Light_Concept_2

Subject Facts

Light and materials — transparent, translucent and opaque

Light can travel through empty space (see concept 1); but when it comes into contact with objects (matter), different things may happen, depending on the material they are made from and the structure of this material. A transparent material (such as air or thin glass) allows Eight to go straight through with little disturbance A translucent material (such as muslin or frosted glass) also allows Eight through. but not straight through. The light ray will be deflected and diffused (broken up) by the structure of the material so that although light does pass from one side to the other, it so 'messed around' that you are unable to make out any distinct image An opaque material (such as wood or metal allows no light to pass through It — which means that you can see the object, as opposed to seeing through it.

The distinctions between transparent translucent and opaque materials are relative. There are degrees of transparency: the thicker a piece of glass, generally, the iess transparent it your hand is normally opaque, but becomes translucent if a very strong is shone through it- These properties of materials, along with some surface properties (strange things can happen as light goes into or comes out of materials), can lead to some interesting optical effects.

Shadows

A shadow formed when an opaque object is placed between a light source and a surface: the shadow falling on the surface has the shape of the object The area of the shadow is unlikely to be completely devoid of light. as in most practical situations light will be scattered off other objects into the shadow area. If you look around now, you will probably see many different shadows with various depths of darkness. So, although an opaque material will block the transmission of light, its shadow will only be in complete darkness f there is only one light source and there are no other objects or surfaces around to redirect light into the shadow which is unlikely, if not impossible. So in reality, a shadow is formed where an opaque object stands in the way of a strong direct light,

Many windows have curtains or blinds that are open in the day (to light h) and closed at night (to stop light getting out). The effectiveness of different fabrics in blocking light varies tremendously. Some are very opaque, making it difficult. to tell whether it is night or day outside; others are merely translucent able to stop voyeurs from seeing inside but not able to stop the sunrise from waking you up.

Seeing things — light scattering and absorption

What happens to the light that is blocked by an opaque material? Where does it go? With most materials, one of two things happens: the light can be scattered (it bounces back off), or it can be absorbed (it is converted to heat w;thin the material) Usually what happens it is a combination of the two: brighter objects scatter more of the light and duller darker ones absorb more (see figure 2).

It is important not to confuse scattering and reflection. Scattered light is returned from a surface in a jumbled state, so that the surface gives no mage of the light source Reflected Eight is returned in precisely the same quantities and positions to have an image of the light source. For example light is scattered from a white plastic teaspoon, but it is reflected from a polished metal one (you can see yourself in it).

Light that is absorbed into materials becomes heat: when something is left out in bright sunlight, it tends to heat up, The duller or darker the object is, the more it heats up, since more of the light that falls onto it is retained as heat. A dull black object will absorb most of the tight that hits it; a shiny white object will scatter most of the light that hits it, and will thus be easy to see,

If you have a reasonable grasp of things so far but don't want to push it you can skip to the next paragraph. Still with us? Good! When the photons or particles of light hit a material they can be either re-emitted (scattered) or absorbed, These photons are energy bearing. When they are absorbed. their energy is transformed from light to heat and thus heats the material.

In summary, the following chain of events leads to our seeing an object. Light is emitted from a source The light strikes an object. The light is scattered from the object. Some of this light is scattered Into your eyes. We see the object.

The different colours which most of us see depend on the proportions of the light scattered and absorbed by different objects.

Colour vision is explored

Reflection

As suggested above, reflection is a special way in which light can bounce off the surface of a material. Instead of being scattered, the light rays return at angles equal to the angles at which they hit the material rather like a snooker ball bouncing off a side cushion (see Figure 3). Not only light coming directly from a source, but light scattered from an object, can be reflected in this way, If the reflective surface is smooth enough, a clear image of an object can be seen (see Figure 4), Mirrors are highly reflective surfaces that return exact images.

Curved mirrors

A flat mirror can give an exact reflection of an object. If the surface of a mirror is curved, the image produced can be distorted: it may be shrunk stretched, or inverted* depending on the curvature, A concave mirror (see Figure 5) can produce a 'stretched' image. A convex mirror can produce a 'squashed' image (see Figure 6). If you compare the path that the light takes in both diagrams, you will see that the angle at which it reflects from the mirror is the key factor in determining the size of the image.

