The statement "The refractive index of diamond is 2.42" means that light travels 2.42 times faster in a vacuum (or air) than it does in diamond. In other words, the refractive index nnn is defined as the ratio of the speed of light in vacuum ccc to the speed of light in the material vvv:

n=cvn=\frac{c}{v}n=vc​

For diamond, with n=2.42n=2.42n=2.42, this implies:

2.42=cv2.42=\frac{c}{v}2.42=vc​

or equivalently,

v=c2.42v=\frac{c}{2.42}v=2.42c​

This means light slows down to about 1/2.42 of its speed in vacuum when passing through diamond

. Additionally, a higher refractive index like 2.42 indicates that diamond bends (refracts) light significantly more than materials with lower refractive indices such as water (1.33) or glass (around 1.5). This strong refraction contributes to diamond's brilliance and sparkle by causing light to change direction sharply inside the stone and reflect internally

. In summary:

• The refractive index of 2.42 means light speed in diamond is about 2.42 times slower than in vacuum.
• It quantifies how much diamond slows and bends light compared to air or vacuum.
• This property is key to diamond's optical effects like brilliance and fire