1. The problem statement, all variables and given/known data My physics coursework hypothesis: "As the thickness of the lens increases, the focal length decreases". I tested this using a ray box, a stand to hold lenses and a screen to see where the light is most focused, which is then measured with a metre ruler. The distance 'u' (light to lens) was kept constant. A concave lens had to be placed directly in front of the ray box to make the rays parallel. However, I am having difficulty differentiating between the equation 1/u +1/v = 1/f and the focal length. The focal length is shown on diagrams as being the distance from the middle of the lens to where the rays intercept? What does the equation, then, state? (I know what u and v are). Also, would the thickness of the lens essentially be the radius of curvature in this scenario, as both sides of the lens have an equal radius? Finally, due to a lack of data online, I'm unsure whether my results are accurate. The data is displayed below: lens (mm) u/focal length (m) power (D) f (equation) 4 0.474 2.11 0.184 5 0.259 3.86 0.139 7 0.166 6.02 0.107 8 0.105 9.52 0.078 15 0.046 21.74 0.040 A thing I should note is that while the order of thicknesses is correct, I'm not highly confident that the value for the thicknesses are correct... We had to measure them ourselves, and the equipment wasn't sophisticated at all so the outcome varied among most people. Note that the distance 'u' is set at a constant 0.3m. The graph for the focal length shows a curve: is this correct, or should the relationship generally be linear? On the other hand, the graph for power is virtually completely linear, which I would assume is correct? 2. Relevant equations 1/u + 1/v = 1/f Power (D) = 1/f 3. The attempt at a solution 1) Is the f value in the equation referring to the point where the rays converge, whereas in our experiment using the screen we were measuring the image? (Apparently they are different?) 2) Or would they be two different values, with the radius of curvature being just 2/thickness? 3) Apart from one bit of data lying a tad to the side of the line of best fit, the points all fit the curve pretty well, so I guess that increases my confidence. They're still not as neat as I would like, though.