A pulse travels along a stretched cord. What decides the pulse's speed?
Oona sends a beam of light from the air into a glass block. In the glass, the light travels at about two-thirds of the speed it had before.
Different media slow light by different amounts. This lesson names the property that compares them.
You saw this rule for mechanical waves: the medium sets a wave's speed.
Light obeys it too. Light travels fastest through a vacuum, and more slowly through every material.
In water, light travels at about three-quarters of its vacuum speed. In glass, at about two-thirds. Different media, different speeds.
Optical density describes one thing only: how much a medium slows light.
It is not the medium's mass density. A medium can be lighter for its size and still slow light more.
Vegetable oil floats on water, so the oil has the lower mass density. Yet light travels more slowly in the oil than in the water. The oil is optically denser.
To compare two media optically, ask one question: in which one does light travel more slowly? That one is the optically denser medium.
Light travels more slowly in sapphire than in window glass. Which medium is optically denser?
Optical density is decided by light's speed in the medium.
The medium where light travels more slowly is the optically denser one.
Light travels more slowly in the sapphire.
The sapphire is optically denser than the window glass.
A cork floats on water: its mass density is lower than water's. Can Petrine tell from this which medium is optically denser?
Optical density describes light's speed in the medium, not mass per volume.
Floating tells us about mass density only.
To compare optical densities, Petrine needs light's speed in each medium.
No. Floating settles mass density, and optical density needs the two light speeds.
Light travels more slowly in sapphire than in water. Which medium is optically denser?
Petrine says: 'Vegetable oil floats on water, so water must be optically denser than the oil.' Is Petrine correct?
Nilo says: 'Light travels more slowly in diamond than in air, so diamond is optically denser than air.' Is Nilo correct?
Which statement defines the optically denser of two media?
Light's speed is measured in three liquids. It travels fastest in the alcohol, slower in the water, and slowest in the glycerin. Which liquid is the most optically dense?
Ethanol has a lower mass density than water: it floats on top. Yet light travels more slowly in ethanol than in water. Which statement is true?