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Have you ever sat in the front seat of a car at night? Road reflectors, street signs and even road paint all shine bright under car headlights. Make your own ‘road paint’ and see if you can make it shine with glass beads!

You will need

• Black card
• White paint
• Palette or scrap of cardboard to put paint on
• Paintbrush
• Clear glass seed beads
• Glitter (you can buy biodegradable glitter from some makeup stores or online)
• Silver pen
• Torch

What to do

1. Start by squeezing out some white paint onto a palette or a scrap of cardboard.
2. Paint 4 thick white lines on your black cardboard. Make sure you use lots of paint!
3. Sprinkle seed beads on the first line.
4. Sprinkle glitter on the second line.
5. Wait for the paint to dry.
6. Shake off any extra glitter and beads.
7. Use the silver pen to colour in the third line.
8. Leave the fourth line alone.
9. We’ve got 4 lines here, and three of them are shiny or reflective. Which one do you think will work best at night?
10. Time to test your lines! Put the cardboard on a table in a dark room. Turn on your torch and shine it at the lines. Which line looks brightest?
11. Move the torch close to your eye but keep it pointed at the lines. Which is brightest now?
12. For a photographic test, take one photo with the flash on, and one with the flash off. Do you notice any difference between the 2 pictures?

What’s happening?

If you look out the front window of a car at night, you’ll notice many things light up brightly. Signs, reflectors and even the paint on the road can glow brighter than seems possible.

The secret is retroreflection. While a mirror only reflects back at you if you’re standing right in front of it, Retroreflectors are different. A retroreflective object reflects light back the way it came, even when it comes in from an angle. That superpower makes them great for road markings. When you’re in a car at night, the light from the headlights is reflected back to the car, making objects glow brightly.

There are many ways to make something retroreflective. The reflectors in the middle of the road and on bicycle wheels use little mirrors arranged in corner shapes, and bounce light around the corners and back the way it came.

Road paint can be made retroreflective by sprinkling tiny glass balls on it. Light gets bent by the glass, reflects off the paint and goes back through the glass again, back the way it came.

In this activity, you tried several ways of creating a shiny line. We found that tiny glass seed beads worked the best. All of the lines reflect to some extent, but only the glass beads retroreflect.

These beads have a hole in the middle, making them a slightly different shape compared with the glass beads used on real road paint. Beads that are not round do not reflect light uniformly, reducing their retroreflectivity. The higher percentage of round beads, the more light is reflected back. Road beads are rounder, making them even more retroreflective, but they’re much harder to buy.

Real-life science

So how do we know that the paint is going to work? Scientists from CSIRO Infrastructure Technologies have a whole suite of tests to make sure the materials are up to the task.

To make sure the glass beads reflect properly, the team measures the size and roundness of the beads, whether they have any colour, and how much the glass bends the light. They also check that there isn’t anything else mixed in with the glass beads.

To make sure the marking goes well, CSIRO also tests how fast the paint dries. And they make sure the glass beads don’t clump and get stuck when sitting in storage. Coatings can also be applied on beads to enhance the glass beads’ properties, such as moisture-proof coatings.

Glass: more than meets the eye

Prepare yourself for National Science Week 2022 with our activities and articles! This year’s theme is ‘Glass: more than meets the eye’.
• Get our glass-themed issue of Double Helix magazine
• Try another activity about bending light: Invisible glass