Everyday Physics

How Spectacles Work

We’ve all heard of how we see things around us when light enters our eyes. But why do some people have to wear spectacles to see either distant things or close-by things clearly?

How your eyes work

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Light reflected off the object enters the eye through the pupil and passes through the lens. After passing through the lens, an image of the object is formed which is inverted, i.e. up is switched with down, left with right and vice versa.

This emerging light falls on the back of the eye which has sensors that turn receive the light and produce a signal correlating to it and send it onwards to the brain, where the information is processed.

diagram

Why Some People Need Spectacles

So why do some people need spectacles?

Some people eyes have lenses which are either curved too much or too little, causing the image formed inside to be either too far in front of or too far behind the sensors that receive the light.

In the image above, the diagram labelled Myopia is the condition when the lens is curved too much and the image is formed at a point which is before the location of the sensors. Similarly Hyperopia is where the lens is curved too little and the image is formed beyond the location of sensors.

How Spectacles Work

Spectacles are just an external extra pair of lenses to correct the early or late formation of the object. The images below show how these lenses work.

Correction of hypermetropia by outwards curving lens

 

Correction of myopia by inward curving lens

 

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