When You Turn Off A Light, Where Does The Light Go?

In space, light will go on, and on, and on… In a windowless room, where does the light go when we switch it off?

Originally posted on Forbes!

Light is a pretty simple beast. In lieu of any interference, it will go on, and on, and on, as we see it doing in the vast, often empty, realms of interstellar and intergalactic space.

Space is a rather unique case, because in between massive objects, light is traveling through something very close to a pure vacuum. This vacuum environment it's traveling through means that there’s very little chance for the light to run into any kind of interference - it's relatively easy for the light to travel enormous distances without anything changing its path or blocking its way.

So what are the options if there is something in the way? Well, light functionally has two options: reflection or absorption. Reflection we’re quite familiar with, as it’s the physics behind seeing yourself in a mirror. This can happen anytime light hits a surface which is smooth, to its perspective. (The smoothness required depends on the wavelength of the light - optical light needs a smoother surface to reflect cleanly off of than radio waves do, which are much longer in wavelength.)

The other option is absorption. This is the process which makes rocks warm in the sun. The rocks are absorbing sunlight over time, and over that time, the energy collected into the rock will warm the surface. Any light can be absorbed, not just the infrared (heat) portion of sunlight. A terrible mirror could absorb enough light that your reflection is only a faint ghost of an image. Unless you’ve got an old filament bulb left in your lamps, you won’t notice an appreciable warming to any of your possessions, because most light bulbs nowadays are designed very specifically not to produce much heat. We can’t see it, and it’s a waste of energy to produce heat which doesn’t help us see the room.

These one-light-year-tall pillars of cold hydrogen and dust, imaged by the Hubble Space Telescope, are located in the Carina Nebula. This image of dust pillars in the Carina Nebula is a composite of 2005 observations taken of the region in hydrogen light (light emitted by hydrogen atoms) along with 2010 observations taken in oxygen light (light emitted by oxygen atoms), both times with Hubble's Advanced Camera for Surveys. The immense Carina Nebula is an estimated 7,500 light-years away in the southern constellation Carina. NASA, ESA, and the Hubble Heritage Project (STScI/AURA); Acknowledgment: M. Livio (STScI) and N. Smith (University of California, Berkeley)

These one-light-year-tall pillars of cold hydrogen and dust, imaged by the Hubble Space Telescope, are located in the Carina Nebula. This image of dust pillars in the Carina Nebula is a composite of 2005 observations taken of the region in hydrogen light (light emitted by hydrogen atoms) along with 2010 observations taken in oxygen light (light emitted by oxygen atoms), both times with Hubble's Advanced Camera for Surveys. The immense Carina Nebula is an estimated 7,500 light-years away in the southern constellation Carina. NASA, ESA, and the Hubble Heritage Project (STScI/AURA); Acknowledgment: M. Livio (STScI) and N. Smith (University of California, Berkeley)

These two options, absorption and reflection, work in tandem with each other, and most materials will do a little of both. Even your standard bathroom mirror absorbs a little of the light that hits it (typically about 10%), and few naturally occurring materials on Earth are perfect light absorbers. Some ultra-black materials are getting very close, but the only truly perfectly absorbing objects around so far are black body objects; an object heated until it glows. (An old filament light bulb would count.)

So, when considering what happens to the light from your light bulb when you switch it off, let’s consider what’s happening when we have the light on. Light is being continually produced by the bulb, which is streaming outwards through the air, mostly unperturbed by having to go through air instead of a vacuum. It will then hit every surface which faces the bulb, and some fraction of it will reflect in the direction of your eyeballs, which will absorb the light, and tell you how bright the room is, along with some information about the objects within the room.

NGC 1999 is an example of a reflection nebula. Like fog around a street lamp, a reflection nebula shines only because the light from an embedded source illuminates its dust; the nebula does not emit any visible light of its own. NGC 1999 lies close to the famous Orion Nebula, about 1,500 light-years from Earth, in a region of our Milky Way galaxy where new stars are being formed actively.  Image Credit: NASA and The Hubble Heritage Team (STScI)

NGC 1999 is an example of a reflection nebula. Like fog around a street lamp, a reflection nebula shines only because the light from an embedded source illuminates its dust; the nebula does not emit any visible light of its own. NGC 1999 lies close to the famous Orion Nebula, about 1,500 light-years from Earth, in a region of our Milky Way galaxy where new stars are being formed actively.  Image Credit: NASA and The Hubble Heritage Team (STScI)

The difference between this situation and switching the light off is simply that you’re no longer replacing the absorbed photons of light with new ones. The last rays of light that the light bulb produced will behave exactly as the rest of the light did: either absorbing into or reflecting off of the various surfaces in your room. The reflected light will bounce until it’s absorbed, but considering how fast the photon can traverse the room, and how few bounces it takes to absorb light, this loss of light is functionally instantaneous.

