Enquiring into sight and light


Benham discs
Bunsen grease-spot photometer
Cylindrical lens
False colours
Fibre optics
An ice lens
Illusions 1
Illusions 2
Illusions 3
Light adaptation
A liquid lens
Measuring refractive index
Pinhole camera
Refracted images
Rumford photometer
A water prism
A zoetrope
Other pages on this site

How to do it

Light adaptation
How long does it take for your eyes to become dark-adapted? Use a standard star as your measure (in the southern hemisphere, try Epsilon in the Southern Cross), and see how long it takes for you to be able to see it after looking at a candle flame, a wood fire, and an electric light, keeping all other conditions the same.

This will help you understand

Pinhole camera
You need a Kodak or similar film cartridge (the dimensions that follow are based on a 126 size cartridge), stiff cardboard, black paint, black tape, a pair of scissors (or knife and cutting board), aluminium foil, a sewing needle, and some rubber bands. Paint the cardboard black, and leave it to dry (or use black cardboard to begin with!). Cut a strip 150 mm long and 32 mm wide. Mark off four sections 36 mm long on this, and score the lines with a sharp knife. Bend the cardboard to form an open box, 36 mm square, and tape together. If you are using a different size film cartridge, vary the dimensions to suit the cartridge: this box has to fit the cartridge neatly.

Cut a second piece of cardboard, 40 mm x 70 mm, with a 10 mm square hole in the centre. Tape a small piece of foil over the square hole, taping it tightly down all round, and use the point of the needle to make a small neat hole in the aluminium. Tape this piece of cardboard to the box, making the whole thing completely light-proof. Now hinge a piece of black paper on the outside, so it hangs down and covers the hole.

Fit the "camera" to the film cartridge: it should be a tight fit. Seal the box to the cartridge with more tape, and use two rubber bands to hold it all together. The film can be wound on with a screw-driver, or with a carved "key" made from an ice-block stick. Cover the pinhole carefully, and only wind on when you are ready to start taking photos. The camera needs to be held still, and is best taped in place for each picture: carry it from place to place wrapped in aluminium foil, or in a light-proof bag. Try exposures of around 1 - 10 seconds with 100 ASA film: different cameras with different size holes will vary considerably, so make notes about the exposure and the brightness of the sun for each shot, so you can evaluate the exposures.

This will help you understand

When something is moving fast in a regular way, it is possible to "stop" the motion by looking at it with a flickering light,

This will help you understand

Making a zoetrope
A zoetrope is a drum, with pictures on the inside. When you look through slits in the spinning drum, you will see the pictures inside, one after another. If the pictures make a sequence, you will see a flickering sort of motion picture. You will need heavy cardboard, some light black cardboard, sticky tape, a very sharp knife, a wood base block, two paper fasteners, and a knitting needle. A strip of thin sheet metal would be useful to make the bracket described below.

Cut a 17.5 cm diameter circle from the heavy cardboard, and drill a hole in the middle of it, large enough to fit the knitting needle. Drill two smaller holes about 2 cm away and on opposite sides, to take the paper fasteners. Make a bracket of cardboard (sheet metal or a strip cut from the side of a PET bottle is better if you can manage it) about 12 cm long, and attach it in place across the centre hole. When the completed drum sits on the knitting needle, it will rotate on this bracket. Cut a strip of black cardboard, 56 cm x 14 cm, and mark lines 2.5 cm and 7.5 cm from one of the long edges. Lay the card on a backing sheet, and draw a series of parallel lines, 4 cm apart, joining the two lines already drawn. Then using the knife, cut a series of slits along these lines, each slit being 4 mm wide.

Put a strip of sticky tape along the edge of the black cardboard, so that half of it sticks out beyond the edge, and cut a series of nicks in the tape. Then fit the black cardboard to the heavy cardboard circle, with the bracket on the outside, and tape it in place to make a drum. Fit the knitting needle to the base, point uppermost, and sit the drum on the needle, like an upside down bucket: the knitting needle goes through the base, and the point pushes against the bracket. (It helps toput a dimple in the bracket for the needle point.) Now make up a strip of figures to go into the drum, each 4 cm from the next, and about 5 cm high. Join these to make a continuous strip, with the figures facing inwards, and stick them in the drum. Now spin the drum, and look through one slit. The zoetrope works better if you can only see through one slit, so arrange a couple of pieces of cardboard to restrict people's view.

This will help you understand

The Bunsen grease-spot photometer
This photometer was once used to compare the intensities of two light sources. It is just a piece of brown paper with a greasy spot on it. When the light on the far side is brighter than the near-side light, the grease spot looks lighter than the surrounding paper.