The greater the curvature of the concave mirror the more it will magnify the image of the object. The Hubble Space Telescope uses a concave mirror to magnify images of distant stellar objects. Another key technological use of such mirrors is to focus light onto a particular small area, greatly increasing the intensity of the light energy in order to heat things very quickly: a 'solar furnace'. A concave mirror (such as the inside of a shiny metal spoon) will produce an inverted image of a distant object: the image will appear upside down, A convex mirror allows the viewer to see the image of a wider view in a smaller area Rear view mirrors on cars often use this approach* enabling drivers to see a wide view of what is behind them with only a small mirror.

Refraction

As mentioned above light travels at different speeds through different transparent materials. It travels more slow!y in water than in air; and more slowly still in glass and perspex. In general, the denser the transparent material, the more slowly fight travels through it. So if tight is travelling through air and then enters some glass which is in its path, it will slow down; but once it has come out the other side, it travel at the speed that it was going at before it entered the glass. Not many science textbooks, including this one, explain how that works! But knowing this property helps to explain why light sometimes appears to bend' — that is, how it can be refracted by glass or water, as in the bending pencil effect (see next section)

A model of refraction

The following model will give you a sense of what happens in refraction. End a wide, smooth ramp and things to prop it up with. Next, join a couple of wooden wheels (of the same size) with an axle made from a piece of dowel Let this simple vehicle roll down the ramp, Now place a strip of corrugated card across the ramp and let the vehicle roll down once more: it will slow down as it hits the strip of corrugated card, but will keep going in the same direction and then speed up again when it is past.

If you now place the strip at an angle across the ramp the wheel that hits the strip first will be slowed down, making the vehicle veer off in that direction. The other wheel will then hit the strip and stow down. The first wheel to hit the strip will also be the first to leave and speed up, causing the vehicle to straighten up again. So it continues down the ramp, but further over to one side than it would have been had the strip not been there (see Figure 7). This is comparable to the way that the path of a light ray is 'bent' by refraction.

Water refraction

If you slip a pencil end-on into a glass of water; it appears to bend (see Figure 8). This is an effect of refraction. The underwater end of the pencil appears to be nearer to you than it actually is, because the tight from that end of the pencil is refracted by the water: More of the light from the pencil is directed towards your eye, so the pencil appears bigger Because the light is going from water to air the 'bent' pencil does not appear to straighten.

You can do some interesting refraction experiments using a clear plastic one-litre box and water. For example, with the box two-thirds full place a pencil along the far edge going down into the water and view it from the diagonally opposite edge. You will see two pencils: one in each side. Now, using less water; tip the box over slightly and rest it on an edge with the pencil underneath. Twist the pencil around: you will see it curve!

Refracting lenses

This is not something you would normally cover in primary work; but it would be an interesting topic for keen Year 6 children to explore. As concave and convex mirrors can change the size of an image, so can concave and convex pieces of glass (or any other transparent material). A dear plastic bottle, filled with water; acts as an effective lens and will do most things that a biconvex lens (one that is convex on both sides) will do.

The light scattered from an object is refracted both when it goes into a biconvex lens and when it comes out The image of an object that is close to the lens will be magnified (see Figure 9). For an object further away, what is seen will depend on how far the viewer from the lens (see Figure 10) If the object is viewed from position a the image is blurred. From position b, it is too blurred to be seen. However if you hold the lens up to a light source and hold a piece of paper at b, the light from the source will be focused into a small, bright dot. Point b is thus called the focal point of the fens, NB This is why you should NEVER look at the Sun through a lens. From position c, the image of the object will appear upside down: the light scattered from the object has been refracted so far that rays coming from the top and bottom of the object have crossed over.

Why you need to know these facts

As much of the information children gain about the word around them is visual, developing an understanding of what happens when light hits different objects is important to their general awareness, Children will have practical experience of windows mirrors and lenses, and will want to be able to make sense of the properties of these materials.

Vocabulary

Absorption when light strikes a surface and retained within it.

Opaque — a material which blocks the passage of light. Reflection when an image is returned from the surface of an object.

Refraction — the bending' of light when it passes from one transparent material to another

Scattering — when light is returned from a surface.

Translucence - a material through which you can see light but not an image.

