Ask students to look through coloured plastic sheets at different coloured objects, or to view drawings they make with coloured markers. Try outdoors.
Depending on the colour of the object/marker and the colour of the filter, different features will be highlighted and will "disappear".
The coloured plastic takes away (absorbs) some of the colours so only one colour reaches your eyes. e.g. a red sheet absorbs all colours except red, so everything has a red tinge. If something has no red in it, it will look black.
Step by step activity to understand the phenomenon:
Ask the students to put the red filter over their eyes. Then lay out coloured papers for them to look at (don't tell the students what colours they are). Ask what colours they appear through the filters. (The red paper will look red through the red plastic; the green paper will look black through the red plastic - see image 1)
Ask the students to take the plastic away so they can see the true colours of the papers. It is a very striking effect.
Discuss: the paper only reflects some colours (that is why it appears a certain colour - the other colours are absorbed). If the plastic does not let the paper colours through, the paper will appear dark.
Notes for teachers on the complexity of this activity:
For coloured acetate and construction paper, both the papers and the filters will allow some other colours of light through other than the colour it appears i.e. green paper will reflect yellow and blue light as well as green (they are next door on the colour spectrum), and a blue filter will allow some green light to pass - hence through a blue filter, green paper appears green and red paper appears black (see image 2).
Some blue items appear purple through the red filter. The red filter is not perfect - it passes some light of other wavelengths besides red. Purple is a mixture of red and blue light - it is a non-spectral colour (violet is a spectral colour next to blue). When objects emitting blue light are seen through our red filter, both red and blue light are perceived, which look purple.
Coloured filters for astronomy studies:
Astronomers use filters to look at images of stars and galaxies, to see the phenomena they are more interested in, while making other phenomena recede. Students can look at composite images of star nurseries, nebulae or galaxies, to see the cooler gas clouds (often imaged in red, so visible through the red filter) separated from the stars and higher energy wavelengths such as X rays (often imaged in blue, so visible through the blue filter).