Image formation with a lens
A set of experiments to introduce real and virtual images formed by a convex lens.
Apparatus and materials
Lens (+7D or 150 mm focal length)
Sheet of plain white paper
Greaseproof paper, small pieces
Health & Safety and Technical notes
If the Sun is visible from the laboratory windows, it is essential for the teacher to remind students that looking at the Sun through a lens will cause blindness.
In the following description, the source of light is referred to as a window. In some cases an electric lamp may be better, as it can be used in a room that is at least half dark. A carbon filament lamp and mounted lampholder are suitable.
a Lenses forming real images
Face away from the window, holding a sheet of paper at arm's length. Hold the lens in front of the paper and move it to and from the paper (towards you and back towards the paper) until you see an image of the window on the paper.
b Real image with and without screen
Hold a lens at arm's length towards the window. Hold a piece of greaseproof paper in the other hand and find the place between the lens and your head where there is a clear image of the window on the paper. You are looking at that image through the paper. Your eyes should be focused on the paper itself.
Remove the paper and look at the image in space. If you cannot find the image put the paper back and repeat the process. It may be helpful to catch half the image on the edge of the paper and the other half in space. Concentrate on the image on the paper and slide the paper away.
c Looking at the real image of a brightly illuminated face
Darken the room and illuminate the face of a student brilliantly with floodlights. Form an image of the model's face and notice how the picture moves farther from the lens and grows in size as you move the lens nearer to the model.
Try this with a piece of thin paper to catch the image, and without it.
d Quick rough estimate of focal length
Hold the lens so that it forms an image of a distant window or lamp on a sheet of paper or a wall and measure (roughly) the distance from lens to image.
e Virtual image
Hold the lens close to your eye and use it as a magnifying glass to look at your thumb.
1 In part a this works best if the laboratory is fairly dark and a blind is opened onto a bright window so that a bright image is produced. (If the Sun is visible from the laboratory windows, it is essential for you to remind students that looking at the Sun through a lens will cause blindness.) The brighter the image, the more colourful it will look. This always surprises some students, before they even notice that the image is upside down. Begin with a +7D lens and then use lenses of different powers. Note the distance between the lens and the screen.
2 Many students will need help to do part b, but the time spent helping them will be worth it. Remind them that they will not see an object clearly when it is very close to their face: they must be a considerable distance from it. If they cannot find the image, put the paper back and repeat the process. Give help by suggesting to them that the image is located in space, just where the paper was. To see it, their eye must be focused at that place.
A real image can be caught on a screen, but the screen isn't necessary for its production. This is just like an image on the film in a camera; the image is upside down and smaller than the object.
You might introduce this to students by asking questions:
What will happen if you take the greaseproof paper away? Will the image still be there?
Remind students not to put their eye at the greaseproof paper. Their eyes should be at least a distance from the paper, as if they were viewing a digital camera.
You could ask the students:
When you face the window and look through the lens, the image appears to be on the lens. But is it?
When you catch the image on greaseproof paper and then move the paper away, sideways, you show that the image is in space between the lens and your eye. The screen is not necessary.
3 In part c, as the lens is moved towards the object being viewed, the image moves back and grows in size.
You might summarize this for students like this:
You see a thing by receiving rays which come straight to your eye from each point on that thing. A lens bends the rays that come from a bright point, and makes all of them pass through another bright point which we call the image. You see that image by receiving rays coming from it to your eye.
That is what a camera lens does. It makes an image of the thing you want to photograph, and the photographic film or CCD element is put just at the image position.
The image sensor in a digital camera is a 'charge-coupled device' but it is not appropriate to explain this term.
4 The focal length of a lens is one way of classifying lenses. The focal length is the distance between the focal point and the lens when viewing a distant object (infinity). The focal length is a constant for the lens.
The power of the lens is 1/f. (1/f gives the change in wave curvature imprinted on any spherical wave by a lens.) The stronger the lens, the shorter is the focal length.
Convex lenses have a positive power, and concave lenses a negative power.
(The word, 'focus', comes from the Greek word meaning 'hearth'. When a lens forms an image of the Sun, then it will burn a piece of paper placed at the focus.)
5 All beginners find the idea of a virtual image more difficult than that of a real image, see part e. Some are helped by being shown a plane mirror forming a virtual image. This is not always as helpful as teachers hope since, to a beginner, the plane mirror acts in quite a different way from a transparent lens. So offer a plane mirror demonstration only as a tentative help to those who are having great difficulty.
You might use the following instruction.
"Hold the lens close to your eye, with your thumb at the right place for looking at it. Whip the lens away and see whether you can see your thumb. Without the lens your thumb is too close for you to look at and see comfortably at that distance.
Now put the lens back. You can see your thumb comfortably and it looks big. Where must the image be? What is the range of places for objects that you can see comfortably? It's from about 25 cm in front of you to right out away at 'infinity'. Where must that image of your thumb be when you can see it comfortably with this magnifying glass? The image you are looking at must be out in front of you, like anything else your eyes can see comfortably. It must be on the same side of the lens as your thumb, but further away.
We call that a virtual image, one that the rays of light seem to come from, but don't actually pass through. You cannot catch that image on a piece of paper."
6 This set of experiments reveals some facts about convex lenses and the links between object and image distance.
- When the object moves towards the lens, the image moves away from the lens on the other side of the lens.
- When the object is at infinity, the image is at the focus.
- When the object is at the focus, then the image is at infinity.
- When the object is between the focus and the lens, then the image becomes virtual and returns behind the lens on the same side as the object.
- The magnification changes and increases as the object distance is reduced.
- When the object is at 2f then the image is at 2f on the other side of the lens and there is no magnification.
This experiment was safety-checked in January 2007