Painting with light

Learn to draw using a diode!

20 minutes
Experiment's video preview



  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

Will the experiment work with other LEDs?

Well, the answer is usually “yes.” But your results will vary widely!

A red LED actually won’t work at all. A green LED will work poorly, but you’ll be able to see the drawing appear gradually. A blue LED will work nicely. A UV LED will yield the best result of all.

But why do they differ? In brief: the light they emit is a form of energy. The different color of each LED represents a different amount of energy being emitted. As it turns out, the reaction that reveals the drawing requires that a certain amount of energy be transferred to the reagents.

The whole paper is blue. What to do?

It seems like your paper was exposed to too much light! This might be due to sunlight, or to using the LED for too long, or because the lights in the room where you performed this experiment are too powerful! Try to reduce any bright lighting and repeat the experiment.

The mixture that I obtained isn’t yellow. What should I do?

If, at this stage of the experiment, you notice blue stripes on a yellow background, take a new absorbent and keep applying the mixture to the paper.

However, if the whole mixture has turned blue, this likely means that you’ve overexposed it to light. But don’t worry! Just discard this plastic cup and start over. When repeating the experiment, keep in mind that the experiment must be performed in very dim lighting.

And if the mixture turned greenish? It’ll still work, but the drawing won’t be as contrasted.

Step-by-step instructions

As we will be dealing with light-sensitive compounds, we need to avoid bright light, such as direct sunlight or strong lamps.


To draw with light we need a light source, such as an LED.


First, prepare light-sensitive ammonium iron(III) citrate by causing an ion exchange reaction between ammonium iron(III) sulfate and citric acid.


Mix ammonium iron(III) citrate with potassium hexacyanoferrate(III). This mixture is the key to making our drawing visible.


Now apply the photosensitive mixture to a paper.


We can initiate a chemical reaction by illuminating the mixture with an LED.


After removing the excess of the photosensitive mixture, you can keep your light-drawing.



Dispose of solid waste together with household garbage. Pour solutions down the sink. Wash with an excess of water.

Scientific description

The light from the LED initiates a series of reactions which eventually produces a dark blue precipitate. But how can light jump-start chemical reactions in the first place? Light is a form of energy, and many substances absorb it readily.

This means that electrons in some atoms get excited by light and perform some “energy jumps” . More often than not they simply “jump” in place, giving the energy back in the form of heat (that’s why objects that absorb a lot of light, that is, dark-colored ones, heat up quickly in sunlight).

In some cases, however, these “jumps” cause electrons to move from atom to atom, resulting in chemical changes.

In our case, when one of the electrons in Fe3+ “jumps” excitedly , its place gets occupied by an electron from a neighboring citrate anion . As a result, Fe3+ turns into Fe2+. This change is not visible by itself, but potassium hexacyanoferrate(III) readily binds with Fe2+ ions, forming an intensely blue compound called Prussian blue.

What is in the photosensitive mixture?

We obtained our photosensitive mixture through a combination of reagents. These consist of three key components:

  • iron(III) ions Fe3+ (from the ammonium iron(III) sulfate solution)
  • citrate ions (from the citric acid solution)
  • hexacyanoferrate ions [Fe(CN)6]3– (from the K3[Fe(CN)6] solution).

What role does light play in chemical reactions?

To start, what is light? It is a form of energy that many substances can absorb. When we talk about the energy absorbed by a substance, we mostly mean that electrons in some of said substance’s atoms receive this energy.

Therefore, electrons get excited by this “power-up” and perform something that we can call an “electron-jump”. In most cases, such a jump ends with a simple emission of the excess energy back into the surrounding area in the form of heat. This is why, for example, dark-colored objects (which absorb a lot of light) heat up quickly in sunlight.

But there is another way to release that energy: the powered-up electrons can jump from one atom to another, resulting in chemical changes. In other words, the light itself causes the chemical reaction to proceed!

Processes that are powered or chemically influenced by light are known as photochemical reactions. One of the most well-known examples of such a reaction is photosynthesis in plants - one of the building blocks of organic life on Earth.

Why does the inscription appear?

The light from the LED initiates a series of reactions. Iron(III) ions Fe3+ interact with citrate ions. They receive one electron and are reduced, transforming into iron(II) ions Fe2+, while citrate ions are oxidized:

Fe3+ + e- → Fe2+

The Fe2+ ions react with [Fe(CN)6]3– ions, yielding the dark-blue precipitate Fe4[Fe(CN )6]3, also known as Prussian blue.

Why do we use a blue LED?

A red LED actually won’t work at all. A green LED will work poorly, but you’ll be able to see the drawing appear gradually. A blue LED work nicely.

But why do their effects differ? In brief: the light they emit is a form of energy. The different color of each LED represents a different amount of energy being emitted. As it turns out, the reaction between iron(III) ions and citric ions requires that a certain amount of energy be transferred to the reagents.

Why doesn’t the drawing wash off of the paper?

The Prussian blue obtained in the experiment is a dark-blue, water-insoluble substance. It settles on the surface of paper well, while the unreacted mixture is easily washed away.

That's interesting!

The secrets of Prussian blue

This blue pigment has many names, such as Prussian blue, Parisian blue, and Turnbull's blue. It is a mixture of ferrocyanide compounds KFe(III)[Fe(CN)6] and Fe(III)(Fe(III)[Fe(II)(CN)6)3.

After its invention, the new pigment rapidly gained popularity thanks to its vivid color, stability, and ease of use. It has many applications, from dyeing cloth to copying technical drawings (i.e. making blueprints).

Indeed, famous artists quickly began working with Prussian blue. Most notably among these are perhaps Dutch post-impressionist Vincent Van Gogh (“Starry Night,” 1889) and Japanese artist, painter and printmaker Katsushika Hokusai (“The Great Wave off Kanagawa,” 1823-31); moreover, a number of artists working in Paris in the 18th (Jean-Antoine Watteau, Nicolas Lancret, Jean-Baptiste Pater) and 19th (Julien Dupré, Edgar Degas, and others) centuries used Prussian blue in their finest paintings.

The Prussian blue in… tea

In the 19th century, tea was still an expensive product, available mostly to the wealthy. And dishonest merchants sometimes used Prussian blue to commit fraud - they dried already-used tea leaves, colored them with Prussian blue, re-dried them, and then sold them as “fresh” tea!

Prussian blue against radiation

In the 20th century, it was discovered that Prussian blue can help people suffering from radiation poisoning. It can bind with cesium and thallium radioactive isotopes in the digestive tract and prevent their absorption. And it is still applied in select cases even today!