Recently I was eating a yellow rice for lunch in a restaurant with only yellow lights. But the rice looked white! I was intrigued by this because I always thought it should look yellow since the yellow pigment reflects only yellow light, but the rice looked really, really white. Why is that? I thought it could be something about the rice, but any yellow object was looking white. The room was full of yellow light bulbs (not normal yellowish bulbs, but very tinted yellow lights) and there was no other color of light to interfere. Maybe there's something to do with human perception?
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9$\begingroup$ Did it look white or did everything look yellowish so that just appeared white? I think it was just an optical illusion. $\endgroup$– JMacCommented May 14, 2017 at 1:50
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8$\begingroup$ This is also what white balance in photography is about. $\endgroup$– RuslanCommented May 14, 2017 at 11:43
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$\begingroup$ If this subject interests you, twitter user twitter.com/AkiyoshiKitaoka frequently posts examples of colour illusions and other oddities in how the brain decodes images. $\endgroup$– Eric LippertCommented May 15, 2017 at 17:05
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1$\begingroup$ It was the other way areound: Your white looked yellow and the yellow rice was the same as the yellow-looking white: yellow. $\endgroup$– Pieter BCommented May 16, 2017 at 7:11
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$\begingroup$ Surely there were more light sources or the bulbs were emitting a large light spectrum, covering yellowish frequencies and others frequencies. If not the yellow rice (objects) and white objects would become yellow, all others non-white or yellow objects would become black, or would show a strange color. I believe there was a source of white light (almos all possible frequencies) with a high color temperature and so it's a optical effect (illusion): en.wikipedia.org/wiki/Color_temperature $\endgroup$– LucianoCommented May 17, 2017 at 14:07
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4 Answers
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Your brain adjusts your perception of color to compensate for lighting that is strongly tinted. This was the reason for the violent conflict some time back about a certain dress. Depending on whether people perceived the dress was being lit by yellow-tinted or blue-tinted light, they saw either a black and blue dress or a white and gold dress. Here's an animated version to show that color happens in the brain, not physics.
Here's another picture to show that your brain interprets colors contextually. The squares marked A and B are exactly the same shade of grey. But, because your brain interprets square B as being in a shadow, it "knows" that the "real" color of the square is lighter. So, you perceive a lighter shade than what is actually there.
Below is an edited version of the checkerboard showing a single color linking the squares A and B (the squares A and B have not been recolored). What I find funny is that half the time I see the squares and line as single color, and half the time I see a gradient from the "dark" square to the "light" square. In the animated picture above, I still see the moving swatch fading from one color to another as it moves.
So, because of the strong yellow lighting in the restaurant, your brain thought the yellow of the rice was due to the lighting and "corrected" your perception. I keep saying the brain does this because all of this visual post-processing happens subconsciously.
Pictures taken from the wikipedia article: https://en.wikipedia.org/wiki/Checker_shadow_illusion
To summarize, the physics of light ends at your retina. Particles of light (photons), each with a a certain energy, hit the cells of your retina, setting off electrical signals that travel to your brain. Your brain then processes these electrical signals to create a coherent image. These processes include factors from memory (what things "should" look like), local contrasts (in color and brightness), cues from the environment (including available light sources), and many others. The result of all this mental post-processing can result in identical photons creating different colors in the mind, as evidenced by the picture above.
Now, who do you trust? Me or your lying eyes?
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12$\begingroup$ @annav It's a computer generated image. If you load it into any image editing program and use the eye drop tool or color picker tool, you will see that they two square are the same shade of grey. Here's an article with an edited version of the picture to show the identity: mentalfloss.com/article/54448/… $\endgroup$– Mark HCommented May 14, 2017 at 4:12
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44$\begingroup$ @annav: the point of the image is that the absolute color of the two squares is the same, but the perceived color is not. We don't see with our eyes, we see with our brains, and our brains' image processing systems sometimes produces funny results. If you erase everything but the A and B squares, your visual cortex no longer has any context to provide corrections to your eye's input, and suddenly the two squares will then be perceived the same. See en.wikipedia.org/wiki/Color_constancy for more examples of your visual cortex doing weird stuff. $\endgroup$ Commented May 14, 2017 at 4:43
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6$\begingroup$ @annav It is the squares neighboring A and B that are creating the illusion. Remove all squares but A and B from the image to allow a direct comparison. $\endgroup$– Mark HCommented May 14, 2017 at 5:28
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18$\begingroup$ Here is the "background" removed i.sstatic.net/PBV01.jpg Please add it to your answer if you wish. $\endgroup$– FarcherCommented May 14, 2017 at 7:09
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6$\begingroup$ @annav To have a very simple proof load up any image software you can imagine and let it show you the RGB values for the color in A and B. If you do, you will see that it is (120,120,120) for both rhombuses. So they absolutely are the same color - that you see a different color is rather the point of the exercise. $\endgroup$– VooCommented May 14, 2017 at 19:56
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As already explained in other answers, it is about perception, not physics.
