Colour - physics fun question
Discussion
To keep this section of the forum alive, I am posting the odd fun question every now and then.
There will be some who will know this answer straight away, so maybe allow a bit of time for others to have a go before firing in the correct answer.
Given that the primary colours of paint (and fabric dyes) are Red, Yellow and Blue. And the primary colours of light are Red, Green and Blue.
When they mix together they 'create' secondary colours.
The question is, if I have a yellow shirt hanging up in a dark room and I shine a BLUE light on to it.
When I look at the shirt, what colour would I see?
There will be some who will know this answer straight away, so maybe allow a bit of time for others to have a go before firing in the correct answer.
Given that the primary colours of paint (and fabric dyes) are Red, Yellow and Blue. And the primary colours of light are Red, Green and Blue.
When they mix together they 'create' secondary colours.
The question is, if I have a yellow shirt hanging up in a dark room and I shine a BLUE light on to it.
When I look at the shirt, what colour would I see?
Toltec said:
You would need to provide spectral profiles for your yellow shirt and blue light source to answer that. It also depends if the blue light can cause any fluorescence in the dyes, material and contaminents in the shirt.
Yep this. The shirt will have a reflection spectral profile.
The light source will have an emission spectral profile.
What your shirt looks like will depend on the overlap between these - coupled with any other effects like the aforementioned fluorescence.
You then also have to factor in the human element in the perception of colour. For example - take the two images below. The central cross piece is actually the same colour in both images (a mid grey - which can be verified by using the eyedropper tool in photoshop).
.Edited by Moonhawk on Thursday 3rd May 13:06
Toltec said:
You would need to provide spectral profiles for your yellow shirt and blue light source to answer that. It also depends if the blue light can cause any fluorescence in the dyes, material and contaminents in the shirt.
Yes, this would be likely in order to gain a recordable value (or set of values) regarding the 'colour' that a device would 'see'. But in more simplistic terms, the question is posed more to what you would simply 'see' in terms of a commonly perceived/recognisable colour.
But I agree, there are many possible things to consider that may affect an answer.
The question is supposed to be purely about colour mixing and the nature of light reflection. (hint)
So to ignoring fluorescence and contaminants for now
Atomic12C said:
But in more simplistic terms, the question is posed more to what you would simply 'see' in terms of a commonly perceived/recognisable colour.
It still not that straight forward. A lamp can look blue whether is monochromatic (like a laser) or has a filtered continuum spectrum (like an incandescent light bulb painted blue) - and many other options besides.How each type of light source renders the colour of a given object very much depends on this (see "colour rendering index")
https://en.wikipedia.org/wiki/Color_rendering_inde...
Atomic12C said:
Yes, this would be likely in order to gain a recordable value (or set of values) regarding the 'colour' that a device would 'see'.
But in more simplistic terms, the question is posed more to what you would simply 'see' in terms of a commonly perceived/recognisable colour.
But I agree, there are many possible things to consider that may affect an answer.
The question is supposed to be purely about colour mixing and the nature of light reflection. (hint)
So to ignoring fluorescence and contaminants for now
If the shirt was set against a light absorbing material or void and in a vacuum or particle free atmosphere so there is nothing to make a light beam otherwise visible and the material of the shirt aborbed all spectra other than yellow you would see nothing. At least not unless the source could heat the shirt enough for it to start emitting visible light.But in more simplistic terms, the question is posed more to what you would simply 'see' in terms of a commonly perceived/recognisable colour.
But I agree, there are many possible things to consider that may affect an answer.
The question is supposed to be purely about colour mixing and the nature of light reflection. (hint)
So to ignoring fluorescence and contaminants for now
If the shirt reflects any blue light at all it will look blue or grey depending on the light intensity.
If the room has walls that can reflect blue light then all bets are off.
Another complicating factor is this.
If you had an object that only reflected yellow light in a very narrow range. Yellow light has a wavelength around 560-590nm.
Now if you shone light onto this object that was in the wavelength range 560-590nm - the object would obviously reflect that light and would appear yellow.
