First slide

Ray optics

Question

According to the thin lens formula, which one of the following is true regarding the focal length of the lens?

Easy

A

f is positive for concave lens
B

f is negative for convex lens
C

f is positive for a diverging lens
D

f is negative for concave lens

Solution

Thin lens formula is given as:
$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
Where u = distance of the object from the optical center of the lens; v = distance of the image from the optical center of the lens; f = focal length of a lens. According to the thin lens formula, f is positive for converging or convex lens and f is negative for a diverging or concave lens.

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