I need a little help with a laboratory class I had about lens. Experiment: We had a light source with a draw of an arrow on it, this light go through a lens of +100 mm (focal length) and a surface on the other side of the lens were we can see the arrow on it. We put the light source at 50cm from the lens (distance s) and we place the surface at a distance were the image looked good, and measure the distance from the lens and the screen and called p. Then we moved the light source 5 cm closer to the lens and adjust the screen again. We do this for about 10 times then we made a graphic 1/p VS 1/s and found a line on the graphic with m=-0.764 and b=0.0883. Later on, in the laboratory manual is asking for the meaning of b (the y-intercept). My question is: What is b? I actually read the physics book and I didn't found anything.