A Beam of Light in Air Is Incident on the Surface

Multiple Choice

A beam of light in air is incident on the surface of a rectangular block of clear plastic (n = 1.40) .If the speed of the beam before it enters the plastic is 3.00 × 10⁸ m/s,what is its speed inside the block?

A) 2.14 × 10⁸ m/s
B) 2.01 × 10⁸ m/s
C) 1.94 × 10⁸ m/s
D) 3.00 × 10⁸ m/s