# Convex Mirror in Backyard to reflect sunlight onto the house

• Lawn/Garden
In summary, the conversation discusses the idea of using convex mirrors to bring direct sunlight onto a house. The person initiating the conversation wants to calculate the size of the mirrors needed and the distance they should be placed from the house in order to cover the entire area of the glass doors with sunlight. Safety concerns are mentioned, as well as the suggestion to start with a flat mirror instead. The purpose of the project is to provide natural light to a dark living area. The person also plans to upload a sketch and pictures for better understanding. The conversation also touches on the possibility of using white paint to reflect sunlight and the potential for irritating neighbors. The geographical latitude of the house is mentioned as a factor that could affect the angle and path of the sun.
Hello-

I am starting a project to get direct sunlight onto the house by placing convex mirrors on the outside stone fence of the backyard since it is the only place of my property that is not shaded by other houses in the neighborhood. For that, I need to calculate the size of the mirrors, given the distance from the stone fence on which they will be placed (on a swivel) to the house as well as the size of the glass doors that open up from the living room to the garden (I would rather that I cover the entire area of the doors with sunlight!). I tried to do some research on my own but I think I'm way over my head on this one. Is there some kind of formula I can use to calculate the area of the light that is reflected onto the house given the size of the mirror and the distance from the mirror to the house? Is there any other parameters that I have missed? Please feel free to ask me for more information if I have missed providing any!

You will get better and more helpful answers if you tell us why you’re doing this. Do you want to warm the house? More economically? Just bring light into the house? Illuminate the exterior? How much?

There are some safety considerations: looking into a mirror that reflects the sun can permanently damage your eyes, and some mirror geometries can concentrate a dangerous amount of heat.

DeBangis21 and Spinnor
What a nice enterprise! I hope you have good fire insurance

If I were you I'd start experimenting with a flat mirror instead of a convex one. Much cheaper and you can already iterate through versions of the swivel mechanism.

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Spinnor
Come on folks, convex mirror won't start a fire, that's not a Fenchurch financial center

Some simple sketch would definitely help, though.

Redbelly98, DaveE, sophiecentaur and 3 others
This sounds similar in concept (but not similar scale) to whole towns in Norway and France that use mirrors to provide sunlight.

https://www.theatlantic.com/photo/2013/10/using-giant-mirrors-to-light-up-dark-valleys/100613/

DeBangis21, harborsparrow, Keith_McClary and 2 others
With a fixed mirror your 'sunspot' will wander over the day. With a convex mirror it still may be OK, but the intensity will be severely reduced.
Maybe a simple white paint or something like this would do better.

Klystron and russ_watters
berkeman said:
And how exactly does a convex mirror concentrate sunlight?
I don't think it is about concentrating the light, nothing like that is said in the OP post. But many years ago I got a lot of light into the room I was spending a lot of time in just by painting an external, opposite wall (actually at an angle) white.

A convex mirror may also succeed in irritating the neighbors.

DaveC426913, russ_watters and berkeman
Is there any other parameters that I have missed? Please feel free to ask me for more information if I have missed providing any!
Your geographical latitude would help. That tells us the angle and path of the sun during the year.

Wow! Thanks for all the replies!

@Nugatory , the purpose is to provide illumination to our living room/kitchen area that is otherwise dark and needs lamps to be turned on during the day, particularly during the winter months.

As for your safety concerns that is something I take very seriously as I have young children! Hence, I had originally thought that it'd be best if the light reflection comes into the house at an angle that is way above their and our height. It seems that, even a small light reflection area can illuminate a much bigger space quite brightly (I got inspired by images like the two on the right here https://www.instructables.com/QuicknDirty-Heliostat-Get-some-sun-into-your-home/)

@BvU I agree that starting out with a flat mirror might be better but I would rather do this once as I will need to custom make this mirror as well as its swivel with a contractor. I would rather, if I can get everything right from the get go, not go through the hassle of changing the mirrors and the extra costs.

@Borek , I will try to make a sketch and upload it. I will also try to take some pictures and upload them.

@Baluncore , I am not sure how to get my latitude but if it helps, you can search google maps for "The Bouquet Residence JVT Dubai". I am on that same row of houses.

As for the fire concerns, as you can see, I live in Dubai, where it can get very, very hot in the summer. Hence, my initial thoughts was to have those mirrors on a swivel that I can point upward or at an angle such that the light is not reflected to the glass doors that open to the garden but rather, a bit above them so that they hit the walls.

Would a convex mirror really start a fire?! Would the heat generated by the mirror's reflection be more intense than that of the direct sunlight (a la a magnifying glass)?

Again guys, thanks for the input! I'm really appreciative of all the comments/help I can get

"The Bouquet Residence JVT Dubai" is within 5 km of 25° North latitude.
In midsummer the Sun will be 1.5° short of overhead.
In midwinter the Sun will be 41.5° above the horizon.

