Help me settle an argument (well a friendly discussion)

[blinky]

Argument sounds so much better but in reality this is just a discussion that myself and my mate are having with my wife and my Astrophysics qualified daughter.  It goes like this....  You turn the light on in your bedroom in the middle of the night, i.e. its pitch dark outside and your curtains are shut, now if you open your curtains, light floods outside and illuminates the street and people can now see you have turned the light on.  So.... the argument - when you open the curtains, does it get darker in your bedroom?

Me and my mate say yes - although its an in theory and in practice we don't think its probably even measurable but... some of the light that was lighting up the bedroom, now lights up the street outside, so surely the bedroom must have got imperceptibly darker right?  The light coming out that single lightbulb in the ceiling (the 'big light' as we call the celling light in our house) doesn't get brighter 'cos you opened a curtain, so the room must get darker as the same amount of light is spread out...

My wife says nonsense (but by her own admission knows nowt about physics) - my daughter says probably nonsense but she would need to do calculations based on the colour or something of the curtains since they would be absorbing the light until they were opened, then they wont be absorbing light so that light would escape.  

Thoughts from the SGL collective?

[Elp]

The light travels so fast compared to the distance you're referring to I don't think it will make any difference.

Your perception however will, your eyes are constantly adapting to changes in light intensity so if you happen to take in any of that open curtain view into the darkness you'll perceive that it has become darker. Also, lighting an area is not just specific to the light sources, you've got diffuse and reflected light all around you, it's why a light walled room will appear brighter than a dark walled room given all light sources being emitted are the same.

[Mr Spock]

The room will be darker - but because you no longer have reflected light from the curtains rather than any other reason.

[AstroKeith]

Assuming the curtains are not pure black, ie when the light is switched on you can see them, then the curtains are reflecting some light back into the room.

When you open the curtains, that light escapes. The room gets darker.

ps: I have a degree in astrophysics too, but this is basic physics.

[blinky]

3 minutes ago, AstroKeith said:

Assuming the curtains are not pure black, ie when the light is switched on you can see them, then the curtains are reflecting some light back into the room.

When you open the curtains, that light escapes. The room gets darker.

ps: I have a degree in astrophysics too, but this is basic physics.

That is exactly my thoughts!

[thomasv]

Would have thought it depended on the reflectivity of the blind. If the blind is ideal black, i.e. was absorbing all of the light that falls on it, and assuming no light is reflected back from the windows after opening the blind, then it should appear the same?

However, if the curtain was made of aluminum foil, it would make a difference opening the blind?

 

[johnturley]

19 minutes ago, blinky said:

Argument sounds so much better but in reality this is just a discussion that myself and my mate are having with my wife and my Astrophysics qualified daughter.  It goes like this....  You turn the light on in your bedroom in the middle of the night, i.e. its pitch dark outside and your curtains are shut, now if you open your curtains, light floods outside and illuminates the street and people can now see you have turned the light on.  So.... the argument - when you open the curtains, does it get darker in your bedroom?

Me and my mate say yes - although its an in theory and in practice we don't think its probably even measurable but... some of the light that was lighting up the bedroom, now lights up the street outside, so surely the bedroom must have got imperceptibly darker right?  The light coming out that single lightbulb in the ceiling (the 'big light' as we call the celling light in our house) doesn't get brighter 'cos you opened a curtain, so the room must get darker as the same amount of light is spread out...

My wife says nonsense (but by her own admission knows nowt about physics) - my daughter says probably nonsense but she would need to do calculations based on the colour or something of the curtains since they would be absorbing the light until they were opened, then they wont be absorbing light so that light would escape.  

Thoughts from the SGL collective?

I suppose if depends on the reflectivity of your curtains, most curtains will have a higher reflectivity than plain glass (except perhaps in the matt black curtains), so it will probably get darker in the bedroom when the curtains are opened.

John 

[blinky]

My argument is there is a finite amount of light being emitted, if you open your curtains, that light escapes and lights up the road outside, so there must be less light inside.  Another analogy, you shing a light on a wall from 6 feet and from 20 feet, 20 feet will be dimmer 'cos the light has spread out further

[globular]

It's never genuinely pitch black outside.

You're comparing;

* the reflectivity of the closed curtains 

with

* the reflectivity of the glass in the windows
* the amount of ambient light passing from outside through the window into the room
* the reflectivity of the, now exposed, window sill - which is often painted white 

I suspect the sum of the different 'open curtain' effects are about the same as the 'close curtain' - which is why we don't notice any material difference.

[blinky]

1 minute ago, globular said:

It's never genuinely pitch black outside.

You're comparing;

* the reflectivity of the closed curtains 

with

* the reflectivity of the glass in the windows
* the amount of ambient light passing from outside through the window into the room
* the reflectivity of the, now exposed, window sill - which is often painted white 

I suspect the sum of the different 'open curtain' effects are about the same as the 'close curtain' - which is why we don't notice any material difference.

