How do magnifying glasses work?

Elwin

A magnifying glass is a great thing to bring on a summer evening walk – you can use it to examine bugs, stones, leaves and anything else you see that warrants some close attention.

But how do these devices work? It all depends on the movement of light.

Light travels at different speeds through different substances. It goes slightly slower through glass and plastic than it does through air.

This speed difference isn’t obvious to the human eye. But it does have another noticeable effect: it can change the angle at which light is travelling. This is referred to as refraction, and it happens as long as the light is hitting the glass at an angle other than 90°.

Gif of waves of light moving across a medium
This animation represents waves of light moving across a medium – slowing the light changes its angle. Credit: Ulflund, CC0, https://commons.wikimedia.org/w/index.php?curid=73784342

It seems odd that changing the speed of light should change its angle – it’s because, rather than moving in a straight line, light moves in waves. Part of the wave is slowing down before the rest has done so. Some people compare this to driving a car off the road at an angle – if the left front wheel hits dirt and slows down while the right wheel is still on bitumen, the car will swing to the left.

Three beams of light shone through convex lens, which is bending the light and making the points converge
These three beams of light shone through a convex lens can visually demonstrate how the lens manipulates light: making points converge, rather than diverge. Credit: User Fir0002 on en.wikipedia – http://en.wikipedia.org/wiki/Image:Large_convex_lens.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1247157

A magnifying glass is usually a convex lens (a lens that bulges outwards), made of either glass or plastic. Light hits the glass at an angle, and it gets refracted towards the centre of the lens. Leaving the glass makes it refract even further, meaning, at some point, these rays of light converge together.

If you’ve held up a glass in front of a cool leaf (for instance), the light from the leaf is getting concentrated, rather than dispersed as it would ordinarily. If you put your eye at the right spot in front of the glass, you’ll see a larger image of the leaf.

This is also why, if you hold a magnifying glass above the ground on a sunny day, you may see a bright spot form. Just as with light coming from other objects, the magnifying glass can concentrate light from the sun.

Because sunlight is much stronger than ambient light, this can be quite dangerous – the magnifying glass can concentrate sunlight enough to set things alight. Make sure never to do this near flammable objects, and don’t leave your magnifying glass lying around outside on total fire ban days.


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