Would a convex mirror really start a fire?! Would the heat generated by the mirror's reflection be more intense than that of the direct sunlight (a la a magnifying glass)?
Assuming you mean concave, yes. A magnifying glass and a concave mirror will both concentrate sunlight from a large area (the mirror or lens) to a smaller area (the spot/image of the sun) unless defocused. Imagine how hot something dark can get when in direct sunlight for a prolonged period of time. It will be even warmer under a concave mirror unless placed well past the focal point of the mirror.
@Nugatory , the purpose is to provide illumination to our living room/kitchen area that is otherwise dark and needs lamps to be turned on during the day, particularly during the winter months.
Modern LED bulbs are extremely efficient and are hands down the most convenient solution to your problem. You should easily be able to light an entire room for less than 100 watts of power usage. They are small, virtually maintenance free, and cheap, whereas a moving mirror large enough to bring sufficient light into your house is none of these things.

Have you considered how this mirror will survive high wind conditions? Or how you'll clean it? Will it resist rocks that might get thrown at it by kids or anyone else?

dlgoff, BillTre and berkeman
Hello @Drakkith

Thank you for the input!

Actually I meant convex, not concave. The idea (if I am not mistaken) is that convex mirrors will illuminate light to a wider area and not focus the light and hence, i would not have to worry about too much heat generated to a small area.

Where I live, there is barely any wind. All year round. The only wind we get rustles some leaves. You never see a tree branch moving.

Since the mirrors will be towards the house, unless my kids throw the rocks (which they're way too young to do), I don't think anyone can do any damage. It is a very friendly neighborhood anyway :)

I will send a picture of the area so that hopefully, it gives a better idea of what I am trying to do.

A convex mirror would indeed solve both the tracking problems and safety issues, but has only really limited usefulness.
As a rule, you get light just as many as the brightness of objects you would see through the mirror. And what would you see through a convex mirror? Mostly, the sky and other buildings. The Sun would be just a small spot => not much extra light.
Painting all the visible surface bright white would yield at least comparable result, by my opinion.

harborsparrow and hutchphd
Since the target(s) are the size of a door, the mirrors can be as big as a door.

Several convex mirrors will not require steering, but will give you pinpoints of sunlight that will temporarily (hopefully) bleech your retina and give you blind spots.

You want light without heat so you should use a rear silvered mirror, or one with a selective coating.

BillTre
Thank you @Baluncore and @Rive

From what you mentioned, it seems that my initial understanding that a convex mirror's reflected light area is wider than a regular mirror due to its bulging outwards is incorrect. Rather, i would get a much smaller reflected light area vs regular mirrors. Did I understand correctly?

Here is a photo of the garden and the house, showing the doors as well as the pillars on which I plan to put the mirrors on a swivel.

Thoughts? Shall I go drop the convex mirrors and focus on regular ones? And for the size, the reflection is always as big as the mirror itself, right?

Lastly, for the heat reflection, if we go with regular mirrors, would the heat reflected to the house be more than that of the direct sunlight (like the other houses in the neighborhood that actually face the sunlight)?

Did I misunderstand anything? Thanks again!

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One more thing @Baluncore , you mentioned that rear silvered mirrors or ones with selective coating would reflect light but not the heat. I tried googling that a bit but could not find more information. Can you please provide more info about those mirrors?

For every mirror, the reflected amount of light is proportional to its shadow. What convex mirror does is, to spread that light wider => since it illuminates wider area, it'll be weaker on a certain spot. On the other hand, the mirror won't have to follow the sun to have some light reflected to a certain spot.
Flat mirror will illuminate a certain spot/direction for only a limited time, but for that time that spot of light will be intensive.

You might look at what coatings are used on window glass where you are.

I assume you want to block reflection of IR and UV, so only visible light remains. If the light will enter the house through windows then that may have been done. If through an open door then a coated mirror might make a difference.

I would first try hanging white sheets on the wall to test the effect. You may only need to whitewash the wall.

Nik_2213
I recommend that you experiment first with a flat mirror using mylar film as the mirror. It would not withstand severe weather, so it would be temporary as an experiment.

The advantage is that you can do the experiment with 0.001 or perhaps 0.0001 times as much effort and money as a permanent rigid mirror. From the experiment you may find the size of the mirror to be adequate, or you may decide that a mirror will not fill your needs. That may override your desire to do it once.

By the way, don't forget severe weather when designing the permanent mirror. If the mirror is glass, don't forget the safety hazard of broken glass.

@BvU I agree that starting out with a flat mirror might be better but I would rather do this once as I will need to custom make this mirror as well as its swivel with a contractor.
anorlunda said:
I recommend that you experiment first with a flat mirror using mylar film as the mirror. It would not withstand severe weather, so it would be temporary as an experiment.

The advantage is that you can do the experiment with 0.001 or perhaps 0.0001 times as much effort and money as a permanent rigid mirror.
A quick alternative to start getting an idea of how much light can be reflected into the dark house would be to borrow one of the mirrors from inside the house to hand-hold in the back yard to see what the results look like.

Every house has a mirror somewhere, and if it's a medicine cabinet door in the bathroom or some other not-too-big mirror from elsewhere in the house, it could be a quick 1-hour experiment to see how much light that size mirror brings into the house, and if you can aim it through a high window like you mentioned to keep it overhead.

