The refractive index of the material of crystal is 1.68 and that of castor oil is 1.2. When a ray of light passes from oil to glass, its velocity will change by a factor
- A$\frac{2}{3}$
- B$\frac{3}{4}$
- C$\frac{5}{6}$
- D$\frac{5}{7}$
✓ Correct answer: D
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