# 04Colours of two kinds

Li04TL of the Light topic
• ## 01 Things you'll need to decide on as you planLi04TLnugget01 Decisions

### Bringing together two sets of constraints

Learners: Distinguishing–eliciting–connecting

How to:

• separate perceptual colour and spectral colour
• Separate altering a colour by adding frequencies from altering a colour by subtracting frequencies
• Identify the role of reflection in colour perception
• connect to how painters mix colours
• connect to coloured lighting
• connect to the colours on a computer screen

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• connect dispersion to refraction
• link colour and frequency
• separate perceptual colour and spectral colour
• link absorption to the colour of an object

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

• ## 02 White light and filtersLi04TLnugget02 Challenge

### 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.

• ## 03 The colour of objectsLi04TLnugget03 Challenge

### Colours come from absorption

Wrong Track: The dress is red because it has that colour and the light in the room brightens it up to let us see it.

Right Lines: The dress is red because when white light falls on it, the dress absorbs many of the frequencies of the spectrum falling on it. The range of frequencies reflected, travelling to your eyes, make the dress appear red.

### Colour depends on the light falling on it

Thinking about the learning

For most people, colour is an innate property of an object. For example, a red dress is red simply because it has that colour.

Thinking about the teaching

The idea of the normal colour of an object is an important one here. We can say that the normal colour of any object is the result of white light falling upon it. If the ordinary light is not white light then the object may appear to be a different colour (e.g. in blue light a red dress will appear black).

In other words, the apparent colour of an object depends on the colour of the light entering the eye. This depends not only on the pigments that colour the object, but also on the light which illuminates that object.

• ## 04 Colours – adding and removingLi04TLnugget04 Challenge

### Lighter or darker

Wrong Track: We've done about colours in art, and the more primary colours you add, the darker it gets.

Right Lines: Adding different coloured beams of light heading towards your eye makes things lighter. Reflecting beams off many different surfaces makes things darker, as with painting.

### More or fewer frequencies of light

Thinking about the learning

You need to be very clear about the distinction between situations where more and more frequencies are added to the light that hits the eye, and those where the frequencies that hit the eye are reduced. The first one happens when you bring beams of light together, the second when diffuse reflection subtracts parts of the incident beam.

• ## 05 Rainbows and spectraLi04TLnugget05 Teaching tip

### Keeping it accessible

Except for individual study for the particularly interested, we do not suggest that you ask pupils to research how a rainbow comes about, nor that you explain it in class. The process is not simple, involving reflection and refraction within the raindrop and the contribution of lots of separate raindrops. By all means enjoy a striking image, but do not explain it to death. Pupils of this age are not likely to enjoy following the necessary detail for so little reward.

• ## 06 It's partly in the mindLi04TLnugget06 Teaching tip

### A small disagreement about colour

David: I wonder if we see all colours as the same?

David: Green to you might be red to me, even through we both call it green.

Sam: Ridiculous!

David: OK, what colour is that rock?

Sam: Blue.

David: Wrong!

• ## 07 Thinking about actions to takeLi04TLnugget07 Suggestions

### There's a good chance you could improve your teaching if you were to:

Try these

• separating perceptual and spectral colours
• introducing and exploiting a simple account of colour vision
• introducing filtering colours as a process of removing certain frequencies
• preferring frequency to wavelength
• explaining in detail, on several occasions, why something appears the colour that it does
• bringing to mind the psychological aspects of colour perception, perhaps by showing a few well chosen illusions

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• conflating adding and removing frequencies
• subtracting colours
• trying to explain the rainbow, for most classes
• reducing the complex processes of colour perception to simple physical principles

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.

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