Ask A Scientist Physics Archive

name         Isabel Q.
status       educator
age          40s

Question -   Thank you for answering:
1) Why, when we spin a color wheel, do we see a whitish blur instead of individual colors?

2) Why do you need to spin color wheel fast in order to get whitish color. Does it relate to the 
speed of light?
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The major factor is by far not the speed of light. Rather, it is the response time of cones and 
rods in the eye. When a wavelength of light strikes the retinal, that "signal" decays in about 
1/25 sec. The superposition of tri-color stimuli (typically yellow, cyan, and magenta) "excites" 
all of the rods and cones, giving the visual perception of  white light (approximately). So the 
sectors of the color
wheel must rotate quickly enough so that each color segment superimposes the other two in 1/25 
of a second or less. The shading will change if the speed is increased or decreased slightly. 
For the distances involved, the speed of light is almost infinite.

Vince Calder
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Isabel,


If the wheel displays the three additive primary colors of light -- red, green, and blue -- it 
should appear white when those colors are blended. You can see the additive result if you use a 
magnifying glass to examine the image on a common color TV screen. You will note that in an area 
where the screen image is white, that area is covered with very tiny pixels (illuminated areas) 
that are either red, green, or blue. The blend of these colors results in white. By changing the 
relative numbers and brightness of the individually illuminated pixels, the set is able to 
produce almost any color in the screen.


If the color wheel is not spun fast enough, individual colors can be seen. As the spin-speed 
increases, the colors will blend, This happens because, as the image is formed on the retina 
of the eye, the retinal sensors become temporarily overloaded with the stimuli provided by the 
three colors. The overload is sometimes referred to as "persistence of vision" -- that is, the 
sensors are unable to respond and clear themselves for the next stimulus if the stimuli events 
occur too quickly. Thus, all sensors are firing
simultaneously and the resulting signal sent to the brain is the blended colors, namely white.


The purity of the blend (white) depends on the purity of the three primaries. This works better 
if the blend is produced by colors derived when white light is passed through a prism.


Regards,
ProfHoff 643
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You see a whitish blur because your eye is too slow in its response to follow the individual 
colors.  If you spin it fairly slowly, you may be able to follow one color with your eyes and 
you will then see the individual colors.


Otherwise your eyes are averaging over all the colors and white light consists of all colors, as 
Newton showed long ago with his prism.


This has to do with the response time of the eye and not the speed of light. The speed of light 
is about 1 foot per nanosecond.  A nanosecond is a billionth of a second\= 0.000000001 seconds.  
With suitable flash equipment, you can take a clear picture of a very rapidly rotating color 
wheel.  On the other hand, the response time of the eye is about 0.05 seconds, which is why 
moving pictures show 20 to 30 frames per second.  The eye then cannot see the individual frames 
and smoothly connects adjacent frames to produce the effect of smooth motion.

Best, Dick Plano  Professor of Physics emeritus  Rutgers University
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Human perception of light is not instantaneous.  This is why a movie can be viewed -- many 
individual pictures are flashed on the screen very quickly and we perceive it as moving images.

In fact, the human eye is very much like a camera.  As with a picture taken with a camera, 
things that move very quickly appear blurred to our sight.  With the color wheel the blurring 
has the effect of overlapping views of all the colors -- this mix is perceived as white light 
(which is a mix of all the colors).

It is not that all the colors arrive together at the eye but that they arrive in sequence (the 
order in which the color wheel is designed) in a very short period of time -- too short for us 
to resolve the individual images.  The mixed images are perceived as white light because they 
contain elements of all colors.

So, this perception has nothing to do with the speed of light, only how quickly our visual 
system can process the images.

Greg Bradburn
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