I intend to show how the position and nature of the image can be worked out using ray diagrams - I will use various formulas to aid me in figuring out the focal lengths.

There are three different types of convex lens, depending on the curvature of each surface:

When drawing ray diagrams, any two of the following rays are needed to fix the image position and size.

Refraction:

If you think of when you look at a straw stuck into a glass of water then you remember how the straw seems to break where it reaches the surface. This bending effect is called refraction. The diagram below shows how a ray of light passes through a glass block. The line at right angles to the block is called a normal. The ray is refracted towards the normal when it enters the block and away from it when it leaves the block. The ray emerges parallel to its original direction (provided that the block has parallel sides).

Prediction

        

        

I think that object one will have a very long focal length as the lines are going to have time to spread out before they reach the lens where they will be bent and so the gradient of the lines will be much less steep than if the object were closer to the lens. With object two the rays would have had less time to spread out and would therefore converge at an earlier point. Objects four and five will not appear on the far side of the lens as they will be going towards the lens at such at angle that they will be reflected by the lens and will therefore appear as virtual images on the same side as the object. I think that object one will be real, and inverted. Object two and object three will appear in the same way, real, and inverted. Objects four and five will appear virtual, the right way up and magnified.

Method

        

For my experiment I am going to set up the apparatus as shown:

The light filament in the bulb is what I am going to try and get focused. I am going to use five different focal lengths for the distance between the light filament and the lens. I intend to measure each different length three times so that I can get a reasonable average. As as soon as I have set the filament at zero on the ruler I will then move the lens to whatever the focal distance is supposed to be I will turn on the light and then move the screen back and forth until the light filament appears sharp and focused on it. Next I will record the distance of the lens from the screen. Repeat this procedure two more times after which I will move the lens to the next distance and do the same thing. As soon as I have all the measurements I will then use the following formula to find the focal lengths:

F =(u * v) / (u + v)

Another thing which I will record is whether the image is virtual or real.

Results

        

        

Evaluation

        There are many faults with this experiment which affected it’s accuracy. The most significant effector to the accuracy of the measurements was the light bulb, and lens. The exact position of the light filament was not sure, nor was that of the lens, with a more accurate way of measuring this (perhaps with a different object to measure other than a light bulb) the difference could have been a few millimeters which would definitely have increased the precision of the experiment. Another thing that affected the measurements by at least a few millimeters each way was the fact that you could never be exactly sure whether the filament was correctly focused. To improve this I would have used something else as an object, possibly just a screen with an arrow-shaped hole in it in front of a ray box or even a picture on a transparency in front of a ray box as this would be easiest to find the exact focus.

        To further experiment I could use different types of convex lenses as there are after all three different types of convex lenses (bi-convex, plano-convex and convex meniscus) the different shapes of each lens would drastically affect the way in which the light rays are bent. Using a ray box would probably also help as it would spread out the light more before it reached the lens.

Conclusion

        When looking back at my original predictions I can see that some, if not all of them were correct. I predicted that objects four and five would not focus on the screen and I was accurate with this statement, as they did not, the light rays were going parallel to each other as they exited the lens. I wrongly predicted that object two’s light rays would converge at an earlier point than the rays of the other objects. This worked exactly opposite to what I had thought, instead of doing this, the rays converged much further on than those of any other object. What I had predicted for object two instead happened for object one, which was once again contrary to my predictions. Despite all of these errors, my prediction for object three was correct. I predicted that it would be real, inverted and the same size as the object and this I predicted correctly.