Optic Lenses

Today I am going to tell you what lenses are, how they work and many more things. Let's dive in!

A lens is an optical device with at least one curved surface to focus or disperse light beams by means of refraction. Some of you may be wondering why I am talking about lenses. Well, it is because of how common they are, for example lenses are used in microscopes, telescopes, cameras, and even your Granma's reading glasses.

So, now that you know what lenses are I can teach you how they work.  In the previous blog post we saw the light moving from air to water. The surface between the two media was flat but the surface of the lens is curved. We can use Snell's law if we think of a curve as a big series of tiny flat lines one after the other. 

For example in my diagram below I have drawn a plano-convex lens and three light rays.

• First, they go through the flat face of the lens. Snell's law predicts that they will go straight through because the sine of 0 degrees is 0
• The light ray in the middle goes straight through the lens without deviating also because the sine of 0 is 0
• The other two rays exit the lens at a point where the surface is inclined. I have drawn a blue line with the inclination of the lens at that point. This is a special line and is called the tangent. Now, we draw the perpendicular line, which is called normal and we can apply Snell's law. The light is going from a slow medium to a fast one so the light ray turns away from the normal   

The following is a photo of 3 laser rays going through a plano-convex lens from my optics kit. You can see that our prediction was correct

You can see that all 3 rays come together at one point. This is called the "focal point". The focal point is the point at which parallel light rays meet after being refracted. Now you might be thinking but what if the rays are diverging not converging. In that case, we would trace the rays backwards to the point were they meet. This is called the virtual focal point. I have drawn it in the following photo.

You might have noticed that the previous lens has 2 curved surfaces. This is called bi-concave. I used this because I don't have plano-concave in my kit. You can also have bi-convex lenses with 2 curved convex surfaces. They work in a similar way.

Another cool thing in my kit is a plano-convex plastic container. You can experiment by putting different liquids and see how the focal point changes. In these photos I started with plain water, then added 1 spoonful of sugar and then a second. You can see that the focal point gets closer as the water gets denser.