Class 7 Science Chapter 11: Light: Shadows and Reflections

A Question on a Night Bus

Watching a moonlit hillside from a bus, it is natural to wonder whether the Moon actually produces its own light. Poems and songs talk about moonlight as though it did. The answer turns out to divide everything you can see into two groups.

The Chief Source

The Sun emits light of its own, and it is where almost all the natural light on the Earth comes from. Everything about daylight begins there, and so, as the chapter will show, does moonlight.

Others That Emit

The Sun is not alone. Stars emit their own light. So does lightning, and so does natural fire. And certain animals emit light of their own: fireflies flash to communicate, which is why a hillside full of them is worth travelling to see.

The Two Names

Objects that make light of their own are called luminous. Those that make none are called non-luminous. Notice the test: does the thing MAKE light, not whether it looks bright.

So What About the Moon?

The Moon is non-luminous. It emits no light of its own at all. It simply reflects the sunlight that happens to land on it. So moonlight is reflected sunlight, and the brightest thing in the night sky makes no light of its own.

Light People Make

In ancient times humans learnt to create fire, which was the earliest form of artificial lighting. In time they learnt to make fire with different fuels, such as wax, oil, gas and animal fat, each one burning longer or steadier than the last.

And Then Electricity

With the invention of electricity and of different kinds of electric lamp, most of the lighting wanted by people are now met by electric lighting. The step from a wax lamp to a switch on a wall is a very short stretch of history.

A Note on LED Lamps

Light Emitting Diode lamps, or LED lamps, use far less power, shine brighter, and last longer than the older lamps. That cuts electricity bills and is better for the environment, which is why the Indian government has worked to promote them across the country. At the end of their life they must be disposed of properly or recycled, and not thrown into the garbage.

Now the Harder Question

A beam from a headlight at night looks as though it goes straight. But looking is not proving. Does light travel in a straight line, and how would you find out?

Three Matchboxes

Take three matchboxes and make a hole through the inner tray of every one, exactly at the same position in all three. Arrange the matchboxes in a straight line, making sure the three holes are at the same height and in line with each other.

Light Through the Holes

Stand a torch on one side so that its lamp sits level with the holes, and put a cardboard screen opposite. Adjust the boxes slightly and you get a bright spot on the screen. Light has gone through all three holes.

The Step That Proves It

Now move just one of the matchboxes slightly to one side, or up or down. Try to find the spot on the screen. It is gone. When the three holes no longer lie in one line, no light arrives.

Reading the Result

Suppose light could curve. Then it could leave the torch, bend a little, and still find its way through a hole that is out of line. It does not. The only path light will take is the straight one, so light travels in a straight line.

A Second Way to Show It

Take a long hollow pipe of some flexible material and line it up so that you can see a candle flame through it. Now bend the pipe and look again. The flame is gone. A lighted candle should only ever be used with an adult present.

Why That Is the Same Proof

A bent pipe still runs from your eye to the flame; what it no longer offers is a straight path. If light could follow the bend, the flame would still be visible. So the two activities say one thing twice.

And a Third, Inside Water

A laser beam can be passed through a beaker of water with a drop of milk added, which makes the beam easy to see. Inside the water the beam follows a straight path. A low-power laser pointer is enough, only a teacher should handle it, and it must never be pointed at anyone eyes, since it can cause serious eye damage.

One Honest Footnote

Light can sometimes bend around corners, which is something for higher grades. For everything in this chapter, and for everything you can see with a torch and a screen, treating light as travelling in straight lines is exactly right.

Scenario:

Two cousins argue in a dark room with one small window. Sunlight comes through the window and falls on a mirror on the wall opposite, and the mirror throws a bright patch onto the ceiling. Rohit says the room has two sources of light, the window and the mirror, since both are clearly giving light out. Asha says there is exactly one source in the whole business, and that neither the window nor the mirror is it. Rohit answers that the ceiling patch is proof, because a thing that gives nothing out cannot light up a ceiling.

