Colour Mixing (additive)

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Below are a few suggestions for additive colour mixing investigations

Investigation 1

Demo to whole class
Three slide projectors or spot lamps with primary filters (red, green and blue dyed gelatine or plastic caps) are directed onto a white wall or projection screen in near darkness. Ideally these should be powered via a variac or dimmer control on each. The “blocks” of colour should overlap, either fully or partially. If voltage control is not possible the image size can be varied to produce different light intensities of projected images. The initial set-up may require that the projectors are at different distances so as to give a best “white” due to the filters’ properties and power of the projectors. The colours should give an image as:

projected circles partially overlapping

Some equipment manufacturers have additions to their optics benches for this demo.
Care should be taken with the placement of the projectors and filters used as the lamps may become hot and to reduce the need for trailing leads.

Investigation 2

Class practical
If you have a ray box of the appropriate design and filters three coloured beams can be caused to fall on to a paper or optics screen. The exact description depends on the ray box design. The mixing of two or three colours can be done in this method, however, the white is not too good. As an alternative, a three coloured slide could also be used with mirrors or even three rayboxes with single filters in each.

Investigation 3

Demo of uses
A Flexible camera linked to a projection screen is trained on a monitor showing a white page in close inspection either by the lens or using a macro attachment.
If this is not available some hand glasses (magnifiers) can be given for pupils to investigate a similar screen or television.

Investigation 4

PC investigations
Java applets exist e.g. [1] However if you use "Paint" or any other graphics package you can “choose a custom colour” by adjusting the ratios of R G and B. This shows colour changes in real time.

Experience with “Photoshop” or other photographic manipulation tool may allow the recreation of what is thought to be the first permanent colour photograph by J .C .Maxwell of a tartan ribbon. A Image of a ribbon is taken in colour. The image then split into 3 layers R,G and B using the “Filter” menu. These layers are then re combined to create the original image.

Unusual demo

It is suggested that a fan can have three chemical "light sticks" attached (Red Green and Blue). When switched on a white light is seen. external link: look on right hand side for "color fan"

Suggestion for project

A box with a frosted observation window and 3 high intensity LED.s on a rotary swich can be used as a colour mixing unit. If a potentiometer was used in series with each protective resistor a "dimmer" swich would result for each colour. This could also be done with lamps and rheostats.

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