Answer: 120 cm.
- A 40 cm
- B 80 cm
- C 120 cm
- D 160 cm
Correct answer: C. 120 cm
Explanation: Using lensmaker: 1/f = (n<sub>2</sub>-n<sub>1</sub>)/n<sub>1</sub> x (1/R<sub>1</sub>-1/R<sub>2</sub>). For curved side (R<sub>1</sub>=20) in water (n<sub>1</sub>=1.33): 1/f = (1.5-1.33)/1.33 x (1/20 - 0) = 0.17/1.33 x 1/20 = 0.0064. f = 156 cm approx 160 cm.
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.