Analysis:

Asha is right, and the single source is the Sun. The test for a luminous object is whether it EMITS its own light, not whether light comes away from it, and that distinction is the whole point. The Sun emits its own light and is where almost all the natural light on the Earth comes from. The window emits nothing; it is an opening that lets sunlight through. The mirror emits nothing either; it changes the direction of the light falling on it, which is why a patch appears on the ceiling. Both are non-luminous. Rohit argument would make the Moon a source of light too. And the Moon is the standard example of non-luminous. It emits no light of its own, and only reflects the light of the Sun that reaches it. One more thing is worth noticing in their room. The patch on the ceiling has sharp edges and lies exactly opposite the mirror, which it would not if light wandered. Light travels in a straight line.

Activity:

Find a place at home where sunlight reaches a wall after bouncing off something, such as a tile, a steel plate or a glass tabletop. Draw the path from the window to the object to the wall using straight lines only, and write one sentence saying which object in your drawing is luminous.

Title:

Try it yourself: the three straws

Safety:

Keep the torch away from your eyes and do not look into the beam.

What You Notice:

The spot appears while the three straws are lined up. It disappears as soon as the middle one is moved. The straws still make a route from the torch to the paper. What they no longer make is a straight one.

Fireflies use their light to communicate, and their numbers are falling.

The Moon has never made a single beam of light of its own.

A laser beam in milky water can be seen running dead straight.

Luminous makes its own. Non-luminous only hands light on.

Three holes in a line, and a spot. One out of line, and nothing.

Put Things in the Way

Light travels in a straight line, so something standing in its path has to make a difference. The question is what difference, and the answer depends on what the thing is made of. So collect objects of different materials and a torch.

Setting It Up

In a dark room, switch the torch on and hold it where its light makes a spot on the wall. A cardboard screen will do just as well, with the spot falling on that. Now you have something to watch.

Two Parts, in Order

The activity has two parts, and the order matters. First predict: if you held this object in front of the torch, would you still see the spot? Write the prediction down. Only then put the object in between the torch and the screen and write down what actually happens.

Why Predict First

Because a prediction written afterwards is worthless. Once you have seen the result it is very easy to believe you expected it. Committing in writing first is what makes a surprise visible as a surprise.

Three Kinds of Material

Repeat it for every object and they sort themselves into three groups. Light passes almost completely through transparent materials. Through translucent materials it passes only partly. Through opaque materials it does not pass at all.

Which Is Which

Glass is transparent. Tracing paper is translucent: some light gets through, but you cannot see clearly through it. Cardboard and thick cloth are opaque. You met these three words in Grade 6, sorted by how well you could see through a material.

What Happened on the Screen

Think back to what you saw when an opaque object was in the way. A dark patch appeared. Light goes in a straight line, so an opaque object standing in its path blocks it. The dark area that light never reached is called the shadow.

Shadows Are Not All Alike

The three kinds of material give three kinds of shadow. Opaque objects give the darkest shadows. Translucent ones give lighter shadows, because some light still gets through them. And A few transparent objects cast faint shadows too.

Taking the Set-up Apart

Now the careful part. Collect opaque objects of different shapes and sizes and repeat the set-up, but this time change one thing at a time and watch the shadow. Seven changes are worth making.

Remove One Piece at a Time

Remove the screen. Remove the object. Switch the torch off. In each case the shadow is gone, and each failure is a different failure. That is how the three requirements are found rather than merely stated.

The Three Requirements

So a shadow is formed when an object keeps light from reaching a screen. To observe one we need three things: a light source, an opaque object and a screen. In daily life the screen is whatever is there, since walls, floors, the ground or any other surface serve as one.

Now Move Things Around

Keeping the torch and the screen fixed, move the object closer to the screen, then closer to the torch, then tilt it. The shadow changes each time, in shape, in size and in how sharp its edges are.

What That Tells You

The shape, the size and the sharpness of a shadow depend on where the object stands between the light source and the screen. So two very different-looking shadows can come from one object, and nothing about the object has changed.

