6. Visible light filters and the observed colour of objects subjected illuminated by variety of light colours

How does a light filter work to produce particular colours?

Why are stained glass windows a complex mixture of light filters?

The diagram is presented in terms of primary and secondary colours only, but the principles described below apply to all the 'intermediate' colours.

Transparent materials allow some of visible light through

Colour filters are so called because they only let some light wavelengths through, but the image is not distorted.

Colour filters are designed to filter out different wavelengths of light, so only certain wavelengths are transmitted.

Tissue paper is translucent because the light is scattered by all the fibres of the paper, so no clear image is seen through it, though all light colours might come through if white paper.

A colour filter absorbs some colours from white light but allows your desired colours to be transmitted.

A basic set of six filters is illustrated above.

1 to 3 are primary colour filters because they only allow the transmission of one of the three primary colour band of wavelengths - red, green and blue on passing a beam of white light through them.

If you view a red object illuminated with white light through a red filter, it will look red because red light is reflected off the surface and will pass back through the filter. If you view it through a green or blue filter it will look black because neither of these colours is transmitted back through a red filter, they are absorbed, so no light seen - black.

You can logically analyse other situations by 'theoretically' viewing primary/secondary coloured objects through primary coloured filters.

 

4 to 6 are secondary colour filters because they allow two of the three primary colours through to produce a secondary colour AND the colour of the filter itself. Looking at the spectrum diagram above, you can see all the primary and secondary colours somewhere in the spectrum.

A cyan filter allows green, blue AND cyan through,

a magenta filter allows red, blue and magenta (sort of purple) through

and yellow filter allows green, red and yellow through,

on passing a beam of white light through them.

If you observe a green or blue object illuminated with white light through a cyan filter you see the object in its true colour (of green or blue). This because a cyan filter allows both green and blue light through.

If you observe the same two objects through a magenta filter the blue object will look blue, but the green object will look black because the magenta filter absorbs green light.

The yellow petals of a daffodil will appear black in blue light, but will appear yellow in yellow light. The petals will appear red in red light and green in green light since they reflect both those colours.

You can logically analyse other situations by 'theoretically' viewing primary/secondary coloured objects through secondary coloured filters.

 

By using various coloured mineral pigments or organic molecule pigments you can produce any shade or any colour you desire.

 

Stained glass windows use mineral pigments that absorb or transmit particular visible light wavelengths so when light streams through you see a selection of bright colours. A stained glass window is essentially a complex arrangement of visible light filters.

St Mary's RC Cathedral, Newcastle - stained glass window of the industrial heritage of north-east England

A wonderful masterpiece of the art of stained glass windows.

 

Many of the pigments are based on coloured transition metal compounds.

 

Now that's what you call a light filter display from stained glass window panels! (medieval Chester Cathedral)

 

I love the 'red devil' in this medieval stained glass window, St Martin-le-Grand Church, York, England

Medieval monks were skilled at mixing mineral pigments to colour the glass - very expensive work!

They made good use of all the primary colours of red, green and blue and the secondary colour yellow.

Over a thousand years ago they were producing yellow and green stained glass using iron oxides, but other colours soon came along. Copper minerals could produce red or blue glass and lots of colour appeared in the great cathedrals of Europe from the late 12th century onwards.

Art and chemistry, what a combination! OK it is physics too! So,

including the wonderful appreciative 'eye', lets called it art + science = manufactured beauty!

 

Light filters are widely used at pop concerts and stage lighting in the theatre.

 

INDEX of my physics notes: The visible spectrum and colour of objects

---

Keywords, phrases and learning objectives for visible light and colour filter effects

Know and be able to explain the effects of visible light filters on the observed colour of objects illuminated by variety of coloured filters.

Be able to predict the observed colour of an object when illuminated with 'filtered' light.