Visible Light, Grade 8
Nothing is visible until light from it reaches your eye.
That sentence is the whole topic, and it is the question a Grade 8 class is almost never asked. A learner can name every light source in a photograph and still have no way to explain how they see the grass.
And once light does reach the eye, something has to focus it. The eye diagram most learners are handed labels the cornea, the pupil, the retina and the optic nerve, and leaves out the part that does the focusing. This page puts it back.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 8 |
| Term | 3 |
| Strand | Energy and change |
| Topic | Visible light |
| Status | Informal activities. Nothing in this topic is prescribed for formal assessment |
| Marks | 50 on our worksheet. The topic revision section is 25 |
Luminous objects, and everything else
A luminous object makes its own light. An illuminated object only sends light back.
| Luminous | Illuminated |
|---|---|
| The Sun, a flame, a bulb, a screen | Everything else in the room |
The Moon is the one that catches a class out. It is bright, it is in the sky, and it makes no light of its own at all.
So how do you see a wooden desk? Light from a lamp or a window falls on it, the desk reflects some of that light, and some of the reflected light reaches your eye. Three steps, and a learner who cannot draw those three arrows has not understood the topic yet.
Light travels in straight lines
Every diagram in this topic rests on it. A ray is drawn as a straight line with an arrow on it, and it does not bend unless it meets a new material.
Shadows are the proof. If light could bend round a box there would be no shadow behind it.
The speed of light
300 000 km per second, or 3 x 108 m per second.
Sunlight takes 8 minutes 20 seconds to reach us. Check it: 150 000 000 km divided by 300 000 km per second is 500 seconds, and 500 seconds is 8 minutes 20 seconds.
You may meet a question asking how far away a star is if its light takes 20 minutes to arrive. The arithmetic works and the astronomy does not. The Sun is 8,3 light minutes away and the next nearest star is over four light years away, so no star is 20 light minutes from here. Ask it about a planet instead: Mars is between roughly 3 and 22 light minutes away depending on where the two planets are, so the numbers are real. That is how our worksheet asks it.
Practical 1: build a pinhole camera
This costs nothing. A shoebox, a pin and a piece of tissue paper.
- Paint or line the inside of the box black. Light bouncing around inside is what washes the image out
- Make one small hole with a pin in the centre of one end
- Cut a window in the other end and tape tissue paper over it. That is your screen
- Put the lid on, stand with your back to a bright window and point the pinhole at something bright
You will see a dim, sharp, upside down image on the tissue.
| Change this | What happens | Why |
|---|---|---|
| A bigger hole | Brighter and blurrier | More light gets in, but from more directions at once |
| A smaller hole | Dimmer and sharper | Each point on the object maps to one point on the screen |
| Move closer to the object | A larger image | The rays spread more inside the box |
Why it is upside down. A ray from the top of the object passes through the hole and carries straight on downwards, landing at the bottom of the screen. A ray from the bottom carries on upwards. Straight lines cannot bend to keep the picture the right way up.
Build this before you teach the eye. The eye does the same thing, and the brain is what turns the result over.
The spectrum of visible light
White light is all the colours together. A prism separates them because each colour bends by a different amount.
| Colour | Wavelength | Frequency | Bent |
|---|---|---|---|
| Violet | Shortest | Highest | Most |
| Indigo, blue, green, yellow, orange | In between | In between | In between |
| Red | Longest | Lowest | Least |
That is also why you cannot state a frequency for white light. White is a mixture, not a colour, and each colour in it has its own frequency.
The seven spectral colours are not the primary colours
You will see violet, indigo, blue, green, yellow, orange and red called the primary colours of light, sometimes twice in the same unit, and then called "the seven colours that make up white light" shortly afterwards. Both cannot be right.
| What they are | How many | |
|---|---|---|
| Spectral colours | What white light separates into in a prism | Seven |
| Primary colours of light | The colours that mix to give white | Three: red, green and blue |
Why it matters more than a name. The topic goes on to ask what happens when seven torches of different colours shine on the same spot. The answer is white, and that only makes sense once a learner knows that mixing light adds up. A class taught that all seven are primary colours has been given no way to understand it.
And red, yellow and blue are the primary colours for paint, not for light. The two sets get swapped constantly.
Practical 2: dispersion through a prism
You need a bright lamp or a phone torch, a piece of card with a 2 mm slit cut in it, a triangular prism from our acrylic prism and lens set, and a sheet of white card.
