The Beer’s Law Lab simulation from PhET lets you make colourful concentrated and dilute solutions and use a virtual spectrophotometer to see how much light they absorb and transmit. You can connect the colour and concentration of a solution to the light that passes through it, which makes Beer’s law easy to picture. It is free, runs in any browser on a phone, tablet or laptop, and it is a great fit for solutions and spectroscopy lessons in Tamil Nadu State Board (Samacheer Kalvi) Chemistry.
What is the Beer’s Law Lab simulation?
This interactive simulation lets you prepare solutions of different concentrations and then shine light through them with a virtual spectrophotometer. You can see how much light the solution absorbs and how much it transmits, and how that changes with the solution, its concentration and the container. Because it runs on HTML5, there is nothing to install, and it works on phones, tablets, Chromebooks and computers.
High school to university
Concentration, absorbance and transmittance
Phone, tablet, Chromebook, PC
English
PhET’s learning goals include relating volume and amount of solute to concentration, explaining how solution colour relates to concentration, calculating molarity, identifying a saturated solution, relating concentration to the light absorbed or transmitted, and relating absorbance, molar absorptivity, path length and concentration in Beer’s law.
Launch the Beer’s Law Lab simulation online
Tap or click inside the player and start exploring. Use the full-screen button in the player for a bigger view.
Credit: University of Colorado Boulder
How to use the Beer’s Law Lab simulation, step by step
- Predict first. Before you change anything, say what you think will happen to the colour of the solution and to the light that passes through.
- Make solutions of different strengths. Compare a dilute solution with a concentrated one and notice the colour.
- Measure the light. Use the virtual spectrophotometer to see how much light is absorbed and how much is transmitted.
- Change one thing at a time. Change the concentration, the solution, the container width or the light source and watch how the light reading changes.
- Look for the pattern. Find how the absorbed light changes when you double the concentration or the width of the container.
Tip for students: record your readings for three concentrations and see whether they follow a straight line. That is Beer’s law at work.
The idea behind the simulation: A = ε × l × c
A = ε × l × c
Here A is the absorbance (it has no unit), ε is the molar absorptivity in L/(mol·cm), l is the path length through the solution in cm and c is the concentration in mol/L. The transmittance is the fraction of light that gets through, and T = 10−A. A higher absorbance means less light is transmitted.
Worked example: A solution has ε = 1000 L/(mol·cm), a path length of 1 cm and a concentration of 0.0005 mol/L. Then A = 1000 × 1 × 0.0005 = 0.5, and T = 10−0.5 ≈ 0.316, so about 32% of the light passes through. Doubling the concentration doubles A to 1.0 and cuts the transmittance to 10%.
Beer’s law key terms explained
These are the words you will meet while using the Beer’s Law Lab simulation, with simple meanings.
Concentration
How much solute is present in a given volume of solution, often in mol/L.
Molarity (M)
Moles of solute per litre of solution (mol/L).
Absorbance (A)
A measure of how much light a solution absorbs. It has no unit.
Transmittance (T)
The fraction of light that passes through the solution.
Molar absorptivity (ε)
How strongly a substance absorbs light at a given wavelength. Its unit is L/(mol·cm).
Path length (l)
The distance the light travels through the solution, which is the width of the container.
Spectrophotometer
An instrument that shines light through a sample and measures how much is absorbed or transmitted.
Cuvette
The small transparent container that holds the sample in a spectrophotometer.
Saturated solution
A solution that cannot dissolve any more solute at that temperature.
Common mistakes: Mixing up absorbance with transmittance (more absorbance means less transmittance), thinking a straight-line relation holds at every concentration, and forgetting to use the path length in centimetres.
Try the Beer’s law calculator (practice what you explore)
Enter the molar absorptivity, the path length and the concentration to see the absorbance and the percentage of light transmitted. Then tap a “what if” button.
What happens to the light when you change a variable
| Change | Effect on absorbance and light |
|---|---|
| Concentration doubled | Absorbance doubles, so less light is transmitted |
| Concentration halved | Absorbance halves, so more light is transmitted |
| Path length doubled | Absorbance doubles |
| Absorbance of 0 | All the light is transmitted (100%) |
| Absorbance of 1 | 10% of the light is transmitted |
Classroom activities for teachers
The Beer’s Law Lab simulation fits a single 40-minute period.
- Predict and test (10 min): students write a prediction before each change, then check it in the simulation.
- Straight-line graph (10 min): students record absorbance for several concentrations and plot the points.
- Find the unknown (10 min): use a known pattern to find the concentration of an unknown solution from its reading.
- Exit ticket (10 min): students explain in two sentences why a darker solution lets less light through.
Looking for the matching chapters? Browse the Science and Chemistry books on TN SchoolBooks and download the Samacheer Kalvi PDFs, then use this simulation to make the solutions lessons visual. More free science simulations and textbook PDFs are on the home page.
Quick quiz: test your understanding of Beer’s law
Score: 0 / 5
Beer’s Law Lab simulation: frequently asked questions
Is the Beer’s Law Lab simulation free?
Yes. PhET simulations are free to run online in a browser. Check PhET’s licensing page for reuse terms.
Does it work on a mobile phone?
Yes. It is built with HTML5 and works on phones, tablets and Chromebooks. Use landscape mode or the player’s full-screen button for a bigger view.
What does Beer’s law say?
Beer’s law says that the absorbance of a solution is proportional to its concentration and to the path length of the light through it. It is written A = ε × l × c.
What is the difference between absorbance and transmittance?
Absorbance measures how much light the solution takes in. Transmittance measures how much light gets through. When absorbance goes up, transmittance goes down.
The Beer’s Law Lab simulation turns concentration and colour into something you can adjust, measure and see. Explore it, take the quiz above, and keep experimenting until predicting the light reading feels easy.