Special Relativity - Velocity of Light

For blue light, a transparent material has a relative permittivity (dielectric constant) of 2.1 and a relative permeability of 1.0. If the speed of light in vacuum is c, the phase velocity of blue light in an unbounded medium of this material is

A. √3.1 c
B. √2.1 c
C. c/√1.1
D. c/√2.1
E. c/√3.1

(GR8677 #03)

Solution:

Permittivity, ϵ = 2.1 ϵ₀
Permeability, μ = μ₀
Speed of light in vacuum:
The velocity of light in medium:  

Answer: D

Electromagnetism – Dielectric

An infinite slab of insulating material with dielectric constant K and permittivity ɛ = Kɛ₀ is placed in a uniform electric field of magnitude E₀. The field is perpendicular to the surface of the material. The magnitude of the electric field inside the material is 

A. E₀/K
B. E₀/Kɛ₀ 
C. E₀
D. Kɛ₀E₀
E. KE₀

(GR0177 #95)

Solution:

In vacuum, E₀ = σ/ɛ₀
In dielectric, E = σ/ɛ
where ɛ = Kɛ₀
Thus, E = σ/Kɛ₀ = E₀/K

Answer: A