# Thread: Why doesn't a room get lighter when a light is switched on?

1. We might have to turn off an electric fire when the room gets warmer, but why doesn't an electric light fill the room with too much light after so long? I assume this has something to do with the absorption of photons.

2.

3. Yep. The walls and furniture absorb different parts of the light, and reflect other parts. Eventually a beam of light bounces around so much there's nothing left. This is how ray tracing programs work, actually. They simulate the path a beam of light must have taken to be visible by the user, with various reflections and absorptions.

4. Originally Posted by ox
We might have to turn off an electric fire when the room gets warmer, but why doesn't an electric light fill the room with too much light after so long? I assume this has something to do with the absorption of photons.
Yes. Photons are absorbed and add to the temperature of the absorbing body (which includes you). That temperature then leaks out at a certain rate. After a while an equilibrium point is reached, after which the room will not get any warmer as the heat input and the heat loss falls into balance.

With light specifically some energy is absorbed each time a photon hits a body. The light from the source is yellowish/white and the color of the objects in the room is an indicator of which parts of the visible spectrum is absorbed. That frequency light that is re-emitted can then be absorbed by something else in turn, until finally only infra red is being reemitted. This is a continuing process while the source is switched on.

5. So if you had a room made of mirrors with a single light source the room would get heavier?

6. Originally Posted by okkaoboy
So if you had a room made of mirrors with a single light source the room would get heavier?
Photons have no mass. So no.

7. Originally Posted by drowsy turtle
Originally Posted by okkaoboy
So if you had a room made of mirrors with a single light source the room would get heavier?
Photons have no mass. So no.
Be careful, because they can add to the relativistic mass, which is nothing but a change in the energy of a system. So adding photons to a box will add to the relativistic mass, and thus, adding more weight to a box. Since wieght is inversely proportional to the mass, the mass is then to have said to have increased.

8. Originally Posted by drowsy turtle
Originally Posted by okkaoboy
So if you had a room made of mirrors with a single light source the room would get heavier?
Photons have no mass. So no.
Actually photons have energy and therefore have mass, so the room would in fact become heavier.

Photons have no rest mass, but that is not the same thing as having no mass.

9. Originally Posted by DrRocket
Originally Posted by drowsy turtle
Originally Posted by okkaoboy
So if you had a room made of mirrors with a single light source the room would get heavier?
Photons have no mass. So no.
Actually photons have energy and therefore have mass, so the room would in fact become heavier.

Photons have no rest mass, but that is not the same thing as having no mass.
No, as far as we can tell, the current theory is that photons do not have a mass since . Rest mass is the kind of mass we associate to invariant mass, which a photon has zero of, but electrons and protons do. Therefore, the mass term is given a zero value for the photon, so how does your mass term work out then?

10. Originally Posted by Manynames
Originally Posted by DrRocket
Originally Posted by drowsy turtle
Originally Posted by okkaoboy
So if you had a room made of mirrors with a single light source the room would get heavier?
Photons have no mass. So no.
Actually photons have energy and therefore have mass, so the room would in fact become heavier.

Photons have no rest mass, but that is not the same thing as having no mass.
No, as far as we can tell, the current theory is that photons do not have a mass since . Rest mass is the kind of mass we associate to invariant mass, which a photon has zero of, but electrons and protons do. Therefore, the mass term is given a zero value for the photon, so how does your mass term work out then?
If you are putting energy into the room, then you are putting mass into the room. The question needs more specification, for instances, is the power source for the light already contained within this room? if they power source is within the room, then there room will not gain mass, if you are putting power into the room from an outside source, then it will gain mass.

energy is mass, mass is energy.

11. Originally Posted by Manynames
Originally Posted by DrRocket
Originally Posted by drowsy turtle
Originally Posted by okkaoboy
So if you had a room made of mirrors with a single light source the room would get heavier?
Photons have no mass. So no.
Actually photons have energy and therefore have mass, so the room would in fact become heavier.

Photons have no rest mass, but that is not the same thing as having no mass.
No, as far as we can tell, the current theory is that photons do not have a mass since . Rest mass is the kind of mass we associate to invariant mass, which a photon has zero of, but electrons and protons do. Therefore, the mass term is given a zero value for the photon, so how does your mass term work out then?
The m in is not rest mass and your equation applies only to bodies with positive rest mass.

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