Double Slit & Diffraction
Change wavelength and slit spacing and see the interference pattern and its intensity graph.
Drag the two slits closer or further apart. Try the Particles view to build the pattern one photon at a time.
About the Double Slit & Diffraction
Free double slit simulator. Drag the slit separation, change wavelength and slit width, and see the bright and dark fringes with a live intensity graph. Built for physics, the double slit & diffraction runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Change wavelength and slit spacing and see the interference pattern and its intensity graph. Use it to explore physics ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the Double Slit & Diffraction
- Use the controls to change Wavelength, Slit separation d, Slit width a. The simulation reacts instantly.
- Pick an option such as Wave, Particles to switch modes or load an example.
- Press "Clear screen" to start, reset or change what is happening.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Change the colour of the light and watch the fringes move.
- Drag the slits closer together and see the pattern widen.
- Make each slit wider and watch the pattern fade away from the centre.
- Read off the fringe spacing and compare with λL/d.
Key ideas you can learn
- Bright fringes occur where the path difference is a whole number of wavelengths: d sin θ = mλ.
- Fringe spacing is Δy = λL / d.
- Narrower slits spread the light out more (diffraction envelope).
Where this is used in the real world
Interference from two slits proved that light is a wave and later that electrons behave like waves too. The same ideas drive interferometers and holography.
Who is this simulation for?
Physics students in middle school, high school and first-year university, teachers who want a quick demonstration for the projector, and anyone revising for exams. It works well for flipped classrooms because students can explore before the lesson.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
Why do we see bright and dark bands?
Light from the two slits overlaps. Where crests meet crests the waves reinforce (bright). Where crests meet troughs they cancel (dark).
What happens if you make the slits closer together?
The fringes spread further apart, because fringe spacing is inversely proportional to the slit separation.
Is the Double Slit & Diffraction free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the Double Slit & Diffraction work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.