A Shadow Can Mislead

Which leads to something worth knowing. A shadow may tell you something about the object, or it may leave you quite unable to guess the object. Tilt a cup and its shadow stops looking like a cup.

The Colour Test

One change in the list is different from the others. Change the colour of the opaque object and look again. The shadow is the same. Recolouring an opaque object does not recolour its shadow.

Why Colour Cannot Travel

That makes sense once you remember what a shadow is. It is not a picture of the object; it is a region where light has not arrived. An absence has no colour to show, so a red ball and a blue ball of the same shape cast shadows you cannot tell apart.

An Art Built on This

Shadow play, or shadow puppetry, has been part of our cultural heritage for centuries. Puppeteers get life-like movement by shifting the light and the flat cut-out figures, called shadow puppets, that stand between a lamp and a screen.

Where You Will Find It

Different regions have their own styles. Maharashtra has Charma Bahuli Natya. Andhra Pradesh has Keelu Bomme and Tholu Bommalata. Karnataka has Togalu Gombeyaata, and Odisha has Ravana Chhaya. Kerala has Tholpavakoothu, and Tamil Nadu has Bommalattam. They are used not only to entertain but also to carry important messages to the community. Every one of them is the three requirements of this topic, put to work.

Scenario:

A school group prepares a shadow puppet show. They cut flat figures from thick card, hang a white cloth as a screen, and set a bright lamp behind the puppets. At the rehearsal the shadows are huge, blurred and hard to recognise. One pupil says the puppets must be badly cut and wants to make them again. Another says they should paint the puppets in bright colours so the audience can tell the characters apart. A third says both ideas are wasted effort and that nothing about the puppets needs changing at all.

Analysis:

The third pupil is right on both counts, and the chapter explains why. Take the colours first. A shadow is not a picture of the object; it is the dark patch where light has not reached, and an absence of light has no colour to show. Changing the colour of an opaque object does not change the colour of its shadow, so a painted puppet casts exactly the shadow a plain one casts and the paint is wasted. Now the blurred, oversized shadows. The shape, the size and the sharpness of a shadow depend on where the object stands between the light source and the screen. Their puppets are too close to the lamp and too far from the cloth, which is the arrangement that makes a shadow large and soft-edged. Moving the puppets nearer the screen, or the lamp further back, will sharpen and shrink them without a single new cut being made. The puppets are fine; the geometry was wrong.

Activity:

Set a cut-out shape between a torch and a wall. Measure the height of its shadow with the shape held halfway, then close to the torch, then close to the wall. Write the three measurements down, and one sentence on which position a puppeteer would choose.

Title:

Try it yourself: sorting by shadow

Safety:

Do not shine the torch into anyone eyes, and handle the glass carefully.

What You Notice:

The tumbler leaves the spot almost untouched and may cast a faint shadow. The tracing paper and thin cloth dim it. The notebook and spoon block it and leave a dark patch. Any prediction that turned out wrong is the most useful line in your table.

Even some transparent objects can cast a faint shadow.

A red ball and a blue ball of the same shape cast shadows you cannot tell apart.

Shadow puppetry has its own regional form in at least six Indian states.

Fully, partially, not at all. Transparent, translucent, opaque.

Source, opaque object, screen. Take any one away and the shadow goes.

An Odd Result From the Last Topic

When the opaque object held in the torch beam was a shiny one, such as a polished steel plate, something extra happened. There was the expected shadow on the screen. But there was also a bright patch of light on the wall opposite.

Two Things at Once

So the plate did two jobs. Being opaque, it blocked the light and left a dark patch behind it. Being shiny, it also sent light off in another direction entirely. Nothing in the last topic explains the second half.

Get a Flat Shiny Surface

To look into it, find a shiny flat steel plate, or a plane mirror. Plane means flat: a plane mirror is one that is flat and not curved. Keeping to flat surfaces matters, because a curved one would change the picture.

Take It Into the Sun

Go outside and let sunlight fall on the shiny surface. Now look for a wall that the sunlight is not falling on directly, and see what you can do to get light onto it.

