K4.3 Light and sound
Seeing and hearing are two of the most important senses by which most of us are able to explore our world and communicate with each other. However the scientific principles involved in these processes are often misinterpreted and/or merely taken for granted. This article explores some of the more important conceptual barriers to understanding about 'light' including light as a form of energy, seeing, shadows and reflections, sources of light, refraction and dispersion (colour). A parallel section is included for 'sound' and presentations and practical suggestions are provided as downloads.
This is one of 17 articles whose main aim is to support the processes of teaching/learning between the science education tutor and the trainee science teachers with a focus on “teachers’ knowledge and understanding”. During a primary or secondary BEd, PGCE or GTP we hope that those learning to become science teachers will be able to challenge their own understanding of science and scientific concepts. Unit K0 specifically explores general issues relating to all the knowledge units - to the learning of science.
Standards: This unit specifically addresses Q14 but, appropriately used can contribute to and provide evidence of competence for many others of the standards especially Q4,6,7,8,18, 22 and 25.
Keywords: Light, Sound, Energy, Seeing, Hearing.
1.0 Introduction: Light and Sound
2.0 The conceptual barriers to understanding ideas about light
2.1 Light as a form of energy
2.3 Shadows and reflections
2.5 Sources of Light
2.6 Refraction and dispersion - the Rainbow
3.0 The conceptual barriers to understanding the ideas about sound
4.0 Giving Practical experiences
5.0 Useful references
There are several parallels between light and sound. Many science courses consider these two together - reflections and echoes; shadows and sound-proofing; the eye and the ear as receptors of incoming energy; both light and sound being wave motion. More difficult is the link between the colour of light and the notes of a scale - both linked with the frequency of vibration of this wave motion. The Doppler (red) shift in light allows us to work out how fast distant galaxies are receding from us, in the same way that the drop in pitch of a police siren as it speeds past you allows you to work out its speed relative to you. However the bow (water) wave of a boat and the pressure (sound) wave of a supersonic plane are phenomena that are not replicated by light waves. We need to point out important differences between light and sound. Light travels so fast that it could circle the earth 8 times in a second, and reaches us from the Sun, through the vacuum of space, in just eight minutes. Contrast this with sound that plods through air only just faster than a jet aeroplane, and would take a day and a half to circle the Earth. The well-known phenomenon of thunder being heard after the flash is explained in this way - light from the flash travels in an instant, but sound takes three seconds for each kilometre (about 5 seconds per mile) giving you a way of working out how far away the storm is. Sound needs a material for the vibrations to be transmitted, but light is slowed down when it hits material. This leads to the bending or refraction of light by transparent objects such as lenses. The rest of this unit concentrates on light, in particular we point out the conceptual problems children face as they come to an understanding of light.
Many of the issues addressed in this section are introduced in this PowerPoint (download 2) which can be used with trainees (both primary and secondary) to alert them to the conceptual barriers to understanding light.
Download K4.3_2.0a 'Lead Lecture - Light'
The idea that light is a form of energy that travels is not an obvious idea - to children light is just ‘there’. By the time they reach the end of secondary school they need to appreciate that light:
- is the visible part of the electromagnetic spectrum (which stretches from low energy radio-waves through TV, radar, micro-waves, infra-red (IR), visible-light (red, orange, yellow, green, blue, violet), ultra-violet (UV), X rays, to high energy gamma-rays).
- can travel through the vacuum of space and any substance that is transparent.
- travels in straight lines from a source, eg the Sun, a bulb or fire and spreads out in all directions.
- travels very quickly (300 000 km per second - 186 000 miles per second so taking 8 minutes to reach us from the Sun (our nearest star) and 4.5 years from the next nearest star.
Light scatters off illuminated objects or spreads out from luminous objects in straight lines, so we 'know' where something is since our eye/brain system is efficient at working out where these lines of light come from (assuming that the light DID travel in straight lines.) This is how we see images - the light appears to be coming out from where the image is. Until children understand how and why we see the real objects around us we cannot expect them to understand images or reflections. Many pupils even at Key Stage 4 will unthinkingly show light coming from the eye and on to the book. The idea of sight rays is a consequence of words such as look, notice etc. that suggest the eye is an active seeker. Many ancient cultures (e.g. Greek and Indian) had this idea of active eyesight. Drawings which show light from the lamp scattering from the book with some rays entering the eye follow the scientific idea of the eye as a receptor. This was recognised by the Islamic school in the eighth century who actually dissected the eye and realised it was a receiver of light (Butt 1991). We should, however, remember that observation is an active process and in order to make any sense of the light entering our eyes, we must match this input with ideas generated from our memory (Guesne 1985). There is a grain of truth in these 'looking' rays which come out of the eyes. Perception is a two-way process of comparing what our senses sense with what our brain has already stored. See download 2
Shade is where the light is blocked by something that is opaque (won't let light through). The word shadow is used when you can identify the shade produced by one particular object by its shape. The area of shade will usually receive some light so it won't be pitch black. If the source of light casting the shadow is diffuse (spread out) the shadow will be fuzzy. For example, on a cloudy day the sunlight hitting the clouds is scattered so there are no shadows since light comes from many different directions.
See Unit K4.4 - Earth and Beyond for work on shadows from the Sun, day and night and phases of the Moon, which are all to do with shadows.
A mirror is usually made of glass with a thin coating on the back of silver or aluminium which reflects most of the light. On a smooth surface the angle of incidence - where the light came from - will equal the angle of reflection (a ball hitting a wall is a good example). Because the light is reflected in one direction rather then being scattered, we think the light is actually spreading out from behind a mirror, and we see a reflection or image of ourselves. Unlike a shadow which is dark and only has an outline, reflections will show all the detail of the object itself, including colour. See download 2 for a discussion of the confusion children make between shadows and images.
