BrightKidz Library
Subjects

Red Is the Colour a Tomato Threw Away

About 11 minutes

Here is a question that sounds too simple to be worth asking. Where is the red in a red tomato?

The obvious answer is that it is in the skin, the way sugar is in a biscuit. Cut the tomato open and the red is right there, so the red must be a thing the tomato has.

It is not, and the way to prove it is to take the tomato somewhere dark.

In the dark it has no colour at all

Walk into a room at night before anyone puts a light on. Everything you can make out is grey. The tomato in the fruit bowl is grey. Your red coat on the back of the door is grey.

Nothing about the tomato changed when the sun went down. What changed is that there is no longer any light arriving.

So the colour was never sitting in the tomato waiting for you. It needs light to turn up first. And that means the colour is really a thing that happens between the light and the tomato, not a thing that lives inside it.

Daylight is not one colour

Sunlight looks colourless, and it is nothing of the kind. It is every colour arriving together, all mixed up, which our eyes read as plain white.

You have seen it split apart. A rainbow is sunlight taken to pieces by raindrops, and every colour that comes out of that rainbow was in the sunlight before it went in.

So when daylight lands on a tomato, all of those colours land on it at once.

Five bands soaked up, one band thrown back A tall upright slab stands on the right, standing in for the skin of an object. Five straight bands travel across from the left and stop dead against the face of the slab, and the inside of the slab is filled with faint diagonal shading to show they went in and stayed there. Each band is drawn a little thicker than the one above it, which is how the picture tells the colours apart without using any colour: the thin bands are the short waves and the thick ones the long waves. One band, thicker than any of the five, leaves the slab and travels back out to the left with an arrowhead on its end. The word in is written beside the five that stop, and the word out beside the single one that returns. in out
Light of every colour arrives at the skin. The thickness of each band stands in for which colour it is, since this page cannot show you colour itself. Nearly all of them go in and stay in. One comes straight back out, and that one is the only one that ever reaches you.

Most of it never comes back

The skin of a tomato is made of molecules, and molecules are fussy about which light they will take in. Each kind grabs certain colours and lets the rest go.

Tomato skin takes in the blue light. It takes in the green. It takes in most of the yellow. All of that arrives, gets absorbed, and turns into a very small amount of warmth. It does not come back out.

Red is the one it will not take.

The red light bounces off, travels to your eye, and is the only part of that sunlight you ever get to see. Which gives an odd little truth:

The colour of a thing is the colour it rejected.

A red tomato is red because red is exactly what it did not want. A leaf is green because green is the part sunlight brings that the leaf has no use for. A white shirt is white because it turns nearly all of it away. And something black has taken in almost everything and given you almost nothing back, which is why a black shirt in summer gets hot.

Imagine a torch that makes red light only — no other colour in it at all. You shine it on things in a dark room. What does each one look like?

  • A red tomato
  • A white plate
  • A pink eraser
  • A blue jumper
  • A green leaf
  • A black shoe

That sorting catches people out, so it is worth walking through.

The white plate turns back every colour it is given. Given only red, it can only turn back red, so it looks red. The blue jumper is blue because blue is the one thing it refuses to keep — but there is no blue in the torch, so it takes in the red and has nothing left to send you. A thing with nothing to send you looks black.

Under a red torch, blue does not look dark blue. It looks like a hole.

Do it for real

  1. Hold the wrapper up to one eye and close the other. Give yourself a moment to settle.
  2. Look at an object the same colour as the wrapper. It stays bright.
  3. Look at an object of the opposite sort — something blue if your wrapper is red. It goes dark, muddy, nearly black.
  4. Take the wrapper away and check the object really is still that colour. Nothing happened to the object.

You have not changed a single thing in the room. You changed which colours were allowed to reach your eye, and objects that had nothing left to send you went dark.

You take a bright yellow jacket into a room lit only by a deep blue lamp. What do you see?

The useful version

Colour is not a label stuck on an object. It is the result of three things meeting: what the light brought, what the object refused to keep, and what your eye does with the leftovers.

Change any one of the three and the colour changes with it. That is why a paint that looked perfect in the shop can look wrong at home, and it is why nobody chooses a jumper in a dark room.