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Formation of Images
Light, Sound and Waves
Pinhole camera and lens camera
for 14-16
The camera is one optical instrument that all students will encounter, even if it is only in their mobile phone! But students are still amazed when they see the image they can produce in a pinhole camera. It is in colour!
Class practical
Constructing a pinhole camera and investigating the action of a lens.
Apparatus and Materials
- For each student or group of students
- Pinhole camera box (15 cm x 10 cm x 10 cm)
- Black paper
- Frosted glass or greaseproof paper
- Spherical lens, +7D or 150 mm focal length. (The focal length should be equal to the length of the box)
- Carbon filament lamps 200 W (up to 8 students may share each lamp)
- Mounted lamp holders (safety pattern)
- Pin for making holes (dress-making rather than optical)
Health & Safety and Technical Notes
The mounted lamp holders should have:
- a double-insulated two-core flex with a 13 A mains plug fused at 3 A;
- a cable anchorage within the mounting;
- a safety-pattern lamp holder which disconnects the pins when the bulb is removed.
Read our standard health & safety guidance
The pinhole camera kits available from equipment suppliers will contain either dismantled card boxes painted black on the inside, or square-section black plastic tubes, with some, or all, of the other items listed above. There are also a number of web pages describing how to make pinhole cameras, although these assume that you will want to use photographic film for a permanent record rather than a translucent screen.
The lenses suggested are designed for a camera approximately 150 mm from front to back. If other sized boxes are used, the lens provided should have a focal length equal to the length of the box.
If the box has a removable lid, students can hold a lens inside, as well as outside the box when investigating the lens camera. Each box should have a square hole cut in it at one end, covered with frosted glass or greaseproof paper, which scatters light over a very small angle. There should be a round hole at the opposite end. This is a very cheap, simple pinhole camera: students can construct their own.
Carbon filament lamps are rated at 200 V. They will last longer if a 200 V supply is available.
Cut a piece of black paper to size. Fasten it over the 5 cm open hole at the end of the box. Glue or Sellotape can be used for this. You may prefer to wrap paper round the end and secure it with an elastic band.
Procedure
- Place the lamps and lamp holders around the laboratory so that up to eight students can work with their cameras about 1.5 metres from a lamp. The laboratory can get congested. Placing the lamps high can alleviate this.
- Pull down the blinds, or otherwise shade the room. Ask each student to do the following.
- Make a small pinhole in the black paper, then point the pinhole end of the camera toward the lamp. Look at the screen while moving the box closer to or farther from the lamp.
- Enlarge the pinhole and repeat the observation.
- Add several more small pinholes and repeat the observation.
- Pepper the whole sheet with pinholes and repeat the observation.
- Give each student a lens and ask them to slide it in front of the pepper of pinholes whilst the box is pointed at the lamp. You may need to tell students to move nearer to the lamp and farther from it and see what happens. They will soon find the position for a single brilliant image.
- Push a pencil, then a finger, through the pepper of pinholes. At each stage, experiment with using the lens in front of the pinholes.
- Try the effect of moving the lens a little away from the camera. Examine the effect when the box is further away from, and when nearer to, the light source.
- You can also have the lens nearer the screen, by holding it inside the box. This is easily done if the box has a removable lid and is held upside down with one hand, while the lens is held inside it from below with the other hand.
- Paste a new piece of black paper on the front of the camera. Make a large pinhole in this paper and then repeat step 8. You are now using a lens camera with a small aperture and students can observe the greater range of focus.
- Finally pull up the blinds so that students can use their
lens cameras
to look at the view through the window.
Teaching Notes
- Try this out first yourself in the laboratory in which you are going to do it. You need to judge:
- How dark the laboratory needs to be
- Where to put the lamps
- How far apart the lamp and the camera with lens need to be to collect the multiple pictures together
- How to get students to navigate around the benches in semi-darkness.
