MadSci Network: Physics

Re: If an apple is in a chamber with a red light, could it be seen?

Date: Mon May 22 17:27:42 2000
Posted By: Harry Adam, Research Associate, Research Division, Kodak Limited
Area of science: Physics
ID: 957491548.Ph

Hello Bryan - a terrific question, this. First, you need to know something about how eyes work. We actually only see differences between things - the cells at the back of our eyes actually send signals to the brain in response to differences they experience from their neighbouring cells. So, in theory - if you could do this experiment - put your eyes in a completely evenly lit sphere with no differences - you would not know if the lights were bright or dim, or even on or off. Of course experimentally this is impossible to do - so there will be differences and we will experience visual sensation.

Your question is like this situation - but with an object (the apple) in field of view and it is a red object. The light is red, too - so the apple will reflect the light. (A green one would absorb it and appear black). Whether you can see the apple will depend on the difference between what it is reflecting and the surrounding light - which if it is also reflected - by the surface of the sphere behind - will also be red - but not necessarily of the same intensity or quite the same hue. That will depend on the nature of the surface of the sphere. A perfectly reflecting sphere (a mirror) will reflect back all the incident light - which the apple (if itís anything like a real one) will not do. Youíll be able to see the apple - certainly its outline, and probably some textural and surface hue variations, too.

All this assumes the light is shining away from the observer, and towards the apple. The apple will certainly cast a shadow which will also be visible depending on the angle of illumination. If the sphere inside is coloured exactly the same as the apple - it will be harder to see - and the only differences allowing you to pick out the shape of the apple will be because the apple is nearer the light and reflects back a higher intensity than the surroundings do - or because it is less reflective than the sphere will reflect back a lower intensity than the surroundings . Youíll just see it as a lighter or brighter apple shape - or a duller apple shape in a red field of view. If the light source is behind the apple - you will simply see the apple as black or almost black against a red background. The exact effect will all depend on a host of factors: light source position, distance from eye to apple, size of sphere, reflectivity of sphere surface to red light and so on.

Can you do an experiment with red light, various colours of card backgrounds and a red apple? Why donít you try it, and write down your observations? Then try to explain them to yourself. If you get stuck - let me know and Iíll try to help. You can e-mail me at

One thing - if you can see nothing but red light for a while - I wonder if you continue to see it as red - there's no other colour to see a difference against...

I hope this helps and doesn't make you see red!!

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