If you take a piece of white paper outdoors on a clear sunny day and look at it, it looks white. If you photograph it with an old-fashioned film camera, the paper will appear white in the finished print. If you repeat the same exercise indoors let's say at night with only incandescent bulbs for illumination, the paper will still look white to your eyes, but in a photograph processed exactly the same way as your outdoor picture, the paper will take on a reddish tint.
The reason is as already explained. Incandescent lights emit a different balance of color than is present in daylight - there is more red. Your brain compensates for the difference because you know the paper is white - it will always be white, so you perceive it as white regardless of the ambient lighting. The photographic film doesn't have this privilege - it captures the light that actually exists.
In your case, your brain notices that everything has a yellow cast due to the yellow lighting, so interprets anything yellow as white. If you had both yellow rice and white rice under the yellow light, they would both look white because the yellow rice is absorbing the non-yellow light (of which there is none) and reflecting all the yellow light; the white rice is reflecting all the light falling on it, of which there is only yellow. So, both kinds of rice are reflecting only yellow light, which your brain deems to be white.
For what it's worth, to do this experiment with a digital camera, you need to be aware that they have a feature known as white balance. In its automatic mode, the camera's white balance does what your brain does - make its best guess as to what is supposed to be white in the picture and make sure it is rendered as white. To use a digital camera to reveal the effect of different lighting conditions, you need to turn off the camera's automatic white balance; if it has an "outdoor" setting, use that for both your outdoor (sunlit) and indoor (artificially lit) pictures.
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$\begingroup$ It's not at all difficult to perform the experiment today. Virtually any respectable camera allows manual white-balance control. $\endgroup$ Commented May 14, 2017 at 23:01
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3$\begingroup$ @whatsisname OK, so to properly perform the experiment, you need to be aware of the camera's white balance control and take it off its automatic setting. $\endgroup$ Commented May 14, 2017 at 23:21
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1$\begingroup$ Of course, this "brain feature" didn't develop for incandescent lights - there weren't any in our ancestral environment (other than "fire" which meant "RUN AWAY FAST", not "try to preserve good colour vision"). I'd guess it developed mostly for diffuse lighting and shadows (Sun shadows are blue-tinted, since they're lit by the blue-tinted sky; forests are green-tinted, because of the light captured by leaves). We rarely notice, since the white balancing works great when the relative colour changes - it only becomes obvious when some wavelengths are lost entirely (e.g. red-and-green glasses). $\endgroup$– LuaanCommented May 15, 2017 at 13:20
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$\begingroup$ White balance does not cope well with strongly tinted light. If there is no blue light at all in the scene, the camera won't be able to guess at what the colors are supposed to be. The white rice will likely be yellow. $\endgroup$ Commented May 15, 2017 at 16:02
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1$\begingroup$ @Luaan: our vision system is an incredibly sophisticated piece of equipment, and many of its functions are shared by other animals, including nocturnal and crepuscular species. "White balance" in the brain is further complicated by changing perception of colors in low-light, and also accounts for diseases of the eye such as cataracts that change the hues perceived. You can't really credit any one factor. $\endgroup$ Commented May 16, 2017 at 21:47
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It is perception. In yellow lighting white and yellow rice will look quite similar, so your brain tends to believe that you see white rice.
White objects reflect light of all colours, while yellow objects reflect blue light much less. In yellow lighting, which lacks the blue component, the difference between white and yellow vanishes.
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I copy all the answers, it is caused by the perception. The brain does wonderful things during the processing the signal from your eyes to your mind.
The brain automatically adjusts the image to collect as much as possible and uses your expirience to adjuts the scene.
In your example you subconsciously expect the rice to be white because rice is usually white, right? All the objecs are slightly yellow-tinted so your brain compensates the overall colour balance and unyellow all yellow objects.
Application of such filters take some time; next time when you enter the restaurant you should see everything yellow-tinted and over the time the tint should fade away. You can try to wear amber goggles. When skiing I can see the snow yellow and after some time I can see the snow white. If I have them on long enough I cannot see blue colour at all when I take them off. Everything is green. I know I have blue skis but I see them green for a while.
The brain does more advanced tricks also. It can locally adjust the sensitivity of the retina. Try to stare long enough on following pictures 1, 2 and then look at plain white paper/screen/wall. You will see the negative.
