Absolute refractive index

The idea

The absolute refractive index of a medium, simply called its refractive index, is taken with respect to vacuum or air: nm = c/v, where c is the speed of light in air and v the speed in the medium.

How slow is slow?

Light is fast in air and slower in water, glass and diamond. A scientist wants a number for each medium that compares its speed with the speed in air.

How would you build such a number for a medium?

• Divide the speed of light in air by the speed in the medium
• Add the two speeds together
• Subtract the speed in the medium from the speed in air

Compare with air

Press Next step. Each bar is the speed of light in the medium. The first bar is the speed c in air.

c divided by v

The shorter the bar, the larger the number c ÷ v. Diamond had the shortest bar and the largest number.

The absolute refractive index of a medium, simply called its refractive index, is taken with respect to vacuum or air: nₘ = c ÷ v, where c is the speed of light in air and v is the speed of light in the medium.

Notes

The absolute refractive index (simply the refractive index) of a medium is nₘ = c ÷ v, the speed of light in air divided by its speed in the medium.

Check yourself

The refractive index of a medium is

Light travels at 3×10⁸ m s⁻¹ in air. In a certain medium the refractive index is 1.5 (an example). What is the speed of light in the medium, in units of 10⁸ m s⁻¹?

Answer: 2 ×10⁸ m s⁻¹

v = c ÷ nₘ = 3×10⁸ ÷ 1.5 = 2×10⁸ m s⁻¹.

Which of these slows light down the most: water (1.33), crown glass (1.52) or diamond (2.42)?

The refractive index of a medium taken with respect to vacuum or air is called its

• the speed of light in air divided by the speed of light in the medium — correct. Yes!
• the speed of light in the medium divided by the speed of light in air. That is the reverse ratio. The refractive index is c ÷ v, with c the speed in air.
• the speed of light in air minus the speed in the medium. It is a ratio, found by dividing, not a difference.
• Water. Water has the smallest of the three refractive indices, so light is slowed down the least in it.
• Diamond — correct. Yes!
• Crown glass. Crown glass slows light more than water, but diamond, with 2.42, slows it more.
• relative refractive index. The refractive index of one medium with respect to another medium is the relative refractive index. With respect to vacuum or air it is absolute.
• optical density. Optical density is a different idea, though a larger refractive index does mean a denser medium. The name of nₘ = c ÷ v is the absolute refractive index.
• absolute refractive index — correct. Yes!