The Molecules and Light simulation from PhET lets you turn on a light source, choose a type of light and a molecule, and watch what happens in the observation window. You can discover which molecules absorb which kinds of light, and how the energy of the light changes the way a molecule moves. It is free, runs in any browser on a phone, tablet or laptop, and it is a great fit for the light, spectrum and greenhouse gas lessons in Tamil Nadu State Board (Samacheer Kalvi) Science, Physics and Chemistry.
What is the Molecules and Light simulation?
This interactive simulation lets you shine different types of light on the molecules found in the atmosphere. You can see whether a molecule absorbs the light and how it moves afterwards, and you can pan and zoom the observation window for a closer look. Because it runs on HTML5, there is nothing to install, and it works on phones, tablets, Chromebooks and computers.
Middle school to university
Light, photon energy, absorption and greenhouse gases
Phone, tablet, Chromebook, PC
English
PhET’s learning goals are to explore how light interacts with molecules in the atmosphere, to see that absorption depends on both the molecule and the type of light, to relate the energy of light to the motion of a molecule, to recognize that photon energy increases from microwave to ultraviolet, and to see how molecular structure affects how a molecule interacts with light.
Launch the Molecules and Light 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 Molecules and Light simulation, step by step
- Predict first. Before you turn on the light, say whether you think the molecule will absorb it.
- Turn on the light source. Choose a type of light and a molecule, then watch the observation window.
- Change the type of light. Try microwave, infrared, visible and ultraviolet light on the same molecule.
- Change the molecule. Try different molecules with the same light and compare what happens.
- Zoom in. Pan and zoom the observation window to see how the molecule moves after it absorbs a photon.
Tip for students: make a table of molecule, type of light, and whether the light is absorbed. Patterns show up quickly.
The idea behind the simulation: E = h × c ÷ λ
E = h × c ÷ λ
Here E is the energy of one photon in joules (J), h is Planck’s constant (6.626 × 10⁻³⁴ J·s), c is the speed of light (about 3.0 × 10⁸ m/s) and λ is the wavelength in metres. The shorter the wavelength, the more energy each photon carries. So photon energy goes up from microwave to infrared, visible and ultraviolet. Low-energy microwaves can make molecules rotate, infrared light can make them vibrate, and high-energy ultraviolet light can even break bonds.
Worked example: Green light of wavelength 550 nm has a frequency of 5.45 × 10¹⁴ Hz. Its photon energy is 6.626 × 10⁻³⁴ × 3.0 × 10⁸ ÷ 550 × 10⁻⁹ ≈ 3.6 × 10⁻¹⁹ J, or about 2.25 eV. An infrared photon of wavelength 10 µm has only about 0.12 eV, roughly 18 times less energy.
Light and molecules key terms explained
These are the words you will meet while using the Molecules and Light simulation, with simple meanings.
Photon
A tiny packet of light energy.
Wavelength (λ)
The distance between two peaks of a light wave. Shorter wavelengths carry more energy.
Electromagnetic spectrum
The whole range of light, from radio waves and microwaves to visible light, ultraviolet and beyond.
Absorption
When a molecule takes in a photon and uses its energy.
Infrared (IR)
Light with longer wavelengths than red light. It makes molecules vibrate.
Ultraviolet (UV)
Light with shorter wavelengths than violet light. It has enough energy to break some bonds.
Greenhouse gas
A gas, such as carbon dioxide or water vapor, that absorbs infrared light and traps heat.
Dissociation
When a molecule breaks apart after absorbing a high-energy photon.
Common mistakes: Thinking all molecules absorb all kinds of light, mixing up wavelength with energy (a longer wavelength means less energy), and believing that the most energetic light is always the most absorbed.
Try the photon energy calculator (practice what you explore)
Enter a wavelength and pick a unit. The calculator gives the frequency, the energy of one photon and the type of light. Then tap a “what if” button.
What happens when you change a variable
| Type of light | Typical effect on a molecule |
|---|---|
| Microwave | Can make some molecules rotate |
| Infrared | Can make some molecules vibrate (greenhouse gases absorb it) |
| Visible | Little absorbed by most atmospheric gases |
| Ultraviolet | High energy, can break bonds |
| Shorter wavelength | Higher photon energy |
Classroom activities for teachers
The Molecules and Light simulation fits a single 40-minute period.
- Predict and test (10 min): students predict whether a molecule absorbs a type of light, then check.
- Absorption table (10 min): students fill in a table of molecules and types of light, and mark where absorption happens.
- Energy ladder (10 min): students order the types of light by photon energy and link each to a molecular motion.
- Exit ticket (10 min): students explain in two sentences why some gases trap heat and others do not.
Looking for the matching chapters? Browse the Science, Physics and Chemistry books on TN SchoolBooks and download the Samacheer Kalvi PDFs, then use this simulation to make the light and atmosphere lessons visual. More free science simulations and textbook PDFs are on the home page.
Quick quiz: test your understanding of light and molecules
Score: 0 / 5
Molecules and Light simulation: frequently asked questions
Is the Molecules and Light 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.
Why do some molecules absorb infrared light?
A molecule absorbs infrared light when its structure lets the photon’s energy make it vibrate. Molecules like carbon dioxide and water do this, and that is why they are greenhouse gases.
Which type of light has the most energy?
Of the types in this simulation, ultraviolet light has the most energy per photon, followed by visible light, infrared and then microwaves.
The Molecules and Light simulation turns the hidden ways that light and molecules interact into something you can watch and zoom into. Explore it, take the quiz above, and keep experimenting until predicting what a molecule does with light feels easy.