Please note that the mirror of concave mirror is not a sphere, but a paraboloid. Our ancestors began to use this principle to make fires as early as the Zhou Dynasty more than 3,000 years ago. Below is an ancient bronze concave mirror named "Yang Sui". ?
In this article, I will teach adults to make a concave mirror that can make a fire with the most common material-cans. First, choose a jar. We will polish the concave surface of the tank bottom to make concave mirror. Pop cans are made of aluminum and iron. Personally, aluminum is better, because it is softer than iron, easier to polish and has better reflective performance. The one on the left is made of aluminum and the one on the right is made of iron. Generally, coke cans are made of iron, and the concave surface on the left in the picture is smooth, which is for beer. ?
First, use sandpaper to polish off the trademark and protective coating on the bottom of the can. The finer the sandpaper, the better. I use 1500 mesh, 32 00 mesh is better. It doesn't matter if there is no such fine sandpaper, because sandpaper will grind itself finer and finer. ?
Then find a soft cotton cloth, dip it in toothpaste, and polish the concave surface repeatedly.
If you feel dry, you can add some water, and if the toothpaste is not enough, add toothpaste. Because toothpaste contains friction agent, it has the function of grinding and polishing. You can constantly change your fingers, position and mode, which will make you feel less tired-because this action will last for a long time ... you can rinse the toothpaste with water in the middle, check the progress of the work and confirm whether there is uneven polishing. How long it takes depends on your frequency and strength, until the bottom of the tank is polished to reflect your shadow as clearly as a mirror. This is a job that requires patience and perseverance. It took me about 2 hours by myself. Actually, watching TV while doing it is not as hard as I thought. Friendly reminder: the hardware store has a special polishing agent to sell, which can replace toothpaste and can greatly save your time and energy. When the polished concave mirror faces the sunlight, you can see a small light spot focusing on your finger. Slowly change the height of the finger until the spot area is the smallest and the finger feels burning. That is the position of the focus. ?
Fix a match in the focal position, so that the sunlight can focus on the matchhead. After a while, you can see that the match head is lit up. ?
Note: Because the area of this concave mirror is very small and the accuracy of the paraboloid is not very high (after all, cans are not made for this), it is not easy to light matches with it. It may take 5 to 10 minutes in summer. At noon, the weather is clear and windless. It should be easier in areas with higher temperatures in the south. Not all cans are suitable for concave mirror. If you find it difficult to focus the light on the bottom of a carefully polished jar, what you need is to change the jar and repeat your work?
Hope to adopt,,,,,