White light and filters
Teaching Guidance for 11-14
Adding or removing frequencies
Wrong Track: When white light goes through a red filter the filter makes the light go red. It adds the red colour to it.
Right Lines: When white light passes through a red filter the filter absorbs or removes all of the colours apart from red, which is transmitted.
White light is a mixture of colours
Thinking about the learning
The central idea to get across here is that white light comes from the Sun and ordinary filament bulbs, and is a mixture of all the colours of the spectrum. Light filters work by subtracting some colours from white light. The filter does not add colour to the light passing through it.
Thinking about the teaching
The teaching challenge here is to emphasise the point that filters work by subtracting colours from the incident light. A good way of helping pupils visualise this is to use diagrams, such as those presented in the Physics Narrative, which show the difference between the colours present in the incident light and those in the transmitted light.
A further point is that the light that is transmitted through a filter is always less bright (of lower intensity) than the incident light, simply because of the absorption of colours by the filter.