How Lenses Work

We all know about refraction. Light changes speed and maybe direction as it goes from (say) air to glass. If the glass is a particular curved shape, we can determine the direction of the refracted rays and get them to meet at a point.

Put properly, a converging or convex lens is a 3D segment of a sphere, thicker in the middle than at the ends. Light near the edge gets refracted a lot, near the middle it hardly changes direction at all. Here’s what happens…


Here, on the left side, the light rays are parallel, so they’re coming from an object a very long way away. The lens is refracting the rays to meet at a point. The distance from the centre of the lens to this focal point is called the focal length.  Fat lenses have short focal lengths, skinny ones have longer focal lengths. The ray that goes right through the centre of the lens doesn’t change direction because it strikes the lens NORMALLY (at 90 degrees to the surface). It’s called the PRINCIPAL AXIS

Words to remember:-

focal length, principal axis, focal point.

TASK 1. Measuring the focal length of a converging lens by holding it a long way from a window and seeing the real, inverted, smaller image produced on the wall. Real images are formed by real rays of light, which is why we can project them on to a wall or a screen and they’re the same colour as the object forming them. 

Check this simulation out. You can change the thickness of the lens and the object position. Look for real and virtual images – the simulation can show both.

Geometric Optics



We can draw diagrams to show what is happening when the object is at a particular distance from the lens. Start with one like this.  The object is more than 2F away from the lens.  Draw two rays, one parallel to the PA, the other through the centre. The image is where they meet.

Task 2. Using a convex lens as a magnifying glass.

Hold your lens close to some writing. Notice what happens as you move the lens a little further from the page. 


We can draw a similar diagram with two rays as before to show what’s happening. Notice that the two rays diverge (get further apart). If we want to find the image we  have to draw dotted lines backwards. Where they apparently cross is where the light appears to come from. This is the image position. This time, the image is virtual, magnified and the right way up.  Writing looks bigger, we’ve created a magnifying glass. Now go back to the simulation and get it to show a magnifying glass for you.


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