An LED is on the central axis 30.0 cm in front of a thin lens. The resulting image, which is virtual, is 10.0 cm from the lens. Determine the focal length of the lens. Using Table 5.3, explain why the lens must be negative even though a positive lens could also form a virtual image.

Table 5.3

Convex Image Object Location Location Orientation Relative Size Type f < s; < 2f 0 > S, > 2f Real Inverted Minified S; = 2f 0 > S; > 2f Real Inverted Same size So = 2f f < $o < 2f Inverted Magnified Real So = f So < f Virtual Erect Magnified Concave Object Image Location Location Orientation Relative Size Type |s:| < |f\, Anywhere Virtual Minified Erect So > s;|