The magnifying glass: qualitative
Simple demonstration of how the magnifying glass works.
Apparatus and materials
For each student
Spherical lens +7D
Health & Safety and Technical notes
If students wear spectacles for reading it is best to keep them on for this experiment.
a Students can be given these instructions:
Hold the lens very close to your eye with one hand. Hold the thumb of your other hand at the right place for looking at it with this magnifying glass. Move the thumb nearer and then farther away until you see it clearly in focus. Then remove the lens and see whether you can still see your thumb.
b Next you should say:
Now put the magnifying glass back. You can see your thumb comfortably and it looks large. You are looking at an image of your thumb. Where must that image be?
1 The positive lens acts as a magnifying glass.
This is a simple observation of the image of an object placed between the lens and its focal plane. The image must be on the same side of the lens as the object. With normal vision it must be between 25 cm and infinity in front of you, and so is further away than the object. This is a virtual image. The light only appears to have come from the image to your eye. You cannot catch a virtual image on a piece of paper.
2 When the lens is removed, your thumb is still there, but it looks fuzzy. It is too close for you to see it in focus, because the eye is unable to form an image of it on the retina. It is much closer than your eye's near-point of comfortable seeing. You cannot change the lens inside your eye enough to focus it.
The magnifying lens allows you to bring the object nearer to your eye, and so the image appears to be bigger than the object. If the image is three times as far away as the object is from the lens, then the image will be three times as tall.
This experiment was safety-checked in January 2007