When the light on the far side is less intense, the spot appears dark, but if the two light sources are balanced, the spot disappears altogether. The photometer is dead easy to make: use ordinary brown paper and butter or margarine, or even the paper which comes around your fish and chips.

If you do not know what the inverse square law is, now would be a good time to look into it.

Here are some more ideas

The Rumford shadow photometer
The Rumford photometer relies on the rather more subjective comparison of two shadows, cast by two lights onto a screen.

Once again, you need to keep in mind the inverse square law and do a bit of calculation before you can find a simple comparison of the power of two lights.

This will help you understand

A water prism
You will need a flat dish such as baking dish, some water, a flat mirror,and access to sunlight inside a room. Half fill the dish with water, and put the mirror in the water on an angle. If the sun is now allowed to shine on the under-water part of the mirror, and reflect back up, it passes through a triangular prism of water, and a spectrum of sorts can be seen.

You can improve the separation by tilting the screen on which the spectrum falls, and also by using a thin slit of light. Two slits in line are even better, as only parallel rays can pass through, but the crudity of the prism probably does not justify this.

This will help you understand

Why it is hard to spear fish
Light is refracted, bent by water. The standard explanation of physics is that light passes more slowly through water than through air, and the "bending" is a side-effect of this slowing down.

When you put a pencil or some other straight object into a glass like this, you will see this sort of bending, but the same bending has a practical importance for anybody trying to spear fish from a rock, a river bank or a stream.

In the interests of the fish, I shall not reveal the secret, but it comes down to the problem that you are aiming a spear which is in the air at a fish which is in the water.

This will give you a bit more of a hint

Estimating the refractive index of water
The apparent depth of water is reduced by refraction in the same way as light is bent. If you can get a deep glass container such as a large measuring cylinder, a fish tank or a vase, and drop a coin into it, you will be able to estimate the refractive index of water.

Set up the equipment like this, and then look down into the water while you move your hand down until you think your finger (outside the container) is level with the coin inside the container.

If you measure the real depth and the apparent depth, the refractive index is just the real depth divided by the apparent depth.

This will help you understand

A liquid lens
The diagram tells you what you need here: mainly a large piece of curved glass, a "clock" glass to chemists, plus a stand of some sort, an assortment of clear liquids, a light source, and a screen.

Set the apparatus up like this, and add one of the liquids to the clock glass, forming what scientists call a plano-convex lens (one side is flat, a plane, and the other is convex). Then move the screen until the light source is focused again.

If you are clever enough, you may be able to measure the distance from the lens to the screen, and relate a set of distances to the refractive indices of the different liquids.

This will help you understand

An ice lens
Take a round bowl, add a small amount of hot water, then seal a piece of cling wrap across the top, raising one edge to let air escape if the plastic bulges up. As the air inside cools, the plastic will bulge inwards, making a nice mould for a plano-convex lens of ice.

All you have to do is pour some boiled water onto the plastic, let it cool, add some more water, and then put the bowl and the water in a freezer.

This will help you understand

Cylindrical lenses
You will need a long clear cylindrical glass jar with a water-tight lid, or a perspex or glass rod. Fill the jar with water and seal it: use the jar both upright and sideways to look at things and people. You can do the same thing with perspex or glass rods.

This will help you understand

Color out of nothing
This is a small and simple device which will make you wonder just what colour is. These are Benham discs, and when you spin one, your eyes will often perceive colour. Using a pair of compasses and heavy paper, make discs like this, about 10 cm across. If you have an old-fashioned record player (the sort that plays vinyl records), you can try spinning the disc on that, or you can mount it on cardboard, fit a small bolt through the exact centre, and spin the disc in a hand drill. When Mr. Benham first invented his discs, more than a hundred years ago, he put these sorts of pattern on the top of a toy top, so that may give you another hint about how to spin it. Spinning the disc in the opposite direction can reverse some of the colours, andd there are other interesting effects to find as well.

This will help you understand

Illusions 1: the arrows
This is a simple illusion which can be used as the basis for a science project. The two horizontal lines that you see here are the same length, but one appears longer than the other. The black line in this drawing is not part of the illusion: it will be explained later.

If you set up one of these as a static drawing, the second arrow can be made as a slide, so that you can get people to try to match the two lines. The front (sliding) piece is the left side of the lower arrow, the black line is a slit through which the slide travels, and the slit sheet has the top arrow and the right side of the lower arrow drawn on it.