- Stand the slit card in front of the lamp so a narrow beam comes through
- Stand the prism in the beam on white paper
- Turn it slowly until a band of colour appears on card held an arm's length away
- Write down the order of the colours, and which end has moved furthest from the original beam
Violet ends up furthest, red least. If nothing appears, the slit is too wide or the room is too bright: 2 mm and the darkest corner you have.
A rainbow is the same thing in a raindrop, three times over. Light bends going into the drop, reflects off the inside of the back of it, and bends again coming out. The two bends separate the colours and the reflection is what sends the light back towards you, which is why your back is always to the Sun when you see one.
Practical 3: shadows, and the one measurement in this topic
Free, and it is the only activity here that produces numbers instead of adjectives.
- Stand a shoebox upright on a large sheet of paper, with a bright lamp 100 cm away
- Trace round the shadow and measure it
- Calculate the area in square centimetres
- Move the lamp to 50 cm and repeat. Then 25 cm
| Lamp distance | Shadow | Area |
|---|---|---|
| 100 cm | Close to the size of the box | Smallest |
| 50 cm | Noticeably larger | Larger |
| 25 cm | Much larger, softer edges | Largest |
Calculating the area is what makes this an investigation. A class that writes "bigger" has made an observation.
Why it happens: light spreads out from a small source, so the closer the object sits to it the wider the cone of light it blocks, and the larger the shadow it throws.
Opaque, transparent and translucent
| Word | What it does | Example |
|---|---|---|
| Opaque | Blocks light. Absorbs or reflects it | A brick wall, foil |
| Transparent | Passes light clearly | Window glass |
| Translucent | Passes light but scatters it, so no image | A frosted bathroom window |
A window does two things at once. Most of the light goes through, which is why you can see out, and a small part reflects off the glass, which is the faint image of yourself you see at night. It is easier to see after dark because there is no bright light coming the other way to swamp it.
Why an object has the colour it has
A material has colour because it absorbs some colours and reflects the rest.
| In white light | Absorbs | Reflects | Looks |
|---|---|---|---|
| A red wall | Everything except red | Red | Red |
| A white page | Almost nothing | Everything | White |
| A black jersey | Most of everything | A little of everything | Black |
You will meet "all black objects absorb light, so no light will travel from that object to your eye", followed within a line or two by "we can see black objects because they reflect some light". The second one is right. A black object absorbs most of the light and reflects a little, and if it reflected nothing at all you could not see it. Something that reflects no light is not black, it is invisible.
You will also meet "a mirror absorbs no light". A household mirror absorbs roughly 5 to 10 % of the light that falls on it. Stand two mirrors facing each other and the reflections fade away to black instead of going on forever. That fade is the absorption.
Put a yellow flower under blue light only and it looks almost black. There is no yellow light in the room for it to reflect, and it absorbs the blue that is there. Ask a class to predict it before you explain it: most of them say it stays yellow.
Mixing light is not mixing paint
This is the best thing in the topic and it is almost never named.
| Start with | Add more | End with | |
|---|---|---|---|
| Light, adding up | Darkness | Each colour adds to the total | White |
| Paint, taking away | White paper | Each pigment absorbs more colours | Muddy brown, then near black |
Two questions sit next to each other in most classroom material: shine seven coloured torches on one white spot, and mix seven paints of the same colours. The results are opposite, and the reason is never given a name.
- Light is additive. Red, green and blue light together give white
- Paint is subtractive. Each pigment removes more colours from the white light falling on it, so more paints leave less light
A hand driven colour disc settles it in five seconds. Spin the seven-colour disc and it goes off-white. Predict the result out loud first, and say that off-white rather than pure white is correct, because printed inks are never pure colours.
Measuring the angle from the normal, and the 35 degree trap
Every angle in this topic is measured from the normal, and this is where marks are lost.
The normal is a line drawn at 90 degrees to the surface, at the point where the ray hits it. The rule is that the angle the ray leaves at equals the angle it arrived at, both measured from the normal, and that both rays and the normal lie in the same plane.
THE TRAP. A diagram in circulation marks 35 degrees between the incident ray and the mirror surface, then asks for the angle the ray reflects at.
35 degrees from the mirror is 55 degrees from the normal. So the answer is 55 degrees, and a learner who writes 35 has read the picture exactly as it is drawn and is marked wrong.
