Speedy Light

It’s a common misconception that light always travels at a constant speed. Light slows down when it passes through solids, gases or liquids. A reader’s question about light speed!

Light zips through empty space at 300,000 kilometers per second — that’s 186,000 miles per second.

But light can only reach that speed in a vacuum — where there’s no matter, only emptiness. When light travels through matter such as water, air, or glass, it moves at a slower pace. In water, for example, it travels at three-quarters of its maximum speed.

Stephen Mill, of Centennial Colorado, writes, “It would seem that light would need to accelerate when it exits a medium. Does light accelerate, or does it remain at a reduced speed?”

Stephen, imagine you’re driving your car at 30 miles per hour and you want to get onto a highway going 60 miles per hour. You’d have to step on the accelerator and wait for a few seconds while your car sped up to the right speed. Light doesn’t behave like that. It does resume its speed when it moves from water, air or glass into a vacuum.

But — unlike you or me or your car or any of the matter all around us — light doesn’t have mass. If light did have mass, it would take time to accelerate from one speed to another. Since it doesn’t, light has the seemingly magical property of changing from one speed to another instantaneously.

Others notes:

The colors we see are a good example of light interacting with the atoms and molecules of a substance. If something looks red, that means that light of all colors except red are being absorbed by the atoms and molecules in the substance. Only red light is reflected. If the substance is a thin piece of plastic, and you are peering through it, then only red light will get through to you. The plastic’s atoms and molecules have been designed to not to absorb red light.

On the smallest level, this means that if the red light is absorbed by an atom or molecule in the red plastic, it is promptly spit back out — or re-emitted — by part of the atom or molecule. This process of being absorbed and then re-emitted is what slows the light down. It’s almost like passing a gallon of water down a line of people with buckets. First you dump the water into the first bucket, and then that person pours the water into the next person’s bucket, and he pours the water into the next bucket, and so on. No person or bucket moves, but the water is passed down the line — more slowly than if a single person ran the water to the finish line.

According to the Web site, “What is the Speed of Light?”, the speed of light in a vacuum is 299,792,458 m/s (meters per second), the speed of light in air is 299,702,547 m/s, the speed of light in water is 225,407,863 m/s and the speed of light in glass is 199,861,638 m/s. So, the speed of light in water is about three fourths of the speed of light in a vacuum.

Astrophysicist Virginia Trimble noted in a May 29, 2003 phone message that, “there are two different kinds of velocities – the phase velocity and the group velocity. And one goes down and the other goes up, so [that] their product is always C squared. But it’s the group velocity, the velocity at which the information is transferred that goes down when you enter a medium and comes back up again when you leave it. But forces and accelerations and energy, I think, are not the right way to think about the process.”

According to the Web site, “Demonstration of Group Velocity“, “The difference between group velocity and phase velocity can be quite difficult to grasp, but at the most basic descriptive level the difference is quite simple. The quantity we commonly refer to as the ‘speed-of-light’ is its phase velocity: the speed at which the wavefronts of a pure frequency tone travel. The group velocity is the speed at which pulses (or ‘wave packets’), that are made up of a range of spectral components, travel. The group velocity and phase velocity are the same in a vacuum, but in any other medium this is not, in general, the case.”

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