Lab: Focal Length and Object Distance

Purpose:

The purpose of this lab is to confirm the thin lens and mirror equation which relates focal length of a mirror or lens to the distance of the object and the distance of the imaged form as a result of reflection, , 1/d0+1/di=1/f. To prove this equation we will use both a concave mirror and a convex lens, completing 16 trials with each object where we will set up a screen, mirror, and light source in that order, moving the mirror until image focuses while the light source is a certain distance away from the screen and measure the distance of the mirror/lens from the screen and light source. The focal lengths of the mirror and lens are known (20 and 10 cm respectively), so the image distance and the object distance are the two measured variables. To prove the formula we will graph 1/d0 vs. 1/di, with 1/di being the dependent variable and 1/d0 being the independent variable in the form 1/di=-m(1/d0) + b, where m is the slope and b is the vertical intercept. The vertical intercept should match the value of 1/f.

Procedure:

1. Set up the apparatus as shown above; place the screen at the very end of the track, the light in the middle, and the mirror of focal length 20 cm on the opposite side.

2. Turn on the light and move the light until the image focuses on the screen. It is at this distance that the light beams that bounce off the mirror meet.

3. Repeat step three for different placements of the mirror. The accumulation of these values will confirm the formula for the mirror equation using the known focal length.

4. Repeat the entire experiment with a lens. Place the screen on one end, the lens of focal length 20 cm in the middle, and the light at the other end. Keep the light and screen at a fixed point and move the lens until the image focuses. For each new measurement move the light to a different point and adjust the lens location. These will determine the thin lens equation and using the known focal length.

Data:

Concave...