The idea is to get people to slide the lower arrow in and out until they feel that the two lines are of equal length. As a general rule, they will pull the lower arrow out until the lower line is longer: record the values for each subject, and maybe a few other variables like their sex and their age, and calculate a mean and standard deviation for each sub-group

You might find it worthwhile seeing what effect it has if you change the angle of the arrow head, keeping everything else constant. There are some more hints here

Illusions 2: how many prongs?
This is another well-known illusion, which sometimes looks like two prongs, or sometimes like three. As a basis for a science project, you could ask people to say how many prongs there are, and see how long it takes for them to realise that there is a conflict.

The version that you see here has some added shading to make the "two" side look more convincing: what effect would it have if you took that shading away?

This may help

Illusions 3: how straight?
The two horizontal lines are completely straight, as you will see when you redraw this diagram for yourself, yet they look to be curved outwards. I have been playing with the idea of one of these, made with black wire, where people can bend the thicker horizontal wires until they "lines" look straight, but I have yet to work out how to measure the bending easily.

Maybe the answer is to have a wheel with a dial and a thread that pulls on the wire? It's your project . . .

This will help you understand

Water fibre optics
Make two small holes in the screw cap of a glass jar, on opposite sides of the lid. Use masking (or gaffer or ducting) tape to attach the bottom of jar to a torch (a flashlight in the USA), so you can shine light through the jar.

Then cover the sides of the jar so light can only get out though the holes in the lid: test the jar in a dark room to make sure this works. Fill the jar with water, screw the cap on tightly, and tip the jar over a sink. If you turn the jar slightly to one side, water will begin to pour from the lower hole as air passes in through the other hole. Get this right, then turn out the lights, turn on the torch, and pour.

This will help you

And now for some help

Light adaptation
Our eyes slowly get more used to the dark, but you will get different results on the night of a full moon, in a city, or just after sunset. Epsilon crucis , the smallest star in the Southern Cross, is the extra star in the Southern Cross on the Australian flag, the one not seen on the New Zealand flag.

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Pinhole camera
Credit where credit is due: I found part of this idea on a Web site, belonging to the Exploratorium in San Francisco. The Exploratorium runs the Science Learning Network , which actually created the page I was looking at.

The main things is to make your camera extremely light-proof, keep it very still, and record every shot, so you know exactly what you did.

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Cylindrical lenses
Try looking at things close up to the lens and things further away, and you will find a point at which the right-way-up image flips over, and appears upside down. Is this related to the diameter of the "lens" you are using?

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The zoetrope
This is an example of persistence of vision, where your brain is fooled into seeing continuous motion from a series of separate images. Why has our brain evolved to perceive flickers as continuous motion?

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Water prism
The light passes through what is effectively a triangular prism made of water. Separation will again be improved if the beam is parallel, but this experiment is fairly robust in the face of sloppy technique.

By the way, if you are looking for the "seven colours of the rainbow", forget it! There are really millions of colours in the rainbow, and our belief that there are seven colours is just a cultural interpretation. Job of Edessa and Aristotle said there were just three colours, date-red, green and yellow, according to Job. Later Aristotelians saw four (unspecified so far as I can discover) colours -- matching the four elements, qualities, humours and seasons. Abu Ali al-Husain ibn Abdallah ibn Sina (Avicenna) went for three -- again I cannot track down what they were.

Roger Bacon favoured five for the simple reason that it distinguished more, but again they are not named: my bet is that he counted red, orange, yellow, green and blue as his five. Theodoric of Freiberg was still faced with a claim for three colours in the 1300s when he asked why, if humans do not have three teeth or three eyes, why should the rainbow have thre colours? No, said Theodoric, the number is four: red, yellow, green and purple. Theodoric also noted that there were many examples which always showed the same colours in the same order. (He is also called Dietrich in German or Thierry in French.) Back to the enquiry | Back to the index

The grease spot photometer

No jokes about greased lightning, though - even I would not sink to that level. You can use this photometer to calibrate the sun's brightness in the early morning or the late afternoon. You could even use it to test when official daylight starts: in the 18th century Royal Navy, daylight was by tradition the time during which you could "see a grey goose at a mile".

Islamic tradition distinguished daytime from night time during Ramadan, the month of fasting, by holding two threads at arm's length, to see if they can be distinguished by the available natural light -- from memory, the threads are black and white. Could a "standard candle" be used to assess the change from day to night in a similar way?

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The Rumford photometer
A photometer like this could be used to compare the strengths of candles and dips made from oil of different quality, or to assess the effect of reflectors, placed behind a flame to increase its apparent brightness.

Go on -- you do some work for a bit!

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Fish spearing and refraction
The problem is that the spear is in the air, while the fish is in the water. If the fish was also in the air, you would have no problems. If you were underwater, using a speargun, there would be no problems. Think about it, realising that there are hints there for you to find, and you may work out what you have to do to hit your fish every time.

