Question

Numerical aperture of an oil immersion objective using oil of R.I. 1.5 is 0.75. Then, its numerical aperture, when kept in air is

• A. 0.25
• B. 0.5
• C. 1
• D. 0.3

Answer 1

Answer: (B)

0.5

Answer 2

Using NA = n sin $\theta$, we get 0 .75 = U5 sin $\theta$ $i,e \,sin \,\theta=\frac{0.75}{1.5}0.5$NA for air = 1 $sin\, \theta = 1 \times 0.5 = 0.5$