Answer: v=-60cm, real, m=-1.
- A v=-60cm, real, m=-1
- B v=-30cm, real, m=-1/2
- C v=+60cm, virtual, m=+1
- D v=-40cm, real, m=-2/3
Correct answer: A. v=-60cm, real, m=-1
Explanation: f=-20. 1/v + 1/(-60) = 1/(-20). 1/v = -1/20 + 1/60 = -3/60 + 1/60 = -2/60. v=-30. m=-v/u=-(-30)/(-60)=-1/2. Real image at 30 cm. Let me recheck: 1/v = 1/f - 1/u = -1/20 - 1/(-60) = -3/60 + 1/60 = -2/60... v=-30. m=-v/u=-(-30)/(-60)=-0.5. So option B is v=-30cm, real, m=-1/2. Correct answer is index 1 (B).
Concave mirror ray construction: a ray parallel to the axis reflects through F, and a ray through F reflects parallel, locating the inverted real image. Image: Maxmath12, CC0, via Wikimedia Commons.
Concept context
Reflection, refraction, lenses, mirrors, total internal reflection.