- Good planning will bring out the
WOW
factor when students see their first picture and experience the delight when the lens collects the multiple pictures together. - It is helpful to get students to look at the same effects together. Work through each stage of the experiment, and discuss what they have seen before moving on to the next effect. Doing this helps to avoid the continuous replacing of the black paper in which the pinholes are made. Once they have worked through it together, students should be free to repeat the experiment on their own.
- Students might need to be warned to hold the screen at
book length
rather than up to their eye. Those used to the LCD screens of modern digital cameras will find this familiar. - Using one pinhole, students should notice that the picture is inverted, and that however much the distance between the lamp and the camera varies, the picture is always in focus; the depth of field is infinite. A slightly bigger pinhole will produce a brighter picture but it may be fuzzier. Eventually students will want to make bigger and bigger holes, but introduce them to the lens effect first. Otherwise, a new piece of paper will need to be fixed to the box.
- If possible, find out beforehand the distance from the lamp to the camera which will make the lens bring all the pinhole pictures in stage 8 together in one bright sharp image. Then arrange students round the lamps in circles of that radius so that the experiment succeeds at once and they enjoy the startling effect.
- The lens placed in front of the pepper-of-pinholes obviously collects up a fan of
rays
(each of which was proceeding to a separate picture on the screen) and bends them all to travel to a single image. - Assuming that these
rays
(really thick 'pinhole-passing' bundles of rays) travel straight in air before and after the lens, you are forced to think that this is what a lens does. It takes a fan of rays from an object point (a source of light) and bends all of them, so that after passing through the lens they all go straight to an image point. - The drawback to using a lens in place of a pinhole is that the camera will only collect the rays together for one particular distance of the camera from the lamp. The depth of field is less for a lens camera than for a pinhole. With one big hole, the depth of field decreases, but the intensity, of what is now called an image, increases. To alleviate this, the lens has to be moved away from the camera itself, to produce a focused image of objects at different distances.
- A wavefronts explanation: an alternative way of thinking about lens action is to consider what happens to wavefronts. A lens changes their curvature.
- Here are some points to emphasize in using the pinhole camera.
- A camera
sees
things in much the same way as the eye does. - Objects emit light in all directions, only some of which enter the camera.
- A small pinhole transmits what can be thought of as a ray of light from the object onto a screen.
- Large apertures produce brighter images but shorter depths of field.
- Students can easily make a pinhole camera at home. It is useful for observing solar eclipses. (Usual warnings about not observing the Sun directly.)
- How Science Works Extension: A pinhole camera can be used to determine the height of an object which it is difficult to measure directly – for example, a flagpole or a tree. Place the camera on the ground, facing the object. Measure the height of the image on the screen, and the distances from the pinhole to the screen and to the foot of the object. The calculation of height is a simple matter of ratios.
- Students should consider how to improve this experiment. Is it better to be close to the object, or farther away? Multiple measurements from different distances will help to reveal how precise the technique is. How long should the camera be? A longer camera will give a bigger image, and this will reduce the error in the result. Have some large cardboard boxes available, together with black cloth drapes which the students can work beneath.
This experiment was safety-checked in January 2007
Resources
Download the support sheet / student worksheet for this practical.
Up next
Making a photo to take home
Class practical
Producing a permanent photographic image.
Apparatus and Materials
- Red or orange safelights
- Floodlamp (preferably with photoflood bulb) For each student or pair of students...
- Pinhole camera
- Black paper
- Pin
- Lens (+7D or 150 mm focal length)
- Bromide paper, many pieces
- Paper clips or hair clips, 2
- Goggles For every 8 cameras...
- Photographic dishes (for developer and hypo), 2
- Dish of water (or sink)
- Developer
- Hypo or fixer
- Large tank or sink for washing
- Newspaper or blotter to dry prints
- Tongs
Health & Safety and Technical Notes
Eye protection (goggles) must be worn for this activity, as the developer is significantly alkaline. The paper must be handled with plastic tongs. Anyone with sensitive skin should wear gloves.