Draw the normal in yourself, on the board, before the class attempts anything like it. It is worksheet question 5.1.
For the measured practical with a protractor, see our mirror practical for the senior grades, which runs it properly.
Practical 4: a mirror, flat foil and crumpled foil
Free. A mirror, a smooth piece of tin foil, and the same piece crumpled and flattened out again.
| Surface | Can you see your face? | What the light does |
|---|---|---|
| Mirror | Yes, clearly | Every ray leaves in the same direction |
| Smooth foil | Poorly | Mostly one direction, some scatter |
| Crumpled foil | No, but it is just as bright | Scattered in every direction |
All three obey exactly the same rule at every point on their surface. What differs is the surface: on crumpled foil each tiny facet faces a different way, so the rays leave in all directions and no image can form. A learner who says the foil "does not obey the rule" has the observation right and the physics wrong.
It is also why you cannot see your face in white paper, which is as bright as a mirror and scatters everything.
Bending light: a glass block and a prism are different things
They get called the same thing, and they do not behave the same way.
| Shape | What the ray does | |
|---|---|---|
| Glass block | Rectangular | Bends going in, bends back coming out, and leaves parallel to how it arrived, shifted sideways |
| Prism | Triangular | The faces are not parallel, so the ray leaves in a new direction and the colours separate |
Going into the glass the ray bends towards the normal. Coming out, away from it. Dashing in the path the ray would have taken with no glass there is the clearest way to show it.
The bent pencil in a glass of water is the same effect. Light from the submerged part bends as it leaves the water, so it appears to come from somewhere it does not.
The full ray-tracing practical belongs to the senior grades. Our glass block practical runs it with pins and a protractor. Do not spend a Grade 8 period on it. Show the effect and move on.
Convex and concave lenses
| Lens | Shape | Parallel rays |
|---|---|---|
| Convex | Thicker in the middle | Brought together at a focus |
| Concave | Thinner in the middle | Spread apart |
A curved surface obeys the same rule as a flat one. The normal is drawn to the tangent at the point the ray hits, and because the tangent points a different way at every point on the curve, each ray bends by a different amount. That is what brings them to a focus.
A magnifying glass is a convex lens a learner can hold, and focusing sunlight to a bright point on paper shows a focal point better than any diagram. Outdoors, on paper on the ground, with the teacher holding the lens.
Convex lenses are in every camera, every pair of reading glasses and every microscope, which is where this topic goes next.
The parts of the eye, including the one that gets left out
| Part | What it does |
|---|---|
| Cornea | The clear front window, and it does most of the focusing. Not just a protective covering |
| Pupil | The hole the light passes through |
| Iris | The coloured ring that changes the size of the pupil |
| Lens | Fine-focuses the image, and changes shape to focus near or far |
| Retina | The light-sensitive layer at the back, where the image forms |
| Optic nerve | Carries the signal to the brain |
Many eye diagrams label the cornea, the pupil, the retina and the optic nerve, describe the cornea as a protective covering, and show no lens at all.
Nothing in that version explains how the image is focused. The cornea and the lens do it together: the cornea provides most of the bending, and the lens changes shape to fine-focus, which is exactly what happens when you look up from this page at something across the room.
A learner who has only seen the four-label diagram has no answer available when asked how the eye focuses.
The image on the retina is upside down, for the same reason the pinhole camera image is, and the brain turns it the right way up.
Practical 5: the hole in your hand
One sheet of A4 per learner, thirty seconds, and every learner can do it at their desk.
- Roll the A4 into a tube and hold it to your right eye
- Hold your left hand, palm towards you, against the far side of the tube, touching it
- Look with both eyes open
There is a hole straight through your hand. The right eye is being shown the view down the tube and the left eye the palm, and the brain combines the two.
It belongs in this topic because it is the cleanest evidence that what you see is assembled by the brain, not delivered whole by the eye.
Why is the sky blue?
Air scatters short wavelengths far more strongly than long ones.
Blue light bounces off air molecules in all directions on its way down, so it arrives at your eye from the whole sky rather than only from the direction of the Sun. Red light mostly carries straight on past.
Violet is scattered even more than blue, and the sky still looks blue, for two reasons: the Sun sends out less violet than blue, and the eye is much more sensitive to blue than to violet.