Young children are likely to show rays of light around a source (like rays from the Sun) or show a glow close to a light (like a glow from a candle). The idea that light will travel for ever (until it is absorbed by an object, which consequently gets warmer) is a difficult one. For example, children of most ages will say that light travels further at night (or in the dark) than it does in the day. The grain of truth in this is that the 'glow' or 'area of brightness' from a torch certainly goes further the darker it is. However each ray of light/photon goes the light itself goes on (at the speed of light) for ever, until it is absorbed by matter. See download 2 for further discussion.
Primary sources of light (luminous objects) need no other source of light to enable you to see them, light spreads out from them; some, like the sun or a candle, are very hot and emit IR as well as visible light. Some sources of light are cold, such as a glow-worm and LEDs. They work by electrons falling between energy levels in the material, so that all the energy is released as light - a much more efficient way of making light than using high temperatures!
Children will find the idea that Sunlight is a source of (visible) energy fairly straightforward:
- It drives photosynthesis in green plants by separating biomass and oxygen. This is the source of energy for almost all other organisms as well - when they need energy they recombine the biomass (food) with oxygen in a process called respiration.
- When you stand in the Sun the light is absorbed by your body and you get hot.
- When light falls on photographic paper it causes chemical changes allowing you to draw using light (graph-photo).
- When light falls on the photo-cell of your calculator, electricity is generated providing power instead of having to use an electrical cell (battery).
Light will change direction as it passes from one medium to another, e.g. air to glass or air to water. It is this effect, refraction, that is used to explain why water in a swimming bath appears to be much more shallow than it really is. Light from the bottom of the pond is bent slightly downwards towards the horizontal as it leaves the surface of the water. Thus it appears to have come (in a straight line) from a point a little higher up. Lenses can be used to bend light and make things appear larger or smaller. Work with ripple tanks making the waves move from deeper to more shallow water can illustrate this effect for KS4 pupils (see section 3 below).
A prism or a drop of water can split white light into its separate colour components (dispersion) because different colours are bent by different amounts as they move from one material into the other. Thus for a rainbow to be produced, water drops and sunshine must be present at the same time.
When light hits a material it might either carry on (material is transparent) or become scattered but get through (material is translucent). If the material is pigmented, then only some colours will get through (see below).
If light cannot get through a material, the material is called opaque. In this case three things can happen:
a) the light will be scattered (from a white surface)
b) the light will be reflected (from a shiny surface)
c) the light will be (selectively) absorbed (from coloured surfaces)
If all light is absorbed the object will appear black, and the light energy will be transferred to heat energy, but if only some light is absorbed the surface can look coloured or grey. Pigments are therefore selective absorbers of light.
Children find it hard to believe that white light is composed of a spectrum of colours (seen as a rainbow when raindrops cause the colours to become separated). Pigments work by selectively absorbing some colours from the white light. That is why you must look at colours in daylight to appreciate them properly, and that is why a blue door (which only reflects blue) looks black at night - the light from sodium vapour street lights has no blue in it, so it is all absorbed by the pigment on the door. It is important to distinguish between coloured pigments that absorb light and coloured lights that add more light.
To understand colour vision, the mixing of pigments by printers and artists and the addition of lights on the stage or on your computer screen, you must appreciate that the red, yellow and blue - the artists’ primary colours - are different colours from the scientists’ primary colours of red, green and blue. There are six colours here, not four, and the names given to them by artists and some teachers are not the same as those given by photographers, printers and scientists. This is discussed in download 2.0a - please refer to the tutor’s notes pages within the PowerPoint.
Many light words [photograph, transparent, penumbra etc] are Greek or Latin and crop up in many different places, so are well worth sharing with your trainees so that they, in turn, can help their pupils get a handle of what are otherwise strange and unfamiliar words.
photo = light (photograph, photocell)
graph = draw (graph paper, graphics)
trans = across/through (transport, Ford Transit van)
umbra = shadow/shade (an in umbrella)
pen = half (as in pen-insular: half an island)
We leave you to look up the rest of these in an etymological dictionary - don't overlook the value (in helping children understand meaning of words) of finding the origin of their meaning.
This download follows the same paragraph headings as in light to show the similarities and contrasts. It identifies some of the misconceptions children have about sound, it has explanations for common experiences, such as echoes, the Doppler effect and musical notes. It can be used as the basis of a hand-out for trainees.
Most practical work can be experienced by trainees in school (and will be on school-based training courses). Download 4 suggests some activities that would be worth doing in central sessions.
Download K4.3_4.0a 'Giving Practical Experiences in Light and Sound'
- Butt, N. (1991) Science and Muslim Societies. London: Grey Seal
- Guesne E (1985) Ch 2 Light in Driver et al (eds) (1985) Children's Ideas in Science Milton Keynes: Open University Press
- Kennedy J (ed) (1997) Ch 10 Light in Primary Science - Knowledge and Understanding London: Routledge
- Nuffield-Chelsea Curriculum Trust (1995) Nuffield Primary Science: Light Teachers' Guide, pp 5-7 London: Collins Educational
- Science Processes and Concept Exploration Project (SPACE) (1989-92) Research Reports: Light Liverpool: Liverpool University Press
Section Developed by: Keith Ross, University of Gloucestershire May 2005
Published: 02 May 2005, Last Updated: 10 Jan 2008