Read our standard health & safety guidance
Procedure
-
Using a pinhole camera
- In a darkened room, load the camera with a piece of normal bromide paper, clipped on the inside of the back window. It is essential to obscure the translucent window at the back of the camera box with opaque paper or metal, otherwise stray light will enter though the window and fog the picture. Make a pinhole and enlarge it with a pencil point to 1 or 2 mm diameter. Cover the pinhole with a finger. Rest the camera on a table and point it at the head and shoulders of a 'subject' sitting about 2 metres away. Direct a bright floodlamp at the subject, and switch it on for 30 seconds or longer with the pinhole open.
- Remove the bromide paper, soak it in developer; watch it and wash it when you see the picture; dip it in hypo; give it a long wash, if possible in a sink with running water. The print can be (partially) dried on newspaper. When fully dry, it is ready to take home.
- Using a lens camera
- Follow the same procedure as above with a fresh piece of bromide paper, but make a hole in the camera's front with a finger. Hold the lens against the hole. (You could use Sellotape.) Turn on the floodlamp and uncover the lens for one or two tenths of a second.
Teaching Notes
- If possible, each student should have their own camera; or at most work in pairs. Anyone who has seen his or her first photographic print emerge in a developing dish will never forget it. But, be warned: get students to write their name on the back of the paper because the best photograph will belong to all of them!
- A darkroom is not necessary if photographic paper is being used (though the image will be a negative). Sheet film can also be used, but a dark room will be needed, or skill with a film-changing bag. For those who understand f-numbers, you can draw up a table of apertures and exposure times for a given film speed. Superb photographic displays can be produced from this simple camera.
- The camera obscura, a relative of the pinhole camera, has a long history of use in the art world and as a curiosity showing the surrounding landscape.
This experiment was safety-tested in January 2007
Up next
Comparing short cameras and long cameras
Class practical
Comparing the curvature and shape of long and short focus lenses.
Apparatus and Materials
For each student
- Plano-convex or biconvex lens + 7 D or 150 mm focal length approx
- Plano-convex or biconvex lens + 2.5 D or 400 mm focal length approx
- Cleaning tissue
- Bath of water and detergent
- Wall, paper or card to form a screen
Health & Safety and Technical Notes
Read our standard health & safety guidance
Soap and water are good for cleaning the lenses. Alternatively, use a soft cloth for cleaning spectacle lenses.
Procedure
- Feel the lens with your fingers, to judge the curvature, and then clean it with the optics cloth or soap and water.
- Hold the + 7 D lens in your fingers and catch the image of a distant window (or lamp) on a wall or a sheet of paper. Repeat with the + 2.5 D lens.
Teaching Notes
- You might ask students:
- "Which lens makes the larger image? Which lens needs a longer camera? What kind of lens would you expect to find in a tiny camera used by a secret agent?"
- The weaker lens produces the larger image but also needs a longer camera.
This experiment was safety-tested in January 2007
Up next
From pinhole camera to lens camera
With a camera, rays of light come straight from each point on a bright filament, brightly lighted face, or whatever the object is that you are photographing.
The picture on the back of a pinhole camera is made by those rays which go straight through the pinhole. The front wall of the camera stops all other rays.
The sketch shows rays of light from just two specimen points on a lamp filament contributing to the picture at the back. Each point on an object provides rays for a little spot in the picture at the back, a spot slightly larger than the pinhole. With a small enough pinhole, you get a fairly sharp picture.
With a large pinhole, you no longer get a point-for-point copy of the object. You get a patch-for-point copy, rather a fuzzy picture.
When there are several pinholes, each lets through rays of light from every part of the object. So each pinhole leads to a whole picture of the object.
Look at the diagram and imagine what the lens does.
For light starting from a single point, the lens seems to collect up the rays that go through different pinholes. It bends them so that they all run together through a point. That point is called the image.
To know what the lens really does, you must let a lens receive many rays of light and see how it deals with them.
