How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Question 417

When you take an eye test at the doctor's office, they use many lenses to find your prescription. Are these lenses cut differently so that to your eye, some objects (on the Snellen chart) look like virtual image and others are real?
The lenses that they place over your eye create virtual images that are closer or farther from your eyes than the object itself. Some lenses are converging (bending rays of light together) and these "magnifying" lenses form virtual images that are located farther away from you than the object itself. If you are farsighted (seeing distant objects well) you will be able to see a nearby object well through such glasses because you will see that object as more distant. Other lenses are diverging (bending rays of light apart) and these "demagnifying" lenses form virtual images that are located nearer to you than the object itself. If you are nearsighted (seeing nearby objects well) you will be able to see a distant object well through such glasses because you will see that object as nearer. If you vision is particularly poor, the lenses you need may not form virtual images at all but will still correct your vision. In that case, you do better to think of these lenses as joining together with the lens of your eye to form a single, image-forming lens. That combined lens works to form a real image on your retina. The other complication with eyeglasses is cylindrical correction (correction for astigmatism). Some people have lenses in their eyes that are not symmetrical and focus light differently up and down or left and right. A water glass is a cylindrical lens, focusing light horizontally but not vertically. To compensate for this cylindrical character, some eyeglasses have the opposite cylindrical character cut into them and rotated into the proper position.
         

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