Optical density

The idea

Of two media, the one with the larger refractive index is optically denser and light travels more slowly in it; optical density is not the same as mass density.

Kerosene and water

Kerosene is lighter than water: it has less mass in the same volume. A student guesses that this also makes light travel faster in kerosene than in water.

Is the student right? Which of the two slows light down more?

• Kerosene slows light more than water does
• Water slows light more than kerosene does
• They slow light equally

Denser or rarer?

Compare each refractive index (Table 9.3) with the 1.33 of water. Put each medium in the right box.

Not the same as mass density

Kerosene has a larger refractive index than water, so light moves more slowly in it, although kerosene has less mass density. Optical density and mass density are different things.

Of two media, the one with the larger refractive index is optically denser, and the one with the lower refractive index is optically rarer. The speed of light is higher in the rarer medium. Optical density is not the same as mass density.

Notes

The medium with the larger refractive index is optically denser, and light is slower in it. Optical density is not mass density: kerosene is optically denser than water.

Check yourself

Which medium is optically denser?

Light travels faster in

Kerosene has a refractive index of 1.44 and water 1.33. Which statement is true?

A ray goes obliquely from an optically rarer to an optically denser medium. It

• The one with the larger refractive index — correct. Yes!
• The one with the greater mass density. Optical density is not the same as mass density. It is told by the refractive index.
• The one with the smaller refractive index. The smaller refractive index belongs to the optically rarer medium.
• the optically denser medium. The speed of light is higher in a rarer medium than in a denser one.
• the optically rarer medium — correct. Yes!
• both equally. The speed of light is different in different media: higher in the rarer one.
• Kerosene is optically rarer than water, because its mass density is less. Optical density depends on the refractive index, not on mass density. The 1.44 of kerosene is larger than the 1.33 of water.
• Kerosene and water have the same optical density. Their refractive indices differ, so their optical densities differ.
• Kerosene is optically denser than water, though its mass density is less — correct. Yes!
• slows down and bends towards the normal — correct. Yes!
• speeds up and bends away from the normal. That is what happens going from a denser to a rarer medium.
• slows down and bends away from the normal. A ray that slows down in a denser medium bends towards the normal.