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Is the speed of a fired bullet the same in space and on Earth?

A bullet fired from a gun travels at supersonic speeds. This picture shows a bullet and the air flowing around it. The bullet is traveling at 1.5 times the speed of sound.  Image credits : Andrew Davidhazy/Rochester Institute of Technology

A bullet fired from a gun travels at supersonic speeds. This picture shows a bullet and the air flowing around it. The bullet is traveling at 1.5 times the speed of sound. Image credits: Andrew Davidhazy/Rochester Institute of Technology

If you were to take a handgun, fire it on the Earth, and then take it into space (which is illegal, by the way), and fire it again, you would have kept everything that produces the speed of the bullet (the mechanism inside the firearm) exactly the same.  So if the gun is the same and the bullets are the same, the “muzzle velocity” - the speed at which the bullet leaves the end of the gun, relative to the gun itself - should be identical in space and on the ground.  For a pistol, this is somewhere around 1000 feet per second.

What changes is what happens after that.  In space, the only force that would act to slow down (or speed up) that bullet after being fired is gravity.  On the ground, we also have gravity pulling the bullet rapidly to the ground, but we also have the atmosphere.  The air around us seems pretty inconsequential, but if you try to move through it quickly enough, the air itself provides a very substantial resistance; meteors encounter our atmosphere almost like a solid wall.

The air around us resists the motion of anything moving through it, particularly at high speeds.  We call this slowing force “drag” or, less inventively, “atmospheric resistance”.  Anything moving through the air will encounter this force, so any bullets fired on the Earth will have their forward motions continuously slowed down by drag.

We can actually calculate how long it would take for the forward motion of a bullet to be stopped completely by atmospheric drag. If you take the initial horizontal speed of a bullet to be 300 meters per second (which is the metric equivalent of 1000 feet per second, and the math is much easier in metric), you can work out the rate at which the bullet is slowed down due to drag.  The effectiveness of the drag force is related to the density of the object you’re pushing through the air (which is reasonably low for a bullet, since the mass is so slight), the surface area of the object being pushed through the air (which also is small for a bullet), the aerodynamic shape of the object (a flat surface offers more resistance than an oval), and then the velocity of the object.

It turns out that bullets are pretty aerodynamic, which is probably no surprise.  Since they’re so aerodynamic, the force of drag doesn’t have that much of an effect, but there is a measurable slowing.  After 3 minutes of flying time (which is implausible, given gravity), the distance travelled by the bullet in a second is 8 centimeters less, entirely due to the effect of drag.  If bullets were shaped like cubes instead of like bullets, this effect would be much greater.

While the drag force does definitely slow down the bullet, gravity has a much larger effect on the bullet’s flight.  If a bullet is fired from a height of 5 feet, precisely horizontally, there’s only a half a second of time before that bullet hits the ground, since gravity is continuously pulling downwards on the bullet. The pull of gravity actually adds speed to the bullet, since it gives the bullet downwards speed, while leaving the forward speed untouched, which means the diagonal speed is higher.  This additional contribution by gravity means the bullet can be going faster when it hits the ground than it was when it left the muzzle of the gun.

So: instantaneously after firing, the speed of a bullet in space and on earth should be the same.  The effect of drag on the bullet will slow it down on earth, which the bullet would not feel in space, but for objects the size and shape of a bullet, this doesn’t have a very large effect on the speed of the bullet. But, if you’re near a strong gravitational force, either in space or on the ground, you can speed up your bullet well past the muzzle velocity.

(A Note from Astroquizzical Management: Thanks for your patience! I’ve moved twice in the past two months, and have one more move coming up, so things have been hectic! Please keep sending your questions my way, I will get to them ASAP!)


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