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The Sky in a Room

Giorgio Carboni, May 1996
English version revised by Ed Vogel



The main component: the converging lens
Lens on a window
Lens on a wooden board
Lens on a roll shutter


This time, we will deal with an optical experiment easy to do, but which, in spite of its simplicity, is able to give you a continuous marvel. What you have to do is simply mount a lens on the bedroom window. In this way, when you wake up in the morning, you will admire the outside scene projected on a wall of your bedroom (figure 1).

If your window is turned to East, you will be able to enjoy the spectacle of dawn and the sunrise. This show changes every day, according to the seasons and the weather conditions, and you will never become tired of observing it. During a storm, lighting will seem to fall in your room. If you live in a City, during the night, you will see car lights to chase each other; the building in front of your house, open one eye or the other; you will see street lamps far away. Some nights, if you have the window facing the favorable direction, you will see the Moon run among the clouds.

For centuries, people experienced the observation of the world behind closed windows. If a knot in the wood of which the window was made just fell, the image of the outside world was formed on the opposite wall. This image is formed because light travels in straight lines. Hence, the rays coming from every object, passing through a thin hole, will attain distinct points on the wall. Those which are coming from a lower point, passing through the hole, will reach a high position on the wall, and vice versa. For this reason, the images produced are inverted.

The optical principle on which this experiment is based is that of the lens which creates images. You can read how this lens works in the section "From Lenses to Optical Instruments" of this Gallery.

This experiment is suitable also to explain the concept of camera obscura and how man has gone from the camera obscura to the photographic camera. The camera obscura is simply a dark room, which can have dimension ranging from many meters to a few centimeters. With a simple hole in a window, an image of the external world will be formed on the opposite wall. Usually, this image is faint. To increase its brightness, you should enlarge the hole. Unfortunately, in doing so, the image becomes more confused. In order to obtain a brighter and a clearer image at the end of the XVI century, G.B. Della Porta suggested the use of a lens. Many artists of the renaissance used this system to draft their views. This method was particularly useful in taking architectural scenes, with the correct perspective.

For long time, people looked for chemical compounds sensible to the light to the aim of allowing the light draw the images by itself. The photographic camera born as a consequence of the success in these attempts. In fact, the photographic camera can be considered a camera obscura with a film: a surface sensible to the light, able to record it.


Now, let's to see how you can make the experiment of the camera obscura, which will allow you to enjoy having the sky in your room and, in some manner, your soul outside. Its main component is a converging lens with a focal length about equal to the distance between the lens and the wall where the image is formed. Usually, this value will be comprised between 4 and 5 meters. You can buy the lens at the optician's. A meniscus lens for eyeglass, and as cheap as possible, is good.

Unlike all the people in the optics field, the eyeglass industry is the only one that measures focal lengths as diopters. So, when you go to the optician to buy the lens for this experiment, he will ask you how much power, or diopters, you want it. This simple formula allows you to pass from the focal length to the diopters:

D = 1/FL

D = diopters
FL = focal length of the lens (expressed in meters!)
Besides,  people place the sign "+" before the power of a converging lens and the sign "-" before the power of a diverging lens. For this experiment you need a converging lens.

Let's make a couple of examples:
- a converging lens of half a meter of focal length has a power of +2 diopters. In fact: D = 1/0,5 = +2
- a converging lens of 4 meter of focal length has a power of +0.25 diopters. In fact: D = 1/4 = +0,25

You cannot find in the market lenses of every value of focal length, but only of fixed values of 1/4 of diopter, so: +0.25  +0.5  +0.75  +1, etc. With difficulty you will be able to find in the market values such as +0.2 or +0.3 diopters which could be handy for you. In any case, ask to the optician for the lens of the theoretical power you need. It is his task to look for the one closer to the ideal measure. To this purpose, it is better to have a focal length shorter than the room width rather than longer. So, at least the objects close to the window could correctly focus on the wall. In terms of diopters, a slightly more powerful lens is better than a less powerful one.