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Measuring the refractive index
OK, we got a bit technical there, didn't we? The refractive index of a clear substance is a number which tells you how much the subsyance slows down light or bends light. The easiest way to measure this is to look at the angle of a light ray coming to a surface (the incident ray), and the angle of that same ray after it passes through the surface (the refracted ray).

These angles are measured against a perpendicular line to the surface, right where the ray hits, but there does not seem to be any mathematical relationship between the two angles, until we look at a mathematical measure called the sine of each angle. But we can avoid even that bit of mathematics if we draw a figure like the one on the right, where the red line with the arrow on it represents a light ray. In this sort of setup, if you compare the lengths of the two purple lines (if you have a monochrome viewing screen, these are the horizontal lines in the figure). As you change the angle of the incident ray, so you change the angle of the refracted ray. Curiously, if you shine a light ray in the opposite direction, it goes back along exactly the same path.

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The liquid lens
You can use almost any clear liquid, so long as it is non-toxic. You will need to stop and think about flammability with most organic liquids, especially if the light is hot. Think about trying a few solutions, and maybe even honey -- I haven't tried this myself, yet.

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The ice lens

The plastic film is pushed in by air pressure as the air inside contracts (you can also just push the plastic down and seal it to get the same effect). Boiled water has less dissolved air in it, so the ice is clear and freer of bubbles than ice from ordinary tap water would be.

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Benham's disc and the false colours

The first account of the disc was a brief and anonymous note in Nature in 1894. It described the disc as a black semi-circle, with a white half divided in four, and with black arcs on it. As the disc turns, people see different colours from the different black arcs. Soon after Benham revealed that if you shine a bright sodium flame on the disc, you will see a very clear blue, and a very clear red. Other people said they could not see this at all. The "official" explanation now says we have three kinds of light receptor in our eyes, in the same way there are three kinds of phosphor in a colour TV. Speaking crudely, these receptors, the cone cells, are all sensitive to just one of red, green and blue. According to the theory, you need all three kinds of cone in the retina of your eye to see colours normally. Somehow, the cones which pick up one of the colours (red, for example) must react differently to flashing lights of a particular frequency. So with different size black bits on the disc, we get different frequency effects, and so our eyes are stimulated to "see" different colours. Well, that's what the theory says, but nothing seems to explain the alleged effects of sodium light.


Some reports said different rotation speeds were needed for different people to see the same effects. Explore this claim, and see what you can discover. There are other patterns for Benham discs, and some of them are better than others. Can you invent a better one?

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How the strobe works

The trick is to keep the wheel spinning at exactly the same speed, which won't be easy. The last time I tried this, I had some scrap sheet lead, so I used scissors to cut a lead disc, and glued it to the back of the carboard with contact adhesive (you can also staple it if you have a heavy stapler).

Then I was able to spin the wheel up, using a hand drill, and know that it would keep its speed much longer, because it had much greater angular momentum.

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The illusory arrows
The measurement will be easier if you set the test apparatus up vertically, and observe from behind as people arrange the arrow. Then you can put a scale of some sort on the back, so that you can read the length off without having to get out a ruler or measuring tape.

It may also be worthwhile noting down whether people have ever seen this illusion before or not. But most importantly, if you are going to try this sort of thing, you will need some expert advice on how to do the statistical analysis. You will probably find differences in the means and standard deviations for groups, but you have to apply tests of significance , and see if the differences are likely to be just chance variations or not.
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How many prongs?
The other factor which affects this illusion is its length. Maybe you could study how long it takes people to notice something is wrong with different lengths of the same illusion.

Once again, it is important to get some good advice on statistical significance, and how you measure it.

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Curved lines

Once again, it is important to get some good advice on statistical significance, and how you measure it.

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Water fibre optics
The thin continuous stream of water captures the light within the stream because the light is coltrolled by total internal reflection. The stream of water will not be visible from the side and you will only be able to see the light when the stream breaks up or hits something.

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This file is http://www.ozemail.com.au/~macinnis/scifun/slight.htm, first created on August 20, 1997. Last recorded revision (well I get lazy and forget sometimes!) was on August 28, 2000.
Worried about copyright? You need to go look at my fine print . Well, maybe you don't after you read the next paragraph, but do it anyhow . . .and to see some more ideas, look at the start of that same page
©The author of this work is Peter Macinnis -- macinnis@ozemail.com.au , who asserts his sole right to the product as it is packaged here, recognising that many of the ideas are common. Any non-profit educational or home use is completely acceptable without let or hindrance. Copies of this whole file or site may be made and stored or printed for personal or educational use. The work used here derives from on-going research and development which will one day lead to a book on brain food ideas.

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