Light, vision and colour
Historically, it was a considerable scientific achievement to understand vision in terms of the eye as a sense organ that receives light that is scattered from objects. Early ideas of vision held by thinkers such as Plato and Aristotle, and then the great Islamic scientists, were of vision as some sort of active engagement of the world by the eye; almost as if some sort of ‘signal’ was sent out by the eye to perceive objects. Students have difficulty in understanding the eye as a passive receiver of light. They also have difficulty with the idea of light as an independent entity that travels through space so that the role of a light source in helping us to see, and the relationship of colour of an object to the colour of the illuminating light, is problematic. Further still, ideas about how images are formed in mirrors and lenses are problematic without scientific notions of vision, and of light as a travelling entity. It is therefore important to be aware of the children’s ideas when teaching them about light, vision and colour.
Key concepts of light, vision and colour
The activities in this topic are designed to explore the following key concepts:
- Sight is a very important sense we use to interpret our world.
- Light is an entity that travels through space in straight lines.
- Some objects (a globe, the sun, a flame) are sources of light; most things we see reflect light.
- Having two eyes is necessary for judgment of depth.
- The image in a mirror is inverted, and symmetrical with the object.
- Curved mirrors cause images that have distorted shapes when compared to the object in front of the mirror.
- Coloured lights and paints can mix to form other colours.
- We see when light is reflected from objects into our eyes.
- Ordinary surfaces reflect or scatter light in all directions. Mirrors reflect light at an equal angle to the incoming light. Many surfaces, such as polished floors, both scatter and reflect light.
- Shadow shapes are areas of no reflected light or areas where the reflected light is less intense that the surrounding area.
- Most objects we see are due to scattered light from the objects.
- Some surfaces reflect more light than others. Black surfaces reflect the least light.
- Our brain puts together the stereo view we have of the world.
- Our eyes and brain can be misled.
- An image is produced when light that is reflected or emitted from an object changes direction before entering the eye to be seen.
- Our image in a mirror is equally far behind the mirror as we are in front of it.
- Light can change direction going into or out of water or glass, which results in an image of the object that may be a distorted shape when compared with the object.
- Glass and water can split light into their constituent colours as they bend.
- White light consists of all the rainbow colours.
- Colours of light and pigments can be mixed together in different ways to give different results. Colours of light mix differently from colours of paint, pencil or crayon.
Students' alternative conceptions of light, vision and colour
Research into students' ideas about this topic has identified the following non-scientific conceptions:
- Young children often make no connection between the eye and the object in the vision process. No explanation for the processes of vision is given: 'we see with our eyes' is sufficient explanation for the vision process.
- Older children often think of vision as something emanating from the eye to the object: a 'visual ray', or 'active eye' model of vision.
- Older children also think that light only needs to illuminate an object for vision of that object to occur; this is the 'general illumination' model.
- It is possible to see in situations where there is no light; it is totally dark.
- Light is only a source (for example, candle flame), an effect (for example, patch of light on a wall), or a state (for example, brightness); there is no recognition that light exists as an entity in space between the source and the effect it produces (young children thinking).
- Light from dim sources remains at the source; light from other sources of light travels away from the source a few metres or more, depending on the brightness of the source (young children thinking).
- Light from a source travels further at night than during the day.
- Shadows are entities independent of light; light allows shadows to be seen, rather than shadows being a result of absence of light (young children).
- Light stays on the mirror during reflection.
- Light does not reflect off non-mirrored surfaces.
- The image of an object in a plane mirror lies on the surface of the mirror.
- Lenses are not necessary to form images.
- The function of a colour filter is to dye white light the colour of the filter.
- White light is pure, not a mixture of coloured light.
- The rules for mixing colour paints and crayons are the same as the rules for mixing coloured lights.
- Colour is an innate property of an object.
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