Total internal reflection has important applications in fibre optic surgery. A long, narrow optical fibre is used in microsurgery for lessening trauma and the chance of infection, and for reducing the healing period. Light within an optical fibre undergoes total internal reflection hundreds of thousands of times without escaping. The critical angle at the glass$-$air interface $(n_{\text{g}\text{l}\text{a}\text{s}\text{s}}=1\mathrm{.}55,n_{\text{a}\text{i}\text{r}}=1\mathrm{.}00)$ is

$33\mathrm{.}7$

$40\mathrm{.}2$

$48\mathrm{.}2$

$56\mathrm{.}3$