Hi there In an experiment I have calculated the radius of curvature for a +0.25D lens using Newtons rings and have obtained a value for 892mm, but the radius of curvature for a +0.25D lens is just over 2 metres. why is the value i got so much lower? do i have to double the value i got for the radius to take into account the 2 surfaces of the lens or not? please help im really stuck! thanks for any help!