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A pinhole in a card can act like a magnifying glass, helping your eye focus on an object that is very close to you. However, by limiting the amount of light that reaches your eye from the object, the pinhole also makes the object appear dimmer.
Hold the card near your eye and look at the lightbulb from several feet away. Move closer to the bulb until you almost touch it, and notice that the writing on the bulb appears to be magnified. Use the pinhole magnifier to examine other small brightly lit objects. You can, for example, examine a computer screen or a television screen up close using a pinhole magnifier.
Try using pins or needles with different diameters to make different-sized holes in the foil. Notice that the smaller the pinhole is, the dimmer your view. As the pinhole is made smaller, the image at first becomes sharper, but then is blurred by diffraction.
You can also form a pinhole by bringing together the thumb and index finger of one hand with the thumb and index finger of your other hand as shown.
The pinhole magnifier works on a very simple principle: The closer you get to an object, the bigger it looks to you. This is because the closer you are to the object, the larger the image the object forms on your retina (click to enlarge the diagram below).
Unfortunately, however, there is a limit to this. If you get too close to the object, your eye is not able to bend some of the light rays enough to obtain a focused image. As a result, the image becomes blurry or fuzzy (click to enlarge diagram).
The pinhole magnifier gets around this problem by limiting the part of the eye's cornea and lens through which the light from the viewed object passes. Therefore, the light behaves more like light passing through the pinhole alone, and the eye's cornea and lens don't need to try to focus the image on the retina. The small aperture means the image is focused at any distance (click to enlarge diagram).
Sadly, there is a trade-off between the resolution, or sharpness, of an image and its brightness. A tiny pinhole produces a very sharp image, but, because it cuts down on the number of rays that enter your eye, the pinhole makes the object look much dimmer.
By using a pinhole magnifier, near-sighted people who normally see things fuzzily at a distance will be able to see them clearly; likewise, far-sighted people who normally see things fuzzily close up will be able to see them clearly.
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Attribution: Exploratorium Teacher Institute