Final answer:
The focal length of the convex mirror is 30 cm, corresponding to a virtual image 20 cm behind the mirror and a real object 60 cm in front of it.
Step-by-step explanation:
To find the focal length of a convex mirror when a bright spot situated at 60 cm forms a virtual image 20 cm behind the mirror, we can use the mirror equation: 1/f = 1/do + 1/di, where f is the focal length, do is the object distance (60 cm here, and should be positive for a real object in front of the mirror), and di is the image distance (-20 cm here, negative because virtual images formed by convex mirrors are behind the mirror).
Plugging the values in, we get: 1/f = 1/60 + 1/(-20) = 1/60 - 1/20 = (1 - 3)/60 = -2/60 = -1/30.
Therefore, f = -30 cm. The negative sign indicates that it is a convex mirror (since convex mirrors always have a negative focal length). So, the focal length of the mirror is 30 cm, which is Option C.