same (controlled).

Materials and procedure are in Materials and the Roadmap.

Observation table

You write thisDraw a table with one column for what you change, one for each repeat reading and one for the average.

Graph
You write thisPlot what you changed along the bottom and what you measured up the side; use a bar chart for categories and a line graph for numbers.
Results
You write thisState what the graph shows in one or two sentences, with numbers.
Conclusion
You write thisSay whether the result supports your hypothesis and why you think so.

Display board

  • Title and your name
  • Question
  • Hypothesis
  • Materials
  • Procedure (with photos)
  • Data table and graph
  • Conclusion
  • What you would do next

Questions judges ask

  • Why add milk?
  • What does a refractive index of 1.33 mean?
  • Why does the beam bend towards the normal?
  • What would you change if you did this again?
  • What was your biggest source of error, and how did you reduce it?
  • Where is this idea used in real life?

Working as a team

Solo or up to 2. Suggested roles:

ResearchSetup and measurementData and graphsDocumentationPresentation

Report and presentation

Report structure

  1. Title and question
  2. Background: the science behind it
  3. Hypothesis
  4. Materials
  5. Method (steps, with a diagram or photo)
  6. Observations (table)
  7. Results (graph and calculation)
  8. Sources of error
  9. Conclusion
  10. What you would do next
  11. References

Presentation structure

  1. The question
  2. Why it matters
  3. The idea behind it
  4. Setup (photo)
  5. Method in three steps
  6. Results (one clear graph)
  7. What the result means
  8. Errors and limits
  9. Conclusion and next step

Viva questions

Try answering before you open each one.

Why add milk?

The fat droplets scatter a little light so the beam's path can be seen.

What does a refractive index of 1.33 mean?

Light travels 1.33 times slower in water than in vacuum.

Why does the beam bend towards the normal?

It slows down on entering the denser medium.

Resources

PhET interactive simulationsphet.colorado.eduNCERT textbooksncert.nic.in

Source and attribution

Original

Written by Technifyed. Free to use for your own school or college project; write the report in your own words. Added 1 Oct 2026.

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