Among the listed media, which has the lowest speed of light?

• A. diamond 2.42
• B. air 1.00
• C. water 1.33
• D. lucite 1.50

Answer: (A)

Light slows down when it moves through a material with a higher refractive index. The speed in a medium is v = c/n, where c is the vacuum speed of light and n is the refractive index. A larger n means a smaller v.

Among the options, diamond has the largest refractive index, about 2.42. That makes the speed of light in diamond roughly c/2.42, around 0.41 times the vacuum speed. The others have smaller indices (air ~1.00, water ~1.33, lucite ~1.50), so light travels faster in them than in diamond. So the lowest speed occurs in diamond.