A type of glass block has a refractive index of 1.8.

(a) Given,

• Refractive index of the glass block = 1.8
• Speed of light in air = 3 x 10⁸ m s^-1

As, refractive index (μ) of the glass block is given by,

$$\begin{aligned}\text μ = \frac{\text {Speed of light in vacuum or air}}{\text {Speed of light in the glass block}} \\[1em] \Rightarrow \text {Speed of light in the glass block} = \frac{\text {Speed of light in vacuum or air}}{\text μ} \\[1em] = \frac{3\times 10^8}{1.8} \\[1em] = \frac{30\times 10^8}{18} \\[1em] = \frac{5\times 10^8}{3} \\[1em] \approx 1.67 \times 10^8 \text { m s}^{-1}\end{aligned}$$

Hence, speed of light in the glass block is approximately 1.67 x 10⁸ m s^-1.

(b) If the width of the glass block is doubled, the speed of light does not change because the speed of light in a medium depends only on the refractive index of the material, not on its thickness or width.

Therefore, the speed remains 1.67 x 10⁸ m s^-1.