Why is a leaf green, and what colour would it look under a red lamp?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Explain why a leaf looks green in white light
Predict the colour an object appears under coloured light
Joseph picks a green leaf and holds it under a red lamp. The leaf looks almost black. In daylight it was green. What changed?
You know white light is a mix of colours. For the sums in this chapter, think of it as three parts. There is a red part, a green part and a blue part. Mixed together they look white.
Light of one colour has only some of the parts. Red light has only the red part. Yellow light has the red part and the green part.
A thing does not make its colour. It sorts the light that falls on it. It sends some colours back, and that is reflection. It absorbs the rest. Absorb means to take in and not send on.
The colours that come back reach your eyes. They give the thing its colour. A red apple reflects the red part and absorbs the green and blue parts.
Draw it first
Draw white light falling on three things
1 White paper reflects all three parts. All of them reach your eyes, so it looks white.
2 Black cloth absorbs all three parts. Nothing comes back, so it looks black.
3 A red apple reflects only the red part and absorbs the rest, so it looks red.
Paper, cloth and leaves scatter light every way. The pictures show just one ray of each colour, to keep them clear.
Now the leaf. In white light, a green leaf reflects the green part and absorbs the red and the blue.
A green leaf in white light reflects the green part. That is the part you see.
Under a red lamp there is only the red part. The leaf reflects green, but there is no green to reflect. It absorbs the red. Nothing comes back, so the leaf looks black or very dark.
A green leaf in red light has no green to reflect. It looks black.
So the colour you see depends on two things. One is the colours the thing reflects. The other is the colours that fall on it.
See it three ways
As a picture
In words
A thing looks the colour of the light it reflects. If the light has none of its colour, it reflects nothing and looks black.
In symbols
colour seen = the colours the thing reflects that the light has.
A filter is a clear coloured sheet. It lets its own colour through and absorbs the rest. A red filter held up to white light lets only the red part pass.
A red filter lets the red ray through and absorbs the green and the blue.A yellow banana reflects the red part and the green part. What does it look like in green light?
Worked example
A yellow banana is in green light. What colour does it look?
Green light has only the green part.
A yellow thing reflects the red part and the green part. Out of 3 parts, it reflects 2.
The light has just the green part, so the banana reflects only the green part.
The green part reaches your eyes. The banana looks green.
Answer: green
Try it
Look through a red filter
You need: a clear plastic bottle of water coloured red with a little beetroot juice or red food colour, a green leaf, a piece of red cloth, a sheet of white paper.
Be safe: Never look at the Sun, and never point the bottle at it. Do not drink the coloured water.
Work in a bright room or in the shade, with daylight coming from a window.
Lay the leaf, the red cloth and the paper side by side on a table.
Look at them with your eyes. Note the colour of each.
Hold the bottle in front of one eye and look through it. Note the colour of each thing again.
What you will see: Through the red bottle, the leaf looks dark. The cloth looks bright red. The paper looks red. The red water is a filter. It lets only red light through to your eye.
If you cannot try it, this is what happens.
A wrong idea many people have
A green leaf always looks green.
Why it feels right: In daylight we always see the leaf as green, so green seems to be a part of it.
The test that breaks it: Look at the leaf through the red bottle. It looks dark. The leaf did not change. The red filter let no green light through for it to reflect.
What is true: The colour you see depends on the light as well as the thing. The leaf only reflects green if there is green light to reflect.
Key words
absorb
to take in light and not send it on.
filter
a clear coloured sheet that lets its own colour through and absorbs the rest.
Back to your guess
The answer is C: A leaf reflects green light and absorbs the other colours.
A leaf reflects green light into your eyes and absorbs the other colours. It makes no light of its own, and it does not reflect the colours it absorbs.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Why does a leaf look green in white light?
See the answer
It reflects the green part of the light and absorbs the other colours.
White light has red, green and blue parts. The leaf sends the green part back to your eyes and absorbs the red and the blue.
If you chose B: A leaf makes no light. It only sends back some of the light that falls on it. Go to S3.9.5.
If you chose C: It is the other way round. The colour you see is the colour that is reflected, not the colour that is absorbed. Go to S3.9.5.
A red apple is in blue light. What colour does it look?
See the answer
Black.
Blue light has no red part. The red apple reflects only red, so it reflects nothing and looks black.
If you chose A: The apple reflects red, but there is no red in blue light. There is nothing for it to reflect. Go to S3.9.5.
If you chose B: The light is blue, but the apple only reflects red. It absorbs the blue. Go to S3.9.5.
If you chose C: Colours of light do not mix in the apple. It reflects only its own colour, and there is none in the light. Go to S3.9.5.
A white shirt is in green light. What colour does it look?
See the answer
Green.
White reflects all the colours that fall on it. Only green falls on it, so it reflects green and looks green.
If you chose A: A white shirt reflects every colour it gets. But the light here is green only, so only green comes back. Go to S3.9.5.
If you chose C: White things reflect all colours. There is green in the light, so the shirt reflects it. Go to S3.9.5.
A yellow banana is in red light. What colour does it look?
See the answer
Red.
A yellow thing reflects the red part and the green part. Red light has only the red part, so the banana reflects red.
If you chose A: Yellow needs both the red and the green parts. Red light has only the red part. Go to S3.9.5.
If you chose B: A yellow banana reflects the red part. There is red in the light, so it reflects it. Go to S3.9.5.
A blue filter is held up to white light. What comes out of the other side?
See the answer
Blue light. The filter absorbs the rest.
A filter lets its own colour through and absorbs the rest. A blue filter lets the blue part pass and absorbs the red and the green.
If you chose B: A filter does not just dim the light. It absorbs the colours that are not its own. Go to S3.9.5.
If you chose C: A filter adds nothing. It only takes colours out of the light. Go to S3.9.5.
From an earlier chapter
Review: which colour of the spectrum does a prism bend the most?
See the answer
Violet.
A prism bends violet the most and red the least. The other colours are in between.
If you chose A: Red is bent the least. It is at the other end of the spectrum. Go to S3.9.4.
If you chose C: Green is in the middle. The ends are red and violet. Go to S3.9.4.
Keep it
Copy this into your exercise book.
A thing reflects the colours it looks and absorbs the rest. White reflects all, black absorbs all. A leaf looks black in red light. A filter lets its own colour through.
Draw: White light falling on an apple. Draw the red ray bouncing back and the green and blue rays stopping.
