Lenses

​Do you need a magnifying glass or glasses perhaps? There's nothing wrong with having glasses, is there? Did you know that the first glasses were glass balls like this one that you looked through? ​ ​ Does that work like glasses? ​ ​What? ​It works like your eyes. Glass lenses do the same thing as these two transparent balls, and as your eyes. ​ ​Ew. ​ ​Yes. A 1000 years ago monks sat at their monasteries and wrote.

When they got old and couldn't see as well, they might have used hemispherical pieces of glass, like these. Reading stones. They developed into the first glasses, the ones you put on your nose, about 700 years ago, and glasses work exactly the same way as your eyes. Here is an eye with good vision. The rays of light pass through the eye's opening, the pupil.

Inside the pupil is a lens. When the light passes through the lens, it changes direction a little. It refracts. The rays of light are gathered together and focused to a point on the retina at the back of the eye. From there, electrical signals travel to the brain, and it's only then that we think that we see something.

If the pupil isn't stretched enough, the rays of light are gathered too much, and they're focused at a point before they reach the retina. This eye is short sighted. It sees well at close distances. In this case, we need glasses. So we put the glasses in front of the eyes, an extra lens made of glass.

This lens is thinner in the middle and thicker at the edges. It's a concave lens. A concave lens spreads out the light, so it is a diverging lens. So we choose a diverging lens that spreads the light out just enough to correct the way the eye's lens gathers together the rays of light just a little too much. We add a little divergence to correct how the eye gathers the light together too much.

Together the two lenses bend the light just enough for it to be focused right on the retina. Now the eye can see well again. As you get older, the opposite problem often affects the eyes. The lens doesn't bend the light enough. In this case the rays of light reach the retina too soon before they are focused.

This eye is far sighted and it sees better at long distances than at short ones. Because the far sighted eye's lens doesn't gather the rays of light together enough, we need a lens in the glasses that helps to gather the rays a little more. We need a converging lens. A converging lens bulges out and is thickest in the middle. It's convex.

Together the glasses converging lens and the eye's lens bend the light exactly enough until the image becomes sharp. An ordinary magnifying glass is a converging lens where both sides are clearly convex. It works by changing the direction of the light as it passes through material of a different optical density. The lens is hit by rays of light that are all traveling in the same direction. They're parallel.

The light rays bend inside the lens and meet at a specific point. The light is concentrated here and if it's sunlight that hits the lens it can get so hot that it starts to burn. The point where the rays meet is called the focal point. The distance between the lens and to the focal point is the focal length. ​ ​If we switch from a convex to a concave lens, we spread the rays of light instead. In this case, the light is not focused on a point on the other side.

But we can extend imaginary lines back through the lens and see where they meet. This point is also called the focal point. The distance between a diverging lens and it's focal point is also called the focal length but it has a negative value. It is the distance with a minus sign in front. If you are short sighted and wear glasses, you might have noticed that minus in the strength of your glasses.

The strength of glasses is given in diopters. And you get the diopter number by dividing 1 by the focal length. If the focal point of your glasses is 2 metres from the diverging lens, then their diopter number is 1/-2 = -0.5 If you don't have glasses, you can get a feeling about them by thinking of something sad because tears lying on the eyes help to bend the light. And if you're lucky, you'll get just the right refraction that you need. I'm telling you it's good to have glasses.