If the phase velocity of a plane wave in a perfect dielectric is 0.4 times its value in free space, then what is the relative permittivity of the dielectric?

• A. 2.5
• B. 1.25
• C. 4.25
• D. 6.25

Answer: (D)

The equation is given by:
Vp (Phase velocity) = (Velocity of light in free space/Phase velocity)²
= (1/0.4)²
= 6.25