Glowing Invisible Ink Secret Messages: A Fun UV Light Science Experiment

This post may contain affiliate links.

Want to write a secret message that magically appears under a black light? This glowing invisible ink experiment is such a fun way to combine science, mystery, and a little magic!

With just a few simple supplies, kids can create hidden messages that are difficult to see in ordinary light but glow when illuminated with a UV flashlight.

It’s a fantastic activity for science lessons, secret agent games, treasure hunts, Halloween activities, and Harry Potter-inspired adventures.

We originally tried this experiment while creating our own version of the Marauder’s Map from Harry Potter. My daughter had the best time making invisible clues that sent her brother on a backyard scavenger hunt!

And the science behind the glowing messages is just as interesting as the activity itself.

How to Make Glowing Invisible Ink

The secret ingredient in this experiment is something you might already have in your laundry room: liquid laundry detergent containing optical brighteners.

Some laundry detergents contain special ingredients that glow under ultraviolet (UV) light, making them perfect for creating hidden messages.

Supplies Needed

• Liquid laundry detergent containing optical brighteners
• A few drops of water, if needed
• A fine paintbrush or cotton swab
• White or light-colored paper
• A UV flashlight or black light
• A small dish for the detergent

Important: Not every detergent will work. Look for a product containing optical brighteners, and test a small sample under your UV flashlight before starting. Adult supervision is recommended. Avoid skin and eye contact with detergent, and never shine UV lights into anyone’s eyes.

Watch it in action here:

Step 1: Prepare Your Invisible Ink

Pour a small amount of liquid laundry detergent into a dish.

If it’s very thick, add just a drop or two of water to make it easier to paint with.

You don’t want the mixture too watery because the writing may spread or become difficult to see under the black light.

Step 2: Write Your Secret Message

Dip a fine paintbrush or cotton swab into the detergent and write a message on your paper.

Try using thin strokes and a small amount of liquid. Thick lines may leave visible marks or take longer to dry.

You can write a secret note, draw a picture, create a hidden clue, or make a treasure map!

Step 3: Let the Message Dry

Allow the paper to dry completely. Depending on how much detergent you use, this may take approximately 15–30 minutes or longer.

Once dry, the writing should be much less noticeable under ordinary lighting, although it may not disappear completely.

Step 4: Reveal the Glowing Message!

Dim the room lights and shine your UV flashlight over the paper. Watch as your hidden message appears!

The optical brighteners in the detergent absorb ultraviolet light and emit visible light, making the writing glow.

It’s such a fun reveal, especially when kids don’t know what the secret message says!

Our Harry Potter-Inspired Marauder’s Map

When we first tried this glowing invisible ink activity, we were inspired by the Marauder’s Map from Harry Potter.

If you’re familiar with the books, you know that the map reveals hidden information with a little magic. We decided to create our own version using science!

My daughter wrote a series of invisible clues for her brother to follow around our backyard.

Her first message began with the familiar phrase, “I solemnly swear that I am up to no good,” and directed him to meet her under the Whomping Willow tree.

From there, she created additional clues leading him on a secret scavenger hunt. It was such a fun way to combine storytelling, outdoor play, and science.

You could easily adapt this idea for a Harry Potter party, a classroom mystery activity, or a family treasure hunt.

We have a little UV flashlight that we used to reveal the messages. This was perfect, but any black light or UV light will do!

The Science Behind Glowing Invisible Ink

So why does laundry detergent glow under a black light? The answer is fluorescence!

Many laundry detergents contain optical brighteners, also called fluorescent whitening agents. These substances absorb ultraviolet light and re-emit some of that energy as visible light, often appearing blue or blue-white. Manufacturers use optical brighteners to help white fabrics look brighter.

In this experiment, we take advantage of that same property to create glowing messages on paper.

What Is Ultraviolet Light?

Ultraviolet light is a type of electromagnetic radiation with wavelengths shorter than visible violet light. Our eyes can’t see UV light itself, but certain materials react to it by producing visible light. That’s why the detergent writing can appear to glow when you shine a UV flashlight on it.

