Analyse the following observation table showing variation of image, distance (v) with object distance (u) in case of a convex lens and answer the questions that follow without doing any calculations:
| s.no | Object-Distance u( cm) | Image-Distance v ( cm) |
|---|---|---|
| 1 | -60 | +12 |
| 2 | -30 | +15 |
| 3 | -20 | +20 |
| 4 | -15 | +30 |
| 5 | -12 | +60 |
| 6 | -9 | +90 |
We can say that the radius of curvature of the lens is 20 cm because when an object is placed at the centre of curvature of a convex lens its image is formed on the other side of the lens at the same distance from the lens. And, we also know that focal length is half of the radius of curvature. Thus, focal length of the lens is + 10 cm.
S.No., is not correct as for this observation the object distance is between focus and pole and for such cases the image formed is always virtual but in this case a real image is forming as the image distance is positive.
Approximate value of magnification for object distance -15 cm ancj image distance
+30 cm is -2
