The Parabolic Mirror

Presentation

In 1997, I was living in a Student residence and I met with JB (Jean-Baptiste), who revealed to have same interest into scientific experiences as me. He went to the next École des Arts et Métiers, where he was studding mechanics. This school had some laboratories with old machines other interesting things and once we found a wonderful parabolic mirror in the big metal box with objects destined to death. The mirror was in the residence in few seconds and then began the experience period.

Physical description

It seems like this, and think it is only a picture.

It is a big mirror of a 1m70 diameter. The weight is about 73kg and it is made of 1,5 centimeter deep glass. When we moved it to the third floor we suffered a lot, first because of the difficulty to gripp it and then because of our fear to break it. It was made in 1937 but it is still without any bigger scratches. Of course for astronomy or industrials it has become too few precise, but for experiences like these we've done, it is still perfect.

Optical effects

JB is one of the owners of this parabolic mirror. He tries to appear interesting while I am taking the picture.

The optical effects are almost the same as with a spherical mirror, but the picture appear deeper. It is difficult to observe objects that are too close but you can see great details if your target is something of further. I will say a few words about it later on this page. Since last year JB has moved to Paris and we have decided to leave the mirror in my room. The very most of people who visit me are totally shocked by these effects, and they say it hurts the eyes. There you can see that life of such experimenters as JB and me is difficult (we have to suffer every day).

Sound effects

Analogue effects can be observed with sound. One of things we've done is to put the mirror at one end of a long corridor to place someone at the other end and to listen in the center of the mirror. The result is a mystical sound coming from the mirror and some louder than expected. The second interesting way to test the sound reflexion is to put a loudspeaker in the center of the mirror. The result effect is a very finely spread sound in the whole room (I mean the sound is very pure as if you were listening next to the loudspeaker.

It is so real, that installing the mirror in afternoon sun shine, Filippos burned his pants. Befor the steel of the srew driver began to melt a great effect surprised us.

Collecting power

The screwdriver after having been a little bit heated.

In fact the whole thing is in the absolute convergence for light coming from infinity. For exemple the spherical mirror is only approximately convergent. This mirror collects the radiation power and reflect it in a sigle point the mirror center. This point is perfect only with a parabolic mirror. The result is a high-energy concentration in this point. For example, if you move a piece of paper (rather quickly) it light itself. You can do a lot of things that seem to be magic for people who have just arrived. Incredible was when we boiled water (with a little coffee to make it black) in less then 2 minutes. But the most wonderful was when we tried to melt a metallic screwdriver. It works rather well.
A little bit of smoke shows how the light converges in the mirror center.
Few days after I discuss it with my physics teacher and we calculated that in a French summer sunshine, a cubic centimeter of steel heets 500 degrees per second. It suppose of course best weather circumstances and it is very difficult to make the mirror absolutely in the solar direction, but the fact that our screwdriver began to melt in 20 seconds shows that it could be true (It was at 5 pm in Lille (north of France) and the sun wasn't very intense. What we thought about was that doing the same thing in Greece (Filippos is Greek) or in Turkey (JB is Turk) would be very dangerous. After this we tried some other tricks but they were not so exciting as this one. For example it is possible to heat a chicken placing it before the mirror center so that the temperature obtained on the chicken surface isn't too high. We didn't test any other animal. I'm thinking about my neighbour but he doesn't appear to have a lot of meat.

Conclusion

It absolutely excellent to have the possibility to test the real power of such a mirror. I recommend everybody to come visit me and ask to demonstrate it, for only $2. (The chicken is for $3) No really, it is cooool.



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