As is known, The best protection against these dangerous rays is glass. Modern high-quality plastic lenses contain a special filter that blocks ultraviolet rays, but it is impossible to determine the strength of this ability in specific glasses by eye. That is why the urgent question is how to determine whether glass or plastic lenses are in the frame you like.
When might it be necessary to check?
When buying glasses in a company store, no one thinks about whether it is glass or plastic; a good brand has all the lenses of excellent quality, and there is no doubt about the safety of the eyes. But if you buy glasses not from official dealers, but in a market or shopping center, there is a need to find out what material the glasses are made of.
How can you tell if your glasses are glass or plastic?
There are several ways to check what material is in front of you. All possible tests are based on the properties of these materials: hardness, weight, melting point, thermal conductivity.
Visual method
In some cases, identifying a plastic lens in glasses can be very easy, you just need to put them on and then lift them by the bow towards your eyebrows. The image should not jump or distort.
By the way! Modern plastic glasses are not amenable to such testing, but cheap plastic will not withstand it.
How to tell by knocking
The most logical way to recognize a glass lens is to tap it with an iron coin. Ideally, it is good to have a glass object with you to compare the sound: it should be sonorous and drawn-out, but with plastic it is dull. On various forums it is suggested that you hit the lens on your tooth; the sound and sensations cannot be confused; how it will look in an optics store is up to you to decide.
By weight
The density of glass is much higher than plastic, which means glass lenses that are the same size as plastic lenses will be significantly heavier. Many people often complain about the discomfort of wearing glasses with glass lenses; under their weight, the nose pads put a lot of pressure on the bridge of the nose and leave marks.
To understand what material the lenses are made of, you need to take with you for comparison glasses with plastic or glass lenses and weigh in your hand alternately those chosen in the store and then your own. You will feel the difference in weight. Or ask the seller to give you plastic glasses for comparison.
Mechanically
A distinctive property of glass is scratch resistance. Take the iron key and run it across the lens. There will be no marks left on the glass surface, which cannot be said about simple plastic.. Modern manufacturers coat plastic lenses with a special protective coating against scratches, so this method may not always work.
Of course, the proposed verification is quite dubious when purchasing in a store.But if the seller assures you that this is glass, then you can offer him to check it mechanically. If you don’t agree, don’t buy from this seller.
Heat
The melting temperature of plastic is approximately 165 degrees, glass - at least 750. The plastic will begin to melt when heated with a simple lighter.. There is a more gentle way to check the temperature of your glasses: take a needle and heat its tip with a lighter, and then prick your glasses with it in an inconspicuous place. If there is no trace of the injection left, there is glass in front of you.
Temperature
Glass has significantly lower thermal conductivity compared to plastic. Therefore, the glass in the room will be slightly cool, and the plastic will be slightly warmer. True, it is quite difficult to feel the difference by touch, but if you have an infrared thermometer, then by measuring the temperature of plastic and glass lenses with it, the difference will be approximately 1–2 degrees (glass is cooler).
Importantso that both couples lie in the same conditions for about 15 minutes, otherwise the experiment will be inaccurate.
What devices determine this and where can I find them?
If the matter is not about the current purchase, but requires an accurate determination, simple methods cannot be used, you need a special device - spectrometer. This sophisticated instrument allows you to perform a spectral laboratory analysis of any material and find out its composition.
The only one, but significant The disadvantage of the method is that the glasses will have to be disassembled, and take a sample (piece) of the lens. This method is relevant in the case of quality control of a specific product. There is no point in keeping this device at home, although it is quite possible to purchase it in specialized stores. Or you can submit a piece of material for analysis to a neighboring laboratory.
For a simple buyer, it is not the material that is important, but its quality.Recently, owners of both sunglasses and corrective glasses are increasingly giving preference to plastic lenses. They are more comfortable to wear, less dangerous and affordable. Check that the seller has a quality certificate for the selected pair and you will be sure that nothing threatens your eye health!
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