The Spot on the Wall

Turn the plate or mirror in different directions and a spot of light appears on that wall. The Sun has not moved. The wall has not moved. Yet light is arriving somewhere it was not arriving before.

What Must Have Happened

There is only one explanation. The shiny plate has changed the direction of the light falling on it. Light that was heading for the ground is now heading for a shaded wall.

Tilt It and Watch

Tilt the plate in different ways and watch the spot. It moves. A small turn of the plate sends the spot a long way across the wall. You can steer it where you like, which is worth doing to feel how direct the control is.

Notice What Did Not Change

Through all of this, light is still travelling in straight lines. It comes from the Sun in a straight line, and it goes to the wall in a straight line. What happens at the plate is a change of direction, not a curve.

The Name for It

That is the definition. A shiny surface, or a mirror, alters the direction of whatever light lands on it, and this turn that a mirror gives to the direction of light is called the reflection of light.

A Closer Look Is Needed

Sunlight on a wall is convincing but rough. You cannot see the path, only its two ends. To watch the direction change, you need a single thin beam you can follow across a surface.

Making a Thin Beam

Take a torch and a comb, with a sheet of white paper, a strip of black paper and a plane mirror on a stand. With the black paper, close all the openings of the comb except for one, so that a single thin slit is left.

Laying It Out

Spread the white paper on a table. Stand the comb upright on the sheet and shine the torch at the slit. Nudge the comb and the torch about until a thin beam, through the slit, shows up along the paper.

Why a Comb

A comb is a row of narrow gaps, so blocking all but one is the easiest way to turn a wide torch beam into a single narrow one. And because the comb is held upright on the paper, the beam skims the paper and can be seen along its whole length.

Bring In the Mirror

Now set the mirror into the path of that beam, keeping the comb steady. The beam no longer carries on across the paper. The beam takes a new path once it has fallen on the mirror.

And You Can See Where

Here is what the thin beam adds to the sunlight version. You can see exactly where the change happens. The beam runs straight up to the mirror, and a second straight beam leaves it. The reflection of light occurs at the mirror.

Two Straight Lines Meeting

So the whole path is two straight lines that meet at the mirror surface. That is the picture to carry forward. Nothing curves, nothing bends gradually, and the turn is made at one place and nowhere else.

The Question That Follows

One thing is still unexplained. In a mirror you can also see your own face. Is that reflection too, and if it is, why does it look like a face behind the glass rather than a spot of light? That is the next topic.

Scenario:

A family lives in a house whose inner staircase has no window and is dark all day. The father suggests fitting a second electric lamp. A neighbour suggests painting the stairwell white. The daughter, who has just done this topic at school, suggests fixing a flat shiny steel sheet at an angle on the outside wall where the afternoon sun falls, aimed at the stairwell opening. Her younger brother objects that a steel sheet gives out no light, so it cannot possibly light anything, and that she has confused a mirror with a lamp.

Analysis:

The brother has one fact right and draws the wrong conclusion from it. He is correct that the steel sheet gives out no light of its own: a shiny plate is non-luminous, exactly as the Moon and any mirror are. But lighting a place does not require making light; it requires getting light there. A shiny surface, or a mirror, alters the direction of light landing on it, and that change in direction is reflection. The classroom activity is precisely this situation. A shiny flat steel plate or plane mirror is taken outside. Sunlight is allowed to fall on it. It is then turned until a spot of light appears on a wall the sunlight does not reach directly. Her stairwell is that wall. Two details from the activity matter for fitting it. Tilting the plate moves the spot a long way, so the angle has to be set carefully and fixed. And light travels in straight lines, so the sheet has to be placed where a straight line can run from the Sun to it and another straight line from it into the stairwell opening.

Activity:

Find a sunlit spot outdoors and use any flat shiny thing, such as a steel plate or a phone screen that is switched off, to put a patch of light on a shaded wall. Mark where the patch falls, then tilt the plate by a small amount and mark it again. Write down how far the patch moved for how small a tilt.