But what happens if you don't shine light that is in the range 560-590nm?
If you shone light that was a combination of 690nm light (Red) and 550nm light (green). According to the additive colour method - we would perceive this light as yellow - however your object wouldn't reflect this light as it doesn't contain any wavelengths that your object can reflect and would therefore appear black.
If you had an object that only reflected yellow light in a very narrow range. Yellow light has a wavelength around 560-590nm.
Now if you shone light onto this object that was in the wavelength range 560-590nm - the object would obviously reflect that light and would appear yellow.
But what happens if you don't shine light that is in the range 560-590nm?
If you shone light that was a combination of 690nm light (Red) and 550nm light (green). According to the additive colour method - we would perceive this light as yellow - however your object wouldn't reflect this light as it doesn't contain any wavelengths that your object can reflect and would therefore appear black.
Edited by Moonhawk on Thursday 3rd May 16:04
Toltec said:
If the shirt was set against a light absorbing material or void and in a vacuum or particle free atmosphere so there is nothing to make a light beam otherwise visible and the material of the shirt aborbed all spectra other than yellow you would see nothing. At least not unless the source could heat the shirt enough for it to start emitting visible light.
If the shirt reflects any blue light at all it will look blue or grey depending on the light intensity.
If the room has walls that can reflect blue light then all bets are off.
Yes good answer.If the shirt reflects any blue light at all it will look blue or grey depending on the light intensity.
If the room has walls that can reflect blue light then all bets are off.
This is more or less what I was looking for.
In simplistic terms the fact that the shirt is yellow means it absorbs most visible light and only 'reflects' yellow.
If a blue light is shone on the shirt, this will be absorbed and not 'reflected'.
If the shirt is in a dark room then the shirt would appear 'dark' to the normal human eye, having none of the light from the blue torch 'reflected' and no yellow light within the torch to 'reflect'.
As is the case with science, one can of course go deeper in to the physics and assume various conditions or the meanings of various answers
Moonhawk said:
Another complicating factor is this.
If you had an object that only reflected yellow light in a very range. Yellow light has a wavelength around 560-590nm.
Now if you shone light onto this object that was in the wavelength range 560-590nm - the object would obviously reflect that light and would appear yellow.
But what happens if you don't shine light that is in the range 560-590nm?
If you shone light that was a combination of 690nm light (Red) and 550nm light (green). According to the additive colour method - we would perceive this light as yellow - however your object wouldn't reflect this light as it doesn't contain any wavelengths that your object can reflect and would therefore appear black.
Yes also good valid answer. If you had an object that only reflected yellow light in a very range. Yellow light has a wavelength around 560-590nm.
Now if you shone light onto this object that was in the wavelength range 560-590nm - the object would obviously reflect that light and would appear yellow.
But what happens if you don't shine light that is in the range 560-590nm?
If you shone light that was a combination of 690nm light (Red) and 550nm light (green). According to the additive colour method - we would perceive this light as yellow - however your object wouldn't reflect this light as it doesn't contain any wavelengths that your object can reflect and would therefore appear black.
This method also explains why the colour pink doesn't really exist
The brain invents the colour.
As the colour spectrum runs from red to blue, each colour in between is easily 'mixed' by the varying small differences of wavelengths of light.
However 'pink/purple' is a mix of red and blue, which occur as the opposite ends of the visible spectrum....therefore no small difference of wavelength mixing can be done...instead the brain invents pink/purple as a 'bridge' between these colours.
FarmyardPants said:
I think the answer that was hoped for is black. A yellow (as in purely secondary yellow) shirt looks yellow in white light (equal mix of RGB) because it reflects 100% of primary red and green and totally absorbs blue. So a purely primary blue light would be wholly absorbed.
I agree, given (only) the information in the question; you'd have to get full marks in a school exam. But I suspect in practice if you shone bright blue light on anything it would look blue.Gassing Station | Science! | Top of Page | What's New | My Stuff