Finding the power of a concave lens by a method of combination

Finding the power of a concave lens by a method of combination
Aims
To find the power of a concave lens by a method of combination, using a convex lens with a power of –4.5D to –5.5D using a convex lens with a focal length of 10

Theory
The power of the length is what opticians prefer to use as a measurement and is the reciprocal of the focal length in metres. The equation for this is



The focal length of a lens is simply the distance from the lens to the point of principle focus. The equation is rearrange from above by taking the reciprocal from each side


The process of calculating the exact power within the –4.5 D to –5.5 D margin will involve initially finding the focal length but in order to find the focal length I need to the power so therefore I will have to devise a means of doing this.
Theory 1)









Theory 2)










The above two theories are both capable of finding the focal length and the one which I will use is theory 1...