How a Converging lens works
The diagram below shows a converging lens. It is called coverging because it brings rays of light together. These lenses are curved so they are fatter in the middle. The line through the centre of the lens at rightangles to it is called the principal axis (or just axis for short). Also shown in the diagram is an object yellow at the top and blue at the bottom.
Click on the left button at the bottom of the diagram to show how yellow light from the top of the oblect fans out to hit the lens. Then click on the right hand button to see how the lens bends the light.
You can see the light starting from A is bent by the lens so that it all passes through a. The curvature of the lens is designed so that this happens. If you move nearer or further from the lens than where a is, then the light is not at a point and so will not be in focus. Next you will see exactly how we know where a is going to be.
Lenses have a property called the focal length. Press the left hand button under the diagram. The focal length is the distance from the centre of the lens to a point where a ray of light parallel to the axis entering the lens crosses the axis on the other side. The lens bends light so that all rays parallel to the axis pass through this same point F. F is called the Principal focus.
The second ray which we can put in is the one through the centre. Because the faces of the lens are effectively parallel here, like a sheet of glass, the light just goes straight on. Press the second button under the diagram to show this.
So you can now see where a is. All the light that passes through the lens from A passes through a.
Next you can put in rays from the bottom of the object. The next diagram shows where we have got to so far. Then click on the left button to show the ray from the bottom that is parallel to the axis. Notice that this must pass through the principal focus F. After that click on the right button to add the ray through the centre.
You can see that the rays from the bottom of the object at B pass through b. So b is the image of B. Click on the third button to add the image.
From this you can see that the inage is upside down and smaller than the object. This is used in a camera and the front objective lenses of telescopes and binoculars. The small image is captured on the small light sensitive chip in the camera. The object is the relatively large subject that you are photographing. Clearly it does not matter that the inage is upside down.
Don't forget that we are only showing the two special rays from A and the two special ones from B. All rays from A that pass through the lens will go through a, as shown in the very first diagram. Similarly all rays from B that go through the lens will pass through b.
There is a huge variety of uses of a converging lens. Different uses depend on what happens when the distance of the object from the lens is changed. This is the subject of the next section. In the meantime think what will happen to the size and position of the image if the focal length of the lens is (a) longer and (b) shorter.
In the next illustration you can see an animation of what happens when the object is moved nearer to the lens. Notice: In all these illustrations we are only showing the two special rays that we know how to draw. The one parallel to the axis passing through the principal focus, and the other through the centre of the lens. Don't forget that all other rays through the lens are bent to cross at the same point - it is just too confusing to show too many rays.
The animation stops when the image is large - just before it gets even larger and goes off to the right. This shows a converging lens being used as a projector. The large image can be seen on a screen. The object (the film, for example) is brightly illuminated by the projector lamp. The image is the opposite way up from the object, so to see the image the correct way up the object must be upside down.
The question now is what happens when the object is brought even closer to the lens?