What the Activity is for

Splitting light with a prism.

This activity is intended to show a really impressive spectrum, made by a prism, from a white light source.

What to Prepare

• quartz-iodine lamp plus 12 V power supply (bright white point source of light)
• high-dispersion prism
• white screen
• a room with good black-out

What Happens During this Activity

Set up the light source and prism as shown, with the screen a good distance away from the prism.

The greater this distance, the larger the spectrum, but you will have to balance this against the brightness of the spectrum, which will be brighter when the screen is closer. How bright you need the spectrum will depend on the darkness in the room. Do not try to do the experiment in a laboratory without blackout. Make sure that the light beam does not go through one of the corners of the prism. Rotate the prism gently so that the direction of the light beam is changed as little as possible. This generally gives the best spectrum.

Enjoy the spectacle. Then draw attention to the ordering of the colours and the path that the light has followed, from source to screen.

What the Activity is for

Division and recombination.

This activity shows white light being split into colours, and then that a prism cannot split one of the colours further. In a second experiment, the colours from the first prism are recombined into white light.

What to Prepare

• A pair of 60 degree prisms per working group
• A light source for narrow beam of light
• A small white screen
• A narrow slot, a few millimetres wide, possibly cut into cardboard, for restricting the beam further

What Happens During this Activity

Pupils arrange the first prism so as to cast a spectrum onto the white screen. We suggest that you check they have managed this before trying out the next steps. Then replace the screen with the slotted card, cutting out all of the colours apart from one that they have selected. They should use the white screen to check that they have managed to select only one colour. Then replace the screen with the second prism, in an attempt to split this colour up further. They will fail! Use the screen to catch the beam after it has left the second prism in order to check that the colour cannot be further subdivided.

Then go back two steps to where the single prism casts a spectrum onto a screen. Now try to insert the second prism so that the colours are recombined into white light. Pupils can be left with this as a puzzle. If stuck, they can be offered a clue: The second prism should reverse the action of the first, so be flipped. Or you could point out that the prisms together need to behave much like the glass block that the pupils looked at earlier and so should be geometrically similar to such a block.

As a result of this experiment, pupils should become more convinced that white light is made up of various different colours.

What the Activity is for

Combining colours of light.

Here you can arrange three overlapping coloured beams of light to fall onto a screen, showing the colours that arise from adding the beams together.

What to Prepare

• A three beam projector (typically this will have three filters (red, blue and green), perhaps with mirrors to generate three separate beams from one lamp)
• A white screen

What Happens During this Activity

Show the three beams initially overlapped, thereby producing white light. Then split them into the three primary beams, finally combining them in pairs to produce the secondary colours.

What the Activity is for

Investigating how colour is made in televisions.

To reinforce that the combination of different coloured beams of light can produce any colour. It can be used as a homework.

What to Prepare

• A magnifying glass and a television (if magnifying glasses are not available, then a wet finger touched against a screen leaves a droplet behind that acts as a magnifying glass)

What Happens During this Activity

Pupils hold the magnifying glass up to the screen, really close, to see the triplets of dots: Red, blue and green. These triplets are used to generate the beams of light that make up the picture. You might get them to explain how any colour is generated from these three dots to an adult, then have the adult comment on the quality of the explanation as a means of having some record of the homework. You might like to ensure that they include an explanation of how black is generated (no light is emitted where none of the triplet will be producing a beam).

What the Activity is for

The colour of an object depends on the illumination.

Here you can draw together ideas about apparent colours, depending on the illumination.

What to Prepare

• A three beam projector (typically this will have three filters (red, blue and green), perhaps with mirrors to generate three separate beams from one lamp)
• Three pieces of mounted coloured paper: Red, blue and green (choose the paper so that each only shows up in the same coloured light, but not in the other two)
• A number of coloured everyday articles, perhaps including some provided by the pupils

What Happens During this Activity

Make sure that the three beam projector is casting three separate coloured beams, which aren't overlapping.

Take the blue paper, shown in white light, and ask for predictions as to what will be seen when it is placed in each of the coloured beams.

Try it and check the predictions. Then overlap two of the beams and again ask for predictions. Again, check the predictions.

Now overlap another pair and repeat the cycle.

Try the whole sequence again with another colour of paper. Repeat until the class is confident with the argument that a blue object looks blue because it reflects blue light, so needs to have blue light falling on it that can be reflected.