Register

Leaving Certificate Physics Practice Test

Access More Questions
Using the lens maker formula for a thin lens in air with n = 1.50, R1 = +20 cm, R2 = -20 cm, what is the focal length f?
Correct Answer:
f = +20 cm
Explanation:
The test is about how a thin lens in air focuses light, described by the lens maker formula: 1/f = (n − 1)(1/R1 − 1/R2). Here the lens material has n = 1.50, so n − 1 = 0.50. The curvatures are R1 = +20 cm and R2 = −20 cm, so 1/R1 = 0.05 cm⁻¹ and 1/R2 = −0.05 cm⁻¹. Plugging in gives 1/f = 0.50(0.05 − (−0.05)) = 0.50 × 0.10 = 0.05 cm⁻¹. Therefore f = 1/0.05 = 20 cm. The positive focal length means a converging lens, which matches a focal length of +20 cm. The other options would require different values of 1/f (for example, 1/f = 0.025 would give f = 40 cm, and a negative 1/f would imply a diverging lens), which isn’t produced by these surface curvatures with this index.

Access more questions from this quiz

Continue to Leaving Certificate Physics Practice Test for more practice questions and the full quiz experience.

Access More Questions