I agree with that, but this was defiantly more focussed around the 'in theory' rather than what we actually would experience

[globular]

6 minutes ago, blinky said:

I agree with that, but this was defiantly more focussed around the 'in theory' rather than what we actually would experience

Yes, a carefully controlled experiment with no ambient light, painted surfaces, real world clutter, etc, just boils down to the differences of reflectivity and scatter of the curtain material verses the window glass.

It's modelling the 'more than you'd have thought' impacts of the real world clutter that makes it more interesting. 

Edited January 24 by globular

[Elp]

1 hour ago, blinky said:

My argument is there is a finite amount of light being emitted, if you open your curtains, that light escapes and lights up the road outside, so there must be less light inside.  Another analogy, you shing a light on a wall from 6 feet and from 20 feet, 20 feet will be dimmer 'cos the light has spread out further

As per my first point. The distances are too small to be almost zero compared to the speed of light it'd be imperceptible to the point of no difference.

[Sunshine]

Interesting question, I am inclined to say that the level of luminosity in the room would not change, light does not accumulate in a closed space then suddenly escape, darkening said space. If this were the case then I believe light would accumulate in a closed space so that eventually it would be infinitely bright until a door is opened allowing loght to flood out. We know light doesn’t behave this way. But I am not a physicist so this is just my layman’s analysis.

[Elp]

No that it true. You usually also need a medium for the light to react with in order to see it (or look directly at the source), otherwise you'd be blinded by light rays wherever you look and we don't. This goes back to what I said about perception and reflected light illumination.

Edited January 24 by Elp

[mikeDnight]

This is easy for me. Whenever there's a "discussion" between a friend and his wife, I always take the side of the wife. I'd like to say its through wisdom to do so, but in reality its probably more to do with cowardice.  😁

[vlaiv]

Just to throw in a wrench ...

Pure glass reflects about 4% of the light that hits it.

If can even become brighter in the room if curtains absorbed more than 96% of light before they were pulled aside.

Edited January 24 by vlaiv
typo

[wulfrun]

Worth pointing out that the human eye is poor at noticing small changes in illumination. The curtains/window represent a small proportion of the area of a room, so regardless of the change between open and closed it would not be much overall change.

I think OP is using wrong logic in thinking of finite amounts of light being emitted. Not the explanation at all.

The difference between open and closed is purely down to how much light is relected off the closed curtains versus the window glass plus the open curtains.

[vlaiv]

1 minute ago, wulfrun said:

I think OP is using wrong logic in thinking of finite amounts of light being emitted. Not the explanation at all.

I thought so too at one point - but argument still stands.

Let me explain.

Since we have dynamic system in equilibrium - there is light source, some photons that are "in flight" and there is a "sink" - which is pretty much anything in the room.

If we think of the photons that leave the room thru the window - those could have been in one of two states - either sunk or "in flight".

Removing them out of the system is not just equivalent of them being sunk - we also remove some of their "in flight" time - and thus we reduce light levels in the room.

[Sunshine]

My brain hurts.

[blinky]

Thanks for all the replies folks - it seems its more complicated than I at first thought!

[iantaylor2uk]

You could argue the room would get brighter. This is because photons hitting the curtains will be absorbed by the curtains (and effectively lost) which will cause the curtains to heat up slightly. However, when the curtains are open, as others have pointed out, some of the photons will be reflected back into the room.

I think the argument that more photons are lost when the curtains are open is wrong, because those photons which are transmitted through the window (and effectively "lost" from the room) would have been those photons that would have been absorbed by the curtains (and also "lost" from the room) 

[vlaiv]

5 minutes ago, iantaylor2uk said:

I think the argument that more photons are lost when the curtains are open is wrong, because those photons which are transmitted through the window (and effectively "lost" from the room) would have been those photons that would have been absorbed by the curtains (and also "lost" from the room) 

That is only valid if we think of curtain being 100% absorbent - pure black body.

Some of photons do get reflected of curtain and contribute to total amount of "in flight" photons that represent illumination of the room. Those that go out of the window (if we assume open window - no glass and reflection) won't spend any more time in the room.

When we are on the subject of black bodies, what are temperatures inside and outside?

Those also contribute some photons and if outside is hotter - those photons will enter the room and contribute to overall illumination

 

[maw lod qan]

All the different opinions don't take in your neighbors gawd awful lights, so in my case it would immediately get brighter!

[saac]

What happens if you don't look, is the room even lit then?  (not a serious question please don't waste any oxygen considering it)  

And if nobody is in the room why is the light even on - have you seen the price of electricity these days - get that light off!

Jim 

Edited January 25 by saac

[GasGiant]

On 24/01/2024 at 13:43, blinky said:

My argument is there is a finite amount of light being emitted, if you open your curtains, that light escapes and lights up the road outside, so there must be less light inside.  Another analogy, you shing a light on a wall from 6 feet and from 20 feet, 20 feet will be dimmer 'cos the light has spread out further

But light isnt 'a mass',  its flowing.  So the constant stream of photons going outside would be instantly replaced by more being emitted by the bulb ?? Maybe 🤣

Edited January 25 by GasGiant