Is the Ink Really Invisible?

Not completely! Depending on the detergent, paper, and amount of liquid used, you may notice faint marks after the message dries. The fun is that the writing becomes much more visible under UV light.

For the best results, try using a small amount of detergent and test different papers to see which creates the strongest contrast.

Tips for Making Glowing Invisible Ink Work

If your message isn’t glowing the way you expected, try these simple troubleshooting tips.

Check your detergent. Not all laundry detergents contain optical brighteners. Test your detergent under UV light before writing a full message.

Try a different paper. Some white papers already contain fluorescent brighteners and may glow under black light, reducing the contrast with your writing. Experiment with different papers to find one that works well.

Use less ink. A thin layer of detergent often creates a less noticeable message when dry.

Let it dry completely. Wet detergent can leave obvious marks on the paper.

Darken the room. The glowing effect is usually easier to see in dim lighting.

Try a different UV flashlight. The strength and wavelength of your light can affect how clearly the message appears.

Part of the fun is experimenting to discover which combination produces the best secret messages!

Download the Invisible Ink Marauder’s Map Now!

Turn Your Invisible Ink into a Secret Code Activity

Want to make the mystery even more challenging?

Instead of writing an ordinary message, write an encoded message in invisible ink!

Kids will have to complete two challenges:

1. Reveal the hidden message using a UV flashlight.
2. Decode the secret message to discover what it says.

This is a perfect activity to pair with my Printable Decoder Wheel.

Kids can use the decoder wheel to solve Caesar cipher messages and create their own secret codes.

Try it with a treasure hunt where each invisible message contains a coded clue leading to the next location! It’s a fun combination of science, problem-solving, literacy, and logic.

More Invisible Ink Experiments to Try

Glowing detergent ink isn’t the only way to create hidden messages!

Different invisible ink experiments use different scientific principles to reveal writing.

Baking Soda Invisible Ink

Mix a little baking soda with water and use it to write on paper.

After the writing dries, brush purple grape juice over the message to reveal it.

The pigments in grape juice can change color when exposed to the alkaline baking soda solution.

This is a fun introduction to acids, bases, and pH indicators.

Lemon Juice Invisible Ink

Lemon juice can be used to write a hidden message that becomes more visible when heated.

The organic compounds in the dried juice brown when exposed to sufficient heat.

Because revealing the message involves heat and can create a fire hazard, this method should be demonstrated only by an adult using appropriate precautions.

Compare Different Invisible Ink Methods

Turn these experiments into a science investigation!

Have kids compare different types of invisible ink and record their observations.

Questions to investigate:

• Which message is hardest to see before revealing it?
• Which method produces the clearest writing?
• Which ink dries the fastest?
• Does the type of paper affect the results?
• How does the amount of ink change the visibility of the message?

Students can make predictions, test their ideas, and record their results.

More Secret Message and Mystery Activity Ideas

There are so many creative ways to use invisible ink!

• Secret Agent Training: Set up a spy school where kids learn to write invisible messages and decode secret instructions.
• Backyard Treasure Hunt: Hide clues around your home or yard and use UV light to reveal the next destination.
• Halloween Mystery: Create glowing clues for a not-too-spooky Halloween activity.
• Science Investigation: Test different detergents, papers, and UV lights to discover which combination works best.
• Secret Code Challenge: Combine invisible ink with a Caesar cipher or other code for an extra layer of problem-solving.

These activities are a great way to encourage kids to experiment, think critically, and use their imaginations.

More Harry Potter-Inspired STEM Activities

If your kids love magical science activities, be sure to try these other Harry Potter-inspired projects:

Harry Potter STEM: Arithmancy Math
Dry Ice Crystal Ball Experiment
Harry Potter Escape Room for Teaching

You can also explore more Harry Potter activities from Babble Dabble Do.

Whether you’re planning a science lesson, a mystery game, or a magical afternoon of fun, these glowing invisible ink secret messages are sure to be a hit.

Mischief managed!