RGB video displays produce red, green and blue light in a range of intensities to imitate a range of colours. Diffraction grating glasses break up the light from a video display to show the red, green and blue components.
The human retina is composed of cones and rods. Cones are sensitive to colour, and most people have three types of cones. Although each type of cone is sensitive across a range of wavelengths in the visible spectrum, they peak in the red, green and blue.
RGB video displays use this fact to produce red, green and blue light in a range of intensities to imitate a full range of colours. In this demonstration we use diffraction grating glasses to break up the light from a video display to show the red, green and blue components. We see that a computer monitor displaying a yellow sun for example is actually not making any yellow light!
A note about “primary” colours
There is nothing at all special about the ranges of wavelengths of electromagnetic radiation we call red light, green light, and blue light. They are “primary colours” only because they correspond to the peak responses of the cones in our eyes. Had we evolved with different kinds of cones we would presumably call different ranges of electromagnetic radiation “primary.” Nor is their anything special about having three primary colours. If we had four types of cones in our eyes we would have four primary colours.
NCEA & Science Curriculum
JNR SCI, PHYS 3.3
Use an incandescent lamp or a thin ray of sunlight in a dark room. Students should be wearing the diffraction grating glasses at left below that are available at low cost. On either side of the light source students will see a continuous spectrum as shown at right below. Fluorescent or energy saving lamps will not produce a continuous spectrum (but that’s worth looking at also as a separate exercise).
Run the spectra.ppt PowerPoint show in the dark room, either on a large bright screen or through a data projector onto a white background The white line splits into defined colour bands rather than the continuous spectrum. Continue to run through the slides, yellow can be seen to be composed of the red and green components and the other secondary colours similarly break up into the primary components as in the image below.
Also, try using a strong magnifying glass (at least 6X) to look at the yellow part of an LCD monitor. It is possible to see the red and green pixels on screen.
Do not look into lasers or other very bright lights with the glasses. Do not look directly into the projector as they can be VERY bright! Look at the projection on the screen or at an LCD monitor.
Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.
Would you like to contribute lesson suggestions? Contact us.
This teaching resource was developed in collaboration with Rory O’Keeffe, a New Zealand Science, Mathematics and Technology Teacher Fellow, 2007, hosted by Victoria University School of Chemical and Physical Sciences. Rory is a science teacher by training and is currently Deputy Principal at Lytton High School in Gisborne, New Zealand. He is also an amateur astronomer. See NZSMT Teacher Fellowships for more information about the Teacher Fellow Program.
This teaching resource was developed with support from