Concave mirror: finding the image
An object is placed 30 cm in front of a concave mirror with a radius of curvature of 40 cm. What image forms?
Answer A: explanation for every choice
A
Correct. f = R/2 = 20 cm. 1/i = 1/20 − 1/30 = 1/60, so i = +60 cm, and m = −60/30 = −2.
B
12 cm comes from adding 1/f and 1/o.
C
A positive image distance means a real image in front of the mirror.
D
An image forms at the focal point only when the object is very far away.
TakeawayMirrors: f = R/2. Object between f and 2f → real, inverted, magnified, beyond 2f.
Equation usedThin lens and mirror equation
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