Light — practice question

What is the speed of infrared radiation in glass with a refractive index of $1.5$?

• A$340\,\text{m s}^{-1}$
• B$2.0 \times 10^8\,\text{m s}^{-1}$
• C$3.0 \times 10^8\,\text{m s}^{-1}$
• D$4.5 \times 10^8\,\text{m s}^{-1}$