Question

The speed of light in a glass slab is ………….. compared to that in the air. Fill in the gap.
(a) larger
(b) smaller
(c) neutral
(d) None

Answer 1

Hint: In this question use the direct formula for speed of light in any medium of refractive index $\eta $ that is $V = \dfrac{c}{n}$ where c is the speed of light. Use the refractive index of air as 1 and that of glass slab as 1.5. This will help comparing the speed of light in different mediums.
Formula used – $V = \dfrac{c}{n}$

Complete Step-by-Step solution:
As we know when the medium changes the speed of the light also changes as the refractive index of different mediums is different.
So the speed of the light depends on the refractive index of the medium.
Let c be the speed of light = $3 \times {10^8}$ m/s.
And V be the speed of light in the medium.
So $V = \dfrac{c}{n}$, where n = refractive index.
As we know that the value of the refractive index is 1 for air and 1.5 for glass.
So the speed of light in air = ${V_{air}} = \dfrac{{3 \times {{10}^8}}}{1} = 3 \times {10^8}$ m/s.
And the speed of light in glass slab = ${V_{glass}} = \dfrac{{3 \times {{10}^8}}}{{1.5}} = 2 \times {10^8}$ m/s.
So as we see that the speed of light in glass slab is smaller compared to that in air.
So this is the required answer.
Hence option (B) is the correct answer.

Note – The trick point that we need to remember is that the lower the refractive index, the faster the velocity of light. Now the refractive index of any medium is dependent upon the density, wavelength and the temperature. It is advisable to remember the refractive index value of glass and air and these are standard values and are most commonly used.