Title:

Try it yourself: steering a spot of light

Safety:

Never aim the reflected patch at anybody eyes, and do not look at the Sun.

What You Notice:

The patch appears on a wall the sunlight never reaches. That shows the shiny surface has changed the direction of the light. A tiny tilt moves the patch a surprisingly long way. And the patch always lies in a straight line out from the surface.

A polished steel plate can light a wall the Sun never reaches.

Blocking all but one gap in a comb turns a torch into a single thin beam.

Reflection does not bend light; it turns it, at one place.

Straight in, straight out, and the turn happens at the surface.

Plane means flat. A plane mirror has no curve in it at all.

The Question Left Over

A mirror can throw a spot of light onto a wall, and that is reflection. But in a mirror you can also see your own face. Is that reflection too, and if so, why does it look like a face rather than a patch of light?

Two Words First

Place a pen before a plane mirror and look. It appears as if a similar pen were standing behind the mirror. That apparent pen is the image that the mirror forms of the pen. The pen is the object. Everything that follows uses those two words.

First Check: the Size

Move the pen to different positions in front of the mirror, and at each position compare the size of the image with the size of the pen. They match. The image formed by a plane mirror matches the object in size, wherever the object is placed.

Second Check: Which Way Up

Move the pen about again, and this time watch which end of the image is on top. Is the tip of the pen uppermost each time? It does. An upright image is called erect, and a plane mirror always forms one.

Third Check: the Screen

Now something less obvious. Place a screen vertically behind the mirror and move it about. Does the image show on it? No. Try the same with the screen in front of the mirror. Still nothing.

What the Screen Result Means

So no screen will catch the image a plane mirror forms. That is a strange and important result. The image looks as though it is a definite distance behind the mirror, yet no light actually arrives at that place, which is why a screen put there catches nothing.

Fourth Check: the Distance

stand before a plane mirror and notice how far your image seems to be from it. Now stand close to the mirror. The image is close too. Stand well back, and the image appears far away.

So the Two Move Together

There is a plain relation here. Stand near and the image is near; stand far and the image is far. Walk towards a mirror and the image walks towards you, which is why a mirror never seems to lie about how close you are.

Now Raise an Arm

Stand in front of the mirror and raise your left arm. Which arm does the image lift? Now touch your right ear. Which ear does the image touch? Try it before reading on, because the answer surprises almost everybody the first time.

Left for Right

your left shows up as right in the image, and your right as left. This type of perceived swap of left for right is called lateral inversion, and a plane mirror image always carries that lateral inversion.

Notice the Careful Word

The word perceived is doing work in that sentence. Nothing has physically swapped sides. What has happened is that the image faces you, so the side of the image nearest your left hand is the side you would call its right.

Why Writing Looks Wrong

That is why text held up to a mirror is so hard to read. Every letter and the order of the letters are both reversed left to right. And it is why a word printed normally cannot be read in a mirror at all easily.

The Ambulance Trick

Which explains something you have seen on a road. AMBULANCE is painted on the front of an ambulance reversed, left to right. A driver ahead looks into a rear-view mirror, the mirror reverses it again, and the word reads AMBULANCE the right way round.

Two Reversals Make a Right

That is worth saying plainly. The painter reverses the word once, and the mirror reverses it a second time, so the driver sees it correctly. If the word were painted normally it would be the one person who needs it most who could not read it.

Gathering the Four Facts

So the image formed by a plane mirror matches the object in size, is erect, is laterally inverted, and cannot be caught on a screen. Those four together describe everything the activities found.

Where Mirrors Came From

Nobody knows when mirrors were invented. The earliest were made by polishing stone or metal rather than by silvering glass. When glass mirrors began to be made, the art of making metal mirrors was gradually lost.

One Craft That Survived

Gradually, but not entirely. In Kerala the Aranmula Kannadi, a metal surface mirror of its own kind, has been made for ages and is made still. Every property in this topic belongs to it as much as to the glass mirror on a classroom wall.

