Detailed Teaching Programme

Introduction

We have developed a successful teaching programme for colour perception that links together material from a number of demonstrations. Depending how the material is presented, we have found it works well with students from intermediate school right through to first-year university.

Instructions

Begin with the one-way mirror box with a red bulb outside and a green bulb inside. But have the red (outside) bulb off and the green one on. Ask students how it is that the light bulb is off but the reflection is on - and it’s the wrong colour!

Students may figure out that this is a ‘one-way’ mirror and that there is a second bulb behind the mirror. If you want to show how so-called one-way mirrors work at this point take a detour and use Candle Under Water and One-way Mirrors to show the mirrors are not one way at all.

When you are ready to continue with colour perception, ask students what would happen if both bulbs were turned on at the same time. They may predict brown (mixing of paints) or blue or something else. Ask the students how many of them have an RGB computer monitor.Usually, few hands go up. But then ask how many have any computer monitor, colour cell phone, colour TV, or anything else that displays a colour image. Explain they are all based on RGB technology and get them to tell you RGB stands for Red, Green, Blue and that these are the primary colours of light or the primary lights.

Red and green combine to make … turn on the lights … yellow!

[Cautionary note: image technology is rapidly developing and some of the newest displays work slightly differently. We will address that in an update.]

But that’s not true. Red and green do not combine to make yellow. We will prove that to you.

Light is a wave. What’s the other wave we use constantly? Students will guess sound. Ask them what happens if you change the frequency of a sound wave. How does it sound different? Some may say louder or softer, but most will know that the pitch changes. Low frequency is a low pitch (make a low pitch sound) and high frequency is a high pitch (make a high pitch sound). What about light? How does light look different if we change the frequency? Students again may guess that it changes the brightness, but it is actually the colour that changes. Red is low frequency, yellow is medium frequency, and green is high frequency. Blue is higher still and we won’t worry about it. So we have red (low pitch) and green (high pitch) combining to make yellow (medium pitch). Let’s see if that works for sound. Get all the male students to make a high pitch, then get all the female students to make a low pitch, and then get them to go together and listen for a medium pitch. None is heard.

It is simply wrong physics to say that a low frequency wave and a high frequency wave combine to make a medium frequency wave. It just is not correct.

So why do red and green make us see yellow?

At this point launch into the three kinds of cones and explain that red and green light cause the same electrical response in the cones that yellow does (see the main page for this demonstration for the details). We call the ‘yellow’ made by the red and green bulbs ‘perceptive yellow’ and true yellow ‘spectral yellow’.

Then put a sodium lamp on top of the one-way mirror RGB box. Adjust the dimmers for the red and green bulbs until the reflection and the sodium lamp appear to be more or less the same colour .

You cannot tell from what you see what frequencies of light are present. We need a device that can separate the light into its constituent colours so we can see what’s really there. A prism does that, but we have something better: the diffraction grating glasses.

Have students use the glasses to view the rainbows from a white incandescent bulb so they get the idea. Then have them look at the sodium lamp and see that the spectrum is just a yellow line (the ‘bar code’ for sodium). Next have a look at the yellow reflection on the RGB box (you may want to shield the outside bulb so they see the yellow reflection only). Students see the spectrum is red and green, not yellow. Real yellow (sodium lamp) and fake yellow (red and green bulbs) look the same to our eyes because of the way our cones work.

You can inject a bit of humour if you want: So what do space aliens see when they intercept our TV broadcasts that are based on RGB? I have no idea, but they won’t see what we see unless their vision is based on RGB cones! In fact, animals and particularly insects have different kinds of cones senstive to different ‘primary’ colours.