Convex Lenses

Here is a ray diagram showing how a convex lens produces an INVERTED IMAGE. The lens and cornea of your eye works in just the same way. The image on your retina (the back of the eye) is upside down, but your brain sorts this out.

lens14.tif (26982 bytes)
In this diagram the magnification is times 2. The image is twice as large
as the object.

Follow the 14 diagrams below to draw your own ray diagram for a convex lens. Make sure that you use a sharp HB pencil and a 30cms ruler. It is best to do the drawing on squared paper so that you get the optic axis at right angles to the lens and the parallel rays of light parallel.

You can change the values for the image height and distance and for the focal length, but some values will give you a smaller image than the object and some values will not give an image at all. I hope that you can read my handwriting.
lens01.gif (2199 bytes)1. Here I have drawn the optical axis (a horizontal line).
lens02.gif (2636 bytes)2. Now I have added the position of the lens (a vertical line).
lens03.tif (26982 bytes)3. This shows that it is a convex lens.
lens04.tif (26982 bytes)4. Now I have drawn a vertical line to show the object position.
lens05.tif (26982 bytes)5. I have drawn a burning candle (this is the object).
lens06.tif (26982 bytes)6. I have added a ray of light (a straight line through the centre of the lens).
lens07.gif (3582 bytes)7. Now another ray of light parallel to the optical axis.
lens08.gif (3544 bytes)8. This ray of light is “refracted” through the centre of the lens.
lens09.gif (3881 bytes)9. A ray of light through the focal point will be refracted by the lens.
lens10.gif (4017 bytes)10. That ray is refracted by the lens parallel to the optical axis.
lens11.gif (4047 bytes)11. Arrows have been added to show which way the light rays travel.
lens12.gif (4402 bytes)12. I have now drawn a dotted line to show where the image is formed.
lens13.gif (5295 bytes)13. Here I have drawn the image of the candle over the dotted line.
lens14.tif (26982 bytes)14. Finally I have divided the image size by the obect size to find the magnification.
When you look at something like another person, the image on your retina
is smaller than the object, so the magnification is LESS than one.

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