@berkeman when I saw that pic I first thought you took a pic of your bathroom and all I could think of was that wished my bathroom stayed that clean.

harborsparrow, chemisttree, berkeman and 1 other person
I can't help thinking that a specific curved mirror surface (convex or concave) will make for a very expensive project. Why not use an array of plane mirrors, made of reflective plastic on plywood? That would cost peanuts (relatively) and the sections could be tilted to produce a suitable result over time, starting with just one (say 2m2) to see its effect. Optical quality is of little importance and the OP could always re-use the timber for something else if the project has no useful effect.
A diagram of the actual layout would allow some simple ray paths to be traced from the Sun in various positions. Rays from Sun to (the defining) mirror edges would show you the coverage on the house. It would give a clue about how to reduce the effect on neighbours and how to fill in for different times of day.
Warning: the neighbours will talk. But hell we are PF. we don't care.

Bystander
Averagesupernova said:
@berkeman when I saw that pic I first thought you took a pic of your bathroom and all I could think of was that wished my bathroom stayed that clean.
I thought @berkeman was being prepared for burns as there is that Alo Vera plant.

sophiecentaur, anorlunda and berkeman
sophiecentaur said:
I can't help thinking that a specific curved mirror surface (convex or concave) will make for a very expensive project. Why not use an array of plane mirrors, made of reflective plastic on plywood?
If cost were an issue I would just go with a lamp and an LED bulb.

Merlin3189, BillTre and dlgoff
Drakkith said:
If cost were an issue I would just go with a lamp and an LED bulb.
That's a bit of a straw man. LEDs are not sunlight and the cost will always be a factor unless you are Elon Musk. Did you cost out the price of a large (how large?) mirror? There are 'issues' and 'issues'.
You have to forgive PF for wanting to introduce Engineering considerations into DIY proposals. It's the way most of us are and you are getting responses from all angles.

BvU
sophiecentaur said:
That's a bit of a straw man. LEDs are not sunlight and the cost will always be a factor unless you are Elon Musk.
What's a straw man? The cost? I hope no one took my statement to mean that cost wasn't an issue at all.

sophiecentaur
Drakkith said:
What's a straw man?
In this case, it's assuming that there are only two choices and then dismissing the one you don't like.
The OP seems to want to see the Sun so Leds won't do.
The largest convex mirror I could find was 800mm diameter and (sale) price £250. That would be for 'security' applications. It could be attractive as a Trompe-l'œil in a garden but I don't know what the curvature would be - probably to small a radius so not the best field (too wide) and a small aperture so illumination would be weak .
Whatever the final choice, I would seriously suggest a test with a good size bathroom mirror for just a few quid.

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sophiecentaur said:
In this case, it's assuming that there are only two choices and then dismissing the one you don't like.
I'm pretty sure that's not a straw man. Besides, that's not what I'm doing. The OP can do what he likes, whether that's use a lamp or install a large mirror or something else. A lamp is simply the cheapest and easiest choice, so much so that if he's going to worry about price then it's worth reconsidering whether the project is even worth doing in lieu of simply buying a lamp.

sophiecentaur and BillTre
Drakkith said:
reconsidering whether the project is even worth doing in lieu of simply buying a lamp.
I think that, to see actual sunshine when none is available would be significant and worth going for. But not at any price. Leds are no better than leaving the lights on indoors and a string of fairy lights wouldn't be a substitute.
As it's not your or my garden then we can't say what the OP should do. All I can say is that a big enough curved mirror would need to be very costly but a plane mirror would be no more than a few pounds. So it gets my vote.

@Hadi Remember that this isn't the early morning or late evening Sun we're talking about, this is the blinding midday Sun. Any surface the light falls on is going to be overwhelmingly bright unless the light is highly diffused or spread out. So bright that most people block the direct light with curtains just so they can see the darker areas inside their house.

Also, any time you look outside the window you're going to be looking into the Sun. It's very bright and very uncomfortable to have in your field of view, so you can consider whichever window you're using for this project to be useless to look out of.

sophiecentaur
Drakkith said:
@Hadi Remember that this isn't the early morning or late evening Sun we're talking about, this is the blinding midday Sun. Any surface the light falls on is going to be overwhelmingly bright unless the light is highly diffused or spread out. So bright that most people block the direct light with curtains just so they can see the darker areas inside their house.

Also, any time you look outside the window you're going to be looking into the Sun. It's very bright and very uncomfortable to have in your field of view, so you can consider whichever window you're using for this project to be useless to look out of.
That is a good point. Perhaps a security mirror would be an appropriate size and have suitable coverage.
I am reminded of playing with a big, gash mirror when I was a lad. The effect was very localised (of course) and just like the shadow image you would get with the Sun shining through a hole. Good for upsetting neighbours iirc.

There is a several years late Kickstarter project attempting to redirect sunlight with a mirror that rotates to follow the sun. Google “Caia” - you may find it interesting.

jumpjack, Merlin3189, berkeman and 3 others
So it's all been done before. More or less an off-the shelf product.
Impossible to think of something original nowadays
Funny to see they can fill whole videos showing the tightening of nuts

harborsparrow and Merlin3189

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