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This activity explores the reasons we see colour the way we do.Vesna Andjic/iStockPhoto / Getty Images

Let’s Talk Science and the Royal Society of Canada have partnered to provide Globe and Mail readers with relevant coverage about issues that affect us all – from education to the impact of leading-edge scientific discoveries. Let’s Talk Science offers a number of fun activities to get youth engaged in STEM.

What does light have to do with colour? Observe some colourful fruit under different conditions to learn why we see the colours we see!

What You Need

· 1 banana

· Flashlight

· 1 green apple

· Red & green cellophane

· Red playing cards

· Shoe box

What to Do

1. Place the apple, banana, and red playing card (face up) into the shoe box.

2. Place the green cellophane (filter) over the top of the shoebox. Shine the flashlight through the hole on the side onto the objects inside. Only green light passes through the cellophane. The red on the playing card goes dark. The yellow banana appears as a darker yellow with a green tinge. The green apple appears green.

3. Remove the green filter.

4. Place the red cellophane (filter) on the top of the shoe box.

5. Shine the flashlight through the hole on the side onto the objects inside. Only red light passes through the cellophane.

6. The red on the playing card disappears. The background of the playing card turns red. The banana turns red. The apple turns red.

What’s happening?

Light travels in the form of a wave, and each colour has a distinct and different wavelength from another. White light is composed of all the different colours added together.

In this activity, various filters are used to filter out parts of the spectrum of white light allowing us to see individual colours. The red filter works by absorbing most of the colours in white light letting only the red light through. This also works similarly with the green filter and green light. We see coloured objects for the same reason. A red tomato appears red because of materials in its skin that only reflect the red light back into our eyes. However, if we were to view the red tomato in blue light instead of white it would appear to be a totally different colour. Colour needs light - there is no colour if there is no light.

Why does it matter?

In the human eye, there is special part called the retina, which is located in the back of the eye. The retina contains two special types of cells – rods and cones. Rods are more sensitive to dim lighting, while cones function best in bright light and allow us to see colour.

Imagine you are staring at a banana. When light reflects off the banana, it hits the retina and depending on the wave lengths of light reflected, it determines what colour we see. Once the light hits the cones in our retina, it sends a signal to our brain and processes the information we are seeing: yellow!

Investigate further

· Try using different colours of cellophane

· Try mixing 2 of the cellophane pieces together. Can you predict what colour the objects will appear?

For more information on this topic check out these Let’s Talk Science resources:

· How do I see colour? (Hands-on Activities) - How is light connected to the colours we see? In this STEAM activity make a colourful light catcher and discover why see different colours?

· How do we see colour? (STEM in Context) - How does the human eye see visible light as colour? And why do some people see more colours than others?

· What is white light? (Hand-on Activities) - Find out what happens if you spin the colours of the rainbow in this hands-on activity.

· How We See (Backgrounder) - Learn about how human vision works as well as some common types of vision problems.

Discover more free, English and French, Let’s Talk Science hands-on STEM activities, resources and events online.