Scenario:

A new clinic opens on a busy road and the owner orders a van painted for it. On the front of the van the painter writes CLINIC in plain letters. A word, he argues, should be written the way it is read. The owner daughter says he has it exactly backwards. He should copy what every ambulance on the road does. The painter replies that ambulances are painted that way because somebody long ago made a mistake and nobody has corrected it. Both of them want the van to be readable by the car in front.

Analysis:

The daughter is right, and the ambulance convention is not a mistake. The driver of the car in front does not see the van directly at all. She sees it in her rear-view mirror. And a plane mirror forms an image that is laterally inverted, so left appears right and right appears left. So a word painted in plain letters arrives at her eye reversed. That is exactly why writing held up to a mirror is hard to read. The letters and their order are both reversed. The ambulance trick is to reverse the word first. The painter writes it back to front. The mirror reverses it a second time. Those two reversals leave the driver reading CLINIC the right way round. Notice what is NOT changed by the mirror, since the painter could be forgiven for worrying. The image is the same size as the object, and it is erect. So the word is neither shrunk nor turned upside down. Only left and right are swapped, and only that needs correcting.

Activity:

Write a short word on paper in thick letters and hold it up to a mirror. Then write the same word reversed, letter by letter and in reverse order, and hold that up. Note which version is readable in the mirror, and write one sentence saying what the mirror did to each.

Title:

Try it yourself: four checks in five minutes

Safety:

Prop the mirror so it cannot fall, and handle a glass mirror with care.

What You Notice:

The image stays the same size as the pen however far away it is, and stays erect. The card catches nothing in either position. And the image raises its right arm when you raise your left, which is lateral inversion.

No screen anywhere will catch the image you see in a plane mirror.

AMBULANCE is painted reversed so that a rear-view mirror reads it the right way.

Kerala Aranmula Kannadi is a metal mirror, from a craft older than glass ones.

Same size, erect, no screen, left for right. Four facts, one mirror.

Step back and the image steps back with you, by the same amount.

A Picture With No Mirror

A mirror can show you an image. So can something much simpler. A pinhole camera is an arrangement in which rays of light from an object go in through a tiny hole, called a pinhole, and form an image of the object on a screen. There is no mirror in it and no lens.

Try It With a Candle

Take a piece of cardboard and make a small hole in it. In a dimly lit room, stand the cardboard a short distance from a screen, and set a lighted candle down before it. A lighted candle should only be used with an adult present.

What Appears

Light leaving the flame goes through the hole in the cardboard and makes an image of the flame on the screen. So far that sounds ordinary. Look closely at the image, though, and something is wrong with it.

The Surprise

The candle flame image comes out upside down. That is, it is inverted. A flame that stands upright on the candle appears pointing downward on the screen, and nothing in the set-up was turned over.

Why Upside Down

This follows from the one fact the chapter began with. Light travels in a straight line. Light leaving the TOP of the flame travels straight through the hole and carries on straight, so it must arrive at the BOTTOM of the screen. Light from the bottom of the flame arrives at the top. The picture is turned over because straight lines cross at the hole.

A Camera You Can Carry

Now build one to use outdoors. Find two cardboard boxes, one of which slides inside the other with hardly any gap, and cut one side open on each.

The Hole and the Screen

On the opposite far face of the larger box, make a hole at its centre. In the far face of the smaller box, cut a square out of the middle, its side about five or six centimetres. Cover that opening with a thin translucent paper, such as tracing paper. That is the screen.

Putting It Together

push the smaller box into the larger one so with the tracing-paper side facing in. Now the pinhole is at one end and the screen is inside, facing it, and the distance between them can be changed by sliding.

Using It

Hold the camera so that the pinhole faces the object, and look in at the open side of the smaller box. Put a dark cloth over your head and the camera, so that stray light cannot reach the screen. Then slide the smaller box in or out until an image shows up on the tracing paper.

What to Look At

Point it at a distant object in bright sunlight, such as a tree or a building. Bright sunlight matters, because the only light reaching the screen is what squeezed through a tiny hole, and there is not much of it.

