Video: Colors of Light

An object appears blue in color when seen in sunlight. The same object is illuminated by a mixture of equally bright red and green light and is viewed through a blue filter. In what color does the object appear?

02:47

Video Transcript

An object appears blue in color when seen in sunlight. The same object is illuminated by a mixture of equally bright red and green light and is viewed through a blue filter. In what color does the object appear?

Okay, so, in this question, we’ve got some object. Let’s say this is our object. And we’ve been told that when seen in sunlight, this object appears blue in color. Now sunlight is white light. And we can recall that white light is made up of all of the colors in the visible spectrum.

In other words, then, if we’re illuminating our object with white light, then we’re technically illuminating with all of the colors of the rainbow, red, orange, yellow, green, blue, indigo, violet, and so on and so forth. We’ve only drawn a selection of the colors here, but let’s just assume that all of the colors in the rainbow are illuminating this object.

Now, when sunlight is falling on the subject, in other words, when all of the colors are illuminating this object, we’ve been told that the object appears blue in color. In other words, then, if we had a human being watching the object from over here, let’s say, then all they’re seeing is blue light coming from the object.

Now it’s worth remembering that the blue light that the person is seeing is actually the light reflected off of the object because this object is just an ordinary object. As far as we know, it doesn’t emit its own light. And so, any light that we do see coming off this object is going to be reflected light.

But then, this means that if we can only see blue light coming from the object, we’re seeing the object as blue, then all of the other colors of the rainbow are absorbed by the object. In other words, red, orange, yellow, green, indigo, and violet are absorbed by the object, and blue is reflected by the object. So, based on this information, we need to work out what will happen when that same object is illuminated by a mixture of equally bright red and green light.

So, let’s take our object again and illuminate it only with red light and with green light. And we’ve also been told that the subject is being viewed through a blue filter. In other words, this is a filter that only allows blue light to pass through it. So, here’s the eyeball of the person staring at this object. And they’re looking at it through the blue filter. So, we need to work out what color the object will look to the person looking at it through the filter.

Well, as we’ve seen already, if we shine all colors of light onto this object, then it absorbs all of them apart from blue, which it reflects. And so, when we shine red light onto it, the red light will be absorbed by the object. And the same is true for the green light. It’s not going to be reflected off the object for it to then go on and go into someone’s eye.

So, basically shining red and green light on the object will result in no light coming off the object. And therefore, the person will see the object as having no color. In other words, it will be black. Now the fact that this blue filter in place is neither here nor there. The blue filter only allows blue light to go through it. But actually, this object is not reflecting any light. So, there is no blue light to go through, for that matter, nor is there any other color of light. And so, to answer our question, in what color does the object appear? Well, it turns out that it will appear in black.

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