Up next
Teaching ray optics
At introductory level, simple experiments can help students to realize that light travels in straight lines and that an object is seen when light from the object enters the eye. A lens bends light rays so that the rays pass through an image point and we think we see the object at that point.
Treated as open-ended experiments they show students the way in which light behaves with real lenses in optical instruments.
Photograph courtesty of Jim Jardine
Most of the experiments described on this website are suitable for intermediate level courses. After completing them, students should be able to draw a diagram of light rays (not formal ray construction diagrams) showing the following.
- Rays travel out from an object point in all directions, going fainter as they go farther.
- All rays from a remote object point pass through an image point.
- Rays from a remote object point which pass through a lens and proceed to a real image point after the lens, continue straight on through that point.
- Rays from an object point which pass through a lens forming a virtual image emerge along lines that appear to come straight from the image point.
- Every ray aimed at a central point in a lens (called the optical centre) passes through undeviated.
The real behaviour of rays falls short of the ideal of passing through images exactly. Students will see this and learn a little about correcting for that aberration
.
The ray optics equipment suggested in these experiments looks simple, but some practical skill is needed to get the best out of it. Teaching notes provided with each experiment will help you ask the right questions of students struggling to get results.
You will be better prepared for student questions if you try out the experiments carefully beforehand. It is also advisable to read traditional textbooks that go beyond what students need to know for examination purposes. For example, knowing that the minimum distance between object and image is four times the focal length of a converging lens will enable a teacher to choose a lens that suits the length of a demonstration bench.
A well-organized cafeteria of equipment
, under teacher control, will encourage students to do their own experimenting. In this way, extension work for faster students can be encouraged.
At intermediate and advanced level, ripple tanks can be brought in when needed, to show reflection or refraction for example. Wave theory predicts that all parts of a wavefront starting from a small light source arrive in phase at the image. This requires all paths from the object to take the same time.
Up next
The longitudinal lens formula and sign conventions
Light, Sound and Waves
The longitudinal lens formula and sign conventions
Teaching Guidance for 14-16
The simple lens formula for thin lenses is included in some advanced level physics courses, though it is rarely used by contemporary optical designers. It provides a source of examination questions and a wrangle about sign conventions. Conventionally, u is the distance from lens to object, v is the distance from lens to image, and f is the focal length of the lens.
If treated lightly, the formula
can be put to good use as an encouragement to students to practise placing virtual images. Nearly everyone uses an optical instrument sometimes. In most optical instruments (telescope, microscope, magnifying glass, spectrometer) the observer looks at a virtual image.
The important thing in the argument about conventions is to choose one convention and stick to it. Advantages and disadvantages of the two common conventions are discussed below.
The Cartesian convention
The Cartesian convention emphasizes the point of view which looks at a lens as changing the curvature of wave fronts going through it. One reason to do this is that it makes good sense of the reciprocal quantities in:
1/ v =1/ u +1/ f
Thinking of a lens as adding curvature, the natural formulation is:
curvature after = curvature before + curvature added
This convention also expresses the fact that the effective power of two thin lenses in contact is found by adding their powers.
In both conventions, diverging lenses have negative powers and converging lenses have positive powers. In the Cartesian system, it is advisable not to restrict the unit dioptre to lens powers alone, but extend it to 1/ v and 1/ u . Indeed, this must be correct if the equation is to have consistent units. Opticians always measure lens powers in dioptres, and so the unit itself is more than respectable.
It is better to express the lens equation in the form above rather than as:
1/ v – 1/ u = 1/ f
where it may be less easy to recall which term is subtracted (though reading this as 'change in curvature = curvature provided by the lens' is quite natural).
In the Cartesian convention, distances to the right are positive and distances to the left are negative, just the same as for cartesian graphs. For a converging lens forming a real image, u is negative and v is positive.
The real is positive
convention
1/ u + 1/ v = 1/ f
In the real is positive
, the symbols are taken at face value and the fact that these reciprocals are related receives no attention. The sign is taken as positive for a real object or image distance, and negative for a virtual object or image distance.