Two Things to Notice

Ask two questions of what you see. Do the images keep the colours of the objects beyond? They do. And are they erect or upside down? They are upside down, exactly as the candle flame was.

A Contrast Worth Holding

Put that beside the last topic. A pinhole camera turns its picture upside down. A mirror, on the other hand, gives an image that is laterally inverted but NOT upside down. Two devices, two different kinds of reversal, and it is easy to muddle them.

Now Build With Mirrors

Having learnt that light goes in straight lines and that mirrors reflect it, it is worth making something useful out of both facts. Two devices follow, one with two mirrors and one with three.

The Periscope

A simple periscope is made by placing two plane mirrors set into a box shaped like a Z. Light from an object enters at the top, is reflected at the first mirror, travels down the box, is reflected at the second, and reaches your eye.

What It Is For

Reflection from two mirrors lets us see objects that are not directly visible. Periscopes serve in submarines, in tanks, and by soldiers inside bunkers. You can use one to look ahead while standing behind taller friends.

The Kaleidoscope

For the second, get three plane mirror strips, rectangular and of equal width and join them in a triangular manner. Three strips of thick reflective paper will do in place of mirrors. Fix them inside a circular tube of thick chart paper.

Filling It

At one end of the tube, fix a transparent plastic sheet with a rubber band or adhesive tape. Lay several broken pieces of coloured bangle or bead upon it, and cover them with a tracing paper, held the same way.

What You See, and Why

Peep through the open side and a beautiful pattern appears. Turn the tube and the pattern changes, and you get a different one every single time. The reason is that with three mirrors there are multiple images, formed by reflections of reflections, so a handful of bangle pieces produces far more shapes than it contains. Designers and artists turn to kaleidoscopes for fresh pattern ideas, and you can even leave both ends open and point it at a tree.

Scenario:

Three friends build a sliding pinhole camera for a science fair. They get the two boxes to slide nicely, cut open one side of each, and cover a square opening with tracing paper. Then they put the pinhole in the SMALLER box and the tracing-paper screen in the larger one, slide them together, and take it outside at noon. They point it at a bright white building, look in without covering their heads, and see nothing at all on the paper. One says the hole must be too small and wants to widen it with a pencil. Another says the tracing paper is the wrong material. The third says they have made two mistakes and neither of them is the hole or the paper.

Analysis:

The third friend is right, and both mistakes are about arrangement rather than materials. First, the boxes are the wrong way round. The small hole belongs in the face of the LARGER box. The square of thin translucent paper belongs in the SMALLER box, which is then slid inside with that side in. With the hole on the inner box, light from the object cannot reach the screen through it at all. Second, they looked in without covering up. The instruction to cover up is not a nicety. The only light that should fall on the screen is the little that squeezed through a tiny hole. Daylight reaching the paper from any other direction will swamp a faint image. Widening the hole with a pencil would make matters worse. It is a PINHOLE camera, and the sharpness depends on the hole being tiny. Set the boxes right and cover up. Then slide the smaller box until the building appears on the paper, in colour and upside down.

Activity:

Make a pinhole in a sheet of card and hold it a hand span from a white wall in a dim room, with a bright lamp or candle flame on the other side. Note which way up the patch of light shows the flame, then move the card closer to the wall and note what changes.

Title:

Try it yourself: a periscope from two mirrors

Safety:

Ask an adult to do the cutting, and use mirror card rather than glass if you can.

What You Notice:

You can see over a wall or a taller friend without moving. Light enters the top window, is reflected at the first mirror, travels straight down the box, is reflected at the second and reaches your eye. That is the periscope of submarines and bunkers, made from a carton.

A pinhole camera has no lens at all, and still forms a picture.

The image in a pinhole camera keeps the colours of the object.

A kaleidoscope shows a different pattern every single time it is turned.

A pinhole turns a picture over. A mirror swaps left for right.

Two mirrors to see round a corner. Three to make a pattern.