Aim: To determine the focal length of a thin convex lens by mounting it on an optical bench and removing parallax between the inverted real image and the search needle.
Determine the focal length f of a thin convex lens by moving the object needle Pโ and search needle Pโ on an optical bench until parallax is completely removed.
Move your eye left and right using the slider. If the two needle tips shift in opposite directions, parallax exists. Adjust the Search Needle until both tips remain rigidly tip-to-tip regardless of where you look!
Formula: 1/f = 1/u + 1/v โน f = (u ยท v) / (u + v)
| Obs # | Object Pos Pโ (cm) | Lens Pos L (cm) | Search Pos Pโ (cm) | u = L โ Pโ (cm) | v = Pโ โ L (cm) | f = (uv)/(u+v) (cm) | Magnification m | Parallax |
|---|---|---|---|---|---|---|---|---|
| 1 | 20 | 50 | 80 | 30 | 60 | 20 | 2x | Zero |