Ask the optician to reduce the lens diameter so that it can settle down precisely in the bottom of a 24x36 film container, as shown in figure 2. This work is commonly performed by opticians to fit lenses into eyeglass frames, therefore do not be afraid to ask that to the optician. After this operation, make a case for your lens as that shown in figure 2. According to the type of window you have, you can mount the lens in different ways, some of which can be permanent, other temporary.

The mentioned below company produces lenses on demand. You can order one or more lenses with the diameter and focal length you need. As the price of one lens doesn't differ greatly from that of more, buy a few of them in order to have them in reserve.
SILO SRL Via di Castelpulci 14/D - Badia a Settimo 50018 Scandicci (FI) -  Lenses and other optical items.


Usually, in USA, houses have windows glasses sliding along the vertical direction. Outdoor light is attenuated by curtains and the room is not completely darkened. In order to make our experiment, it is necessary apply to these windows an opaque roll shutter which entirely halt the light. In fact, light must pass through the lens only. To this purpose, the shutter has to be fully opaque, and it must slide along two lateral guides. In commerce, you can find a shutter of this kind and you have only to saw it to the right width and then insert it in the window opening as shown in figure 3. If you do not find this type of shutter, you can buy an anti-mosquito roller web, and to replace the web with a black plasticized tissue, or a black plastic strong film.

After mounting the lens in its case, you have to insert it in the hole of the lens holder which you have to fasten to the window (fig. 3). When you will have inserted the shutter on the window opening, you must open a hole in the tissue in order to allow light entering the room. Take care to close every other light source, otherwise your faint image will lose contrast.

This method is easy to perform, and allows you readily open the window to aerate the bedroom during the day.



In some other countries, the houses are constructed with heavy rolling shutter already fitted onto the windows. Usually, these shutters are made of wooden or plastic slats, which are hooked each other and slide along two lateral guides. When these shutters are completely down, never a light ray is entering the room. In this case, you can mount the lens in two manners: a temporary one and a permanent one.

The temporary solution involves mounting the case with the lens in a hole made in a wooden board. Place the board on the window-sill and pull down the shutter as you can see in figure 4. Take care of not let light to filter among the slats. As the board must be shorter than the windows width in order to be put in place, you have to close the remaining opening also.


Permanent solution requires the authorization of your wife: a thing not easy to obtain! The question is to make a hole in a slat in order to insert the case with the lens in it. Figure 5 indicates how carry out this hole and fit the case in it. The plastic slats have two walls, one inside, the other outside. So, you have to make two coaxial holes of different diameter (fig. 5). You can easily make the internal hole with an annular drill. You can do the external hole with a circular path of little holes, removing the central part, and finishing with a half-round file. When you finish working the hole, place some glue and insert the lens case into the slat.

I have been successful in obtaining from my wife the authorization of making the hole in my shutter, and I have been very glad of this solution. From an aesthetical point of view, the work is not nasty to see and even my wife has not grumbled. The shutter works as usually, carrying and rolling the lens without any problem.

The solid corner of the light yielded by the lens is near 180. This means that it will project the image on every wall of the room, floor and ceiling comprised. The only wall excluded is that of the window. Any way, the image will be distinct only on the wall opposite to the window. The other walls will send reflected light which will lower the contrast of the principal image. To get round this problem, at about 10 cm of distance from the lens, apply a mask with a rectangular hole. In my case, as the glass of my window is at this distance, I have been able to mount this mask directly on it. In doing so, I used an adhesive sheet of black plastic. This mask is useful also to stop the light which otherwise would arrive on your eyes when you are sleeping.


Every morning, when I wake up, I see in my bedroom the panorama of the external world. I know if there is the Sun, or if the sky is cloudy. Not only, but I watch also people walking in the park and trees swaying in the wind.

To have the external world inside your room, even only as an image, will improve remarkably the sleep quality because you feel yourself in some way out of the room and that will give you a great sense of freedom. As that fine song said: "The room has not more walls...". I think that this experiment has also a therapeutic capability for many people who are depressed or who have sleeping problems. However, pay attention when you are sleeping, because if the Moon disk should lie on your skin during a night of full Moon, you could be enraptured in a fantastic dream.

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