At sunset the light has to travel through far more air to reach you, so almost all the blue has been scattered out sideways before it arrives, and what is left is red and orange.
Safety
- A laser is a teacher's tool at the front of the room, or it is not used at all. Never hand one to a learner, never at or near eye level, never aimed at a person or at a shiny surface at head height
- Never look at the Sun, and never through a lens, a prism or a pinhole. Point the pinhole camera at a bright window or a lit scene
- Focusing sunlight belongs outdoors, on paper on the ground, with the teacher holding the lens. It will set paper alight, which is the point, and it burns skin just as well
- Check glass prisms and lenses for chipped edges as they go out and come back
- Lamps get hot. Let them cool before they are packed away
Published material suggests a laser pointer for the mirror work and gives no safety instruction with it at all. There is no reason a Grade 8 class needs one. A lamp behind a 2 mm slit in card does everything a laser does here, at a fraction of the risk and a fraction of the price.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| The pinhole image is a grey blur | Hole too big, or light leaking in | Smaller hole, tape the seams, black inside |
| No pinhole image at all | Not enough light on the subject | Back to a window, box pointed at something bright |
| No colours from the prism | The slit is too wide | 2 mm, and hold the card an arm's length away |
| Colours, but very pale | Room too bright | Curtains, or the darkest corner |
| The shadow will not trace | Lamp too far away or too diffuse | One small bright source. A phone torch beats a ceiling light |
| The class answers 35 degrees | They read the angle off the mirror | Draw the normal in first, every time |
| The colour disc goes grey, not white | Printed inks are never pure | Off-white is the correct result. Say so before you spin it |
| Nobody sees the hole in the hand | Hand not touching the tube, or one eye shut | Both eyes open, hand against the tube |
How the 50 marks are made up
| Part | Marks |
|---|---|
| Seeing, sources and straight lines | 10 |
| The spectrum and dispersion | 10 |
| Shadows, opaque and transparent | 8 |
| Colour, absorption and mixing | 10 |
| Mirrors, glass blocks and lenses | 8 |
| The eye | 4 |
Nothing in this topic is prescribed for formal assessment. The topic revision section is 25. Our worksheet is 50 and the split above is ours.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 50 marks, including the 35 degree question, the shadow area table and the planet version of the speed of light sum
- Marking memorandum, with what to accept for each answer and the eight answers that look right and score nothing
Related practicals
- Measuring reflection with a mirror and a protractor. The senior version of the mirror work on this page, done as a measurement
- Tracing a ray through a glass block. Where the bending on this page is measured properly, with pins and a protractor
- Electric circuits, Grade 8. The Term 3 neighbour, and a bulb turns electrical energy into light
- Static electricity, Grade 8. The other Term 3 topic that needs almost no apparatus
What you need to run it, and what we cannot supply
Four of the five practicals on this page cost nothing. The pinhole camera needs a shoebox, a pin and tissue paper. The shadow measurement needs a lamp and a sheet of paper. The foil practical needs a mirror and tin foil. The hole in the hand needs one sheet of A4.
We will be straight with you about the rest. There is no class set for this topic in our range at the moment.
| What a class set needs | Where we stand |
|---|---|
| Plane mirrors, six of them | We do not stock a plane mirror line at all |
| Ray boxes | Out of stock, every model |
| Single convex lenses | Out of stock. Our concave lenses are in, which is the wrong way round for teaching |
| Torches and colour filters | Not stocked. A phone torch and a slit in card do the job |
All four are being sourced. Until then, here is what we can actually send you, and it is a demonstration set for the front of the room rather than a set per group:
| Item | Price | What it is for |
|---|---|---|
| Acrylic prisms and lenses, 6 pieces | R760 | Dispersion, and a convex and concave lens in the same box |
| Hand driven colour disc | R500 | Mixing light. Nothing else in the range demonstrates it |
| Glass lenses, set of six in a case | R500 | Convex and concave together, in glass |
| Magnifying glass, 50 mm | R35 | A convex lens a learner can hold |
| Crookes radiometer | R250 | Optional. Light carries energy, and it spins to prove it |
If you buy one thing off this page, buy the colour disc. Mixing light is the single thing this topic explains worst, and a spinning disc settles it in five seconds for a class of thirty.
For a learner building a project at home, the laser alarm kit at R66 has the mirrors and the light sensor in one packet, and it is a reflection project that ends with something that actually works.
The rest of the range is in light and optics.