Transparent - a material through which you can see an image.

Amazing facts

Most materials, even transparent ones, can produce some kind of shadow. On a sunny day, when the sunshine is streaming through the window onto the floor, open a window so that some of the sunshine is going directly onto the floor and some is going through the glass. Compare the brightness of the floor in the two areas. If glass were perfectly transparent, it would be invisible. Can you imagine a glass-shaped amount of wine apparently floating above the table?

Common Misconceptions

You can get a reflection from any shiny object. A reflection is really an image. Some 'shiny' surfaces scatter light very effectively but do not reflect. Some surfaces can produce a reflection if you catch them at the right angle in the right light. "Reflectors} on cycles and cars scatter hght, but in a particular direction.

Shadow misconceptions.

Asking the children to draw picture which includes the Sun, a tree and the tree's shadow should indicate how well they appreciate the relationship between a light source, an opaque object and a shadow. There are three crucial questions to ask as you look at the picture: is the shadow on the opposite side of the object from the light source? Is the shadow a similar shape to the object? Does the shadow touch the object? If these questions are not all answered with 'yes', a walk outside on a sunny day will be helpful. I have often watched five-year-olds sprinting along the playground and stopping suddenly with the expectation that their shadow will keep going — as in a cartoon! Empirical evidence realIy does help to reinforce correct ideas.

Why do rainbows appear in the sky?

The answer *because sunlight is refracted through droplets of water' is only useful if you know about refraction! The best way to approach this question is to produce a spectrum in the classroom using a strong light source (perhaps the Sun), a tray of water and a mirror. Using a plant mister to spray water through a beam of sunlight is also a good way to produce a spectrum and this is similar to what happens with rainbows: water droplets act as lenses, breaking white light up into a spectrum of colours.

The colours that appear in oily puddles and bubbles are due to the fact that the varying thickness of a layer of oil or bubble means that it reflects different colours (that is, wavelengths) of light.

On a hot day, why do objects in the distance appear to shimmer?

To understand this, you need to know about refraction. When light moves from a dense material to a less dense one (or vice versa), it is refracted and so changes direction. On a hot, sunny day, the air just above a sand concrete or tarmac surface becomes heated. The heated air rises, because it is less dense than the air around it. So the light scattered from a distant object passes from denser air to less dense air. As it does so it refracts, causing the images of distant objects to shimmer. To explain this in terms that children will be able to grasp, It may be useful to compare it to the 'bent pencil. If the water is you get a fairly stable image; but if you make a few little ripples, it appears to dance all over the place. The warm air rising creates a similar effect.

When I look out through a window, why do I sometimes see what's inside the room? Transparent materials usually have smooth surfaces, which tends to make them reflective as well, The reflection from a glass window is more obvious if you view it from an acute angle, because you will not be blocking it,The reflection is also more obvious if there is more light on your side of the glass (being reflected) than on the outside (being let through).

Teaching ideas

Testing transparency (investigating and sorting)

The children can use a torch to test whether a sample of 49 material is opaque or translucent. Can they tell when the torch switched on by looking at {t through the material? Pant out to them that the thickness of the material and the brightness of the torch will affect the results. Of those materials that allow light through, can they say which are transparent (allow objects to be seen through them?)

Shiny and dull (sorting and ordering)

Ask a child to collect five to ten objects or materials from around the room and then place them in order of shininess. Record the order. Do the others agree with the order? Test the objects by placing them in a lightproof box (see page 08) and shining a light through a small gap: which one is the brightest now?

Bendy mirrors (exploring and observing at KS2)

The children can explore the effect on reflections of bending plastic mirrors, observing how the images change and trying to explain how the light is being reflected to make these images appear, A light box could be used to produce thin beams of light in order to trace the reflected paths.

Lenses and prisms (exploring and observing at KS2

The children can investigate a selection of prisms and lenses in the same way as in 'Bendy mirrors'. Sectional convex or concave prisms are easier to use with light boxes than disc-shaped lenses.

Refracting prisms (exploring and explaining)

The children can take a regular shaped lump of glass or perspex with flat faces (they usually come as triangular-based or square based prisms), and look through it, rotate it, they will see images changing position, disappearing and reappearing. Can they tell you where the images are coming from? (More information about prisms is given below.)