Skip to Content Go to sign in Skip to Primary Navigation Skip to Secondary Navigation Skip to Page Navigation Skip to Header Navigation Skip to Footer Navigation Read more about accessability options and our navigation

Physics Classroom is making strides to make our site accessible to everyone. Our site contains 6 navigation areas. The Primary, Secondary, and Page Level navigations have a screen reader version of their nav structure that allows using the left and right keys to navigate sibling navigation items, and up or down keys to navigate parent or child navigation items. The others can be navigated using tabs. The Primary Navigation handles the first 2 levels of site pages. The Secondary (which is not always available) handles the 3rd and 4th level of structure. The Page level navigation allows you to navigate the current page's headings quickly. The Header Navigation contains the Light/Dark Mode toggle, Search, Notifications and account login. The Breadcrumb Navigation contains the breadcrumb of the current page. If the current page has a breadcrumb, you can get to it by skipping to the content and tabbing in reverse (shift plus tab). The Footer Navigation contains links such as Privacy, Contact, about and terms. Some resources contain an Audio Player that can be activated by holding down the T key for 3 seconds, and then using K to pause and resume. While not every area of Physics Classroom is usable purely from keyboard and screen reader, we are committed to continue work on making this possible. If you have questions or need additional help, please use this link to contact us.

Return to screen reader navigation
Home Minds On Light Waves and Colors Mission LC5 Color Subtraction Help Help 10

Mission LC5 Color Subtraction

The Question

Three colored spotlights - red, green and blue - with equal intensities may be turned ON and OFF in order to illuminate a shirt with different colors of light. Suppose that a shirt which appears cyan under white light is placed under the spotlights and then appears as green. From this information, one cannot make any conclusions as to whether the ____ spotlight(s) is/are turned ON or OFF.

(Note: the actual colors listed are generated at random and may vary from the above.)

Dig that Diagram

Many students of physics have seen a diagram similar to the one shown at the right. The diagram depicts three circles colored with the primary colors of light - red, green and blue. The primary colored circles overlap to produce other colors of light - known as the secondary colors of light: cyan, magenta and yellow. Complementary pairs of light colors are those colors that are exactly opposite each other on the diagram: red and cyan, magenta and green, and blue and yellow.

Minds On Time

The color a shirt appears is related to the wavelengths (colors) of light that it reflects. And of course to reflect a color of light, that light color must be shining on the shirt. From the appearance of the shirt in the presence of white light, we can infer what the shirt can do to each of the three primary colors of light. If a shirt appears cyan under white light, then the shirt is capable of reflecting cyan light. Cyan light is a combination of green and blue light (see the Dig that Diagram section). So the shirt is capable of reflecting green and blue light (whenever green and blue light shine upon it) and absorbing red light (whenever red light shines upon it).
 
Now suppose the same shirt is placed underneath the red, green and blue spotlights - any one of which could be on or off. The shirt is observed to be of green appearance. Green light is being reflected by the shirt so the green spotlight MUST be turned on. What about the red and blue spotlights? The blue spotlight MUST be turned off. After all, if blue light was shining on our shirt, then it would be reflecting it and the appearance would have some blue to it. As for the red spotlight, it is impossible to conclude as to whether it is on or off. Our eyes can only detect the light which gets reflected from the shirt. And the red light would not be reflected by this shirt, whether it was shining on it or not.

Tune In! Listen!

Click the button below to play an audio file.

Hot Link