Advantages: The merit of this convention is that it makes the lens equation simple and easy to remember. Double negatives, which can confuse students, do not arise in as many cases as with the Cartesian system.
Disadvantages: The real is positive
sign convention is not used at all in professional work in optics, nor in ray tracing software which is readily available, and it obscures what is going on underneath. Some students get confused between the sign of (-1/ u ) and the sign of u itself.
Thanks to Dave Martindale for pointing out an error on this page, now corrected. Editor
Up next
Dissectible model eye
A model eye and a sectional drawing of the human eye are useful to show students, and links with the biology department can usefully be made when you are teaching how cameras work.
The eye and the camera have a lot, structurally, in common.
- The eye pupil is just a hole which lets the light in.
- The iris is an adjustable diaphragm which controls the amount of light allowed into the eye.
- The retina is a fine network of nerves which are sensitive to light. Most of the adjustment of the eye, between bright sunlight and complete darkness, is done by changes to the sensitivity in the retina so that the range in sensitivity may vary over a million times by storing up light sensitive chemicals for use in low light levels.
- A camera is painted black inside; so is the eye, to lessen trouble due to stray light being reflected.
- Most cameras have a flat film or CCD plate at the back. Some cheap ones hold the film on a curved back to allow for curvature of the image field of their simple lens. The eye has a curved back.
The camera lens must be moved in and out to focus the image of objects at different distances. The eye changes the power of its crystalline lens system by means of ciliary muscles which change the curvature of the lens. The power of an eye differs from one person to another. In round terms, the bending of the cornea is about 40D and the total power may be 60D or more.
The most important thing about the lens is that it can be squeezed to change its power. In an average eye
the lens can increase by 4D when it is pulled into greater power to focus something nearby. That change for focusing objects at different distances is called accommodation.
Bear in mind that eyes do not have air in them. The materials inside an eye differ from one another in refractive index. The aqueous humour between the cornea and the lens is salty water that carries chemicals to nourish the cornea. It also presses the cornea outwards to keep it fully rounded. The vitreous humour is a less dense, clear, watery jelly which helps to keep the eyeball fully rounded.
The main refraction of light by the eye is at the front surface, the cornea, hence the success of contact lenses. Since most astigmatism is due to unequal curvature of the cornea, a spherical contact lens can remove
astigmatism.
Three parts of the retina are worthy of note;
- There is an interesting patch on the retina where all the nerves of the retina are bundled together into the optic nerve to go to the brain. There are no nerve endings there so it is a blind spot. That gives no trouble in seeing because our eyes scan all the time so you never notice your eye missing anything which falls on the blind spot for an instant. It is easy to show that the blind spot is really there by staring fixedly at X with their right eye, closing the left eye...
- ...while bringing the page nearer. The spot disappears when its image falls on the blind spot. The blind spot is some way from the yellow spot for best seeing(see 3, below.)
- The retina is fed by blood vessels which are just in front of the nerves so that light forming an image goes through them before reaching the nerves. Some people observe a difference in colour due to that red filter when they lie on one side on a sunlit lawn and compare the hues they see with the upper and lower eyes.
- The human eye has a small patch of retina where there are no blood vessels in front. That is the patch used for accurate seeing, as in reading, and is called the fovea or yellow spot. You can learn about the size of it by staring at a book with one eye open and the other closed and estimate the longest word which can be read without moving the book or your eye. However, the eye has a power of 60D or so, the real yellow spot is less than the length of the word. It is at least 20 times smaller and probably less still as it is difficult to keep your eye still when looking at the word. Measurements show it is less than a 1/4 mm wide.
A cheap, fixed-focus camera has such a small aperture that, even if an object is out of focus, the cone of rays from the lens to each image point is so narrow that it makes only a small blur patch if the film catches it too soon or too late. The eye pupil closes down in bright sunlight, thus giving some depth of field. In an emergency, a person who has lost his or her spectacles can read the telephone directory by putting a card with a pinhole in front of the eye and moving closer to the page.