The speed of light in vacuum is $3 \times 10^8 \,\mathrm{m/s}$ and that is a universal constant. Apart from that, the speed of light in a definite and particular optical medium is constant ( however in some cases, during birefringence, the speed of light does change inside a medium, depending upon direction of propagation) with small changes in the magnitude depending on the pressure, temperature and a few other physical factors.

The speed of light in vacuum is $3 \times 10^8 \,\mathrm{m/s}$ and that is a universal constant. Apart from that, the speed of light in a definite and particular optical medium always varies ( In some cases, during birefringence, the speed of light changes inside a medium depending upon direction of propagation) with small changes in the magnitude depending on the pressure, temperature and a few other physical factors.

The speed of light in vacuum is $3 \times 10^8 \,\mathrm{m/s}$ and that is a universal constant. Apart from that, the speed of light in a definite and particular optical medium is constant ( however in some cases, during birefringence, the speed of light does change inside a medium, depending upon direction of propagation).

The speed of light in vacuum is $3 \times 10^8 \,\mathrm{m/s}$ and that is a universal constant. Apart from that, the speed of light in a definite and particular optical medium is constant ( however in some cases, during birefringence, the speed of light does change inside a medium, depending upon direction of propagation) with small changes in the magnitude depending on the pressure, temperature and a few other physical factors.