A ray of light is incident at the glass-water interface at an angle i, it emerges finally parallel to the surface of water, then the value of μg would be

For  glass−water  interface  gμw=sin  isin  r  .  .  .  .  .  iFor water−air  interface  wμα=sin  rsin  90o  .  .  .  .ii⇒gμw×wμα=sin  isin  r×sin  rsin  90o=sin  i⇒μwμg×μαμw=sin  i⇒sin  i⇒μg=1sin  i