What everyday household chemicals (cleaners, paints, detergents, etc.) contain l…

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What everyday household chemicals (cleaners, paints, detergents, etc.) contain large enough amounts of phosphor to glow under black light?

Fluorescent paints and many laundry detergents contain fluorescent chemicals-chemicals that absorb ultraviolet light and use its energy to produce visible light. Fluorescent paints are designed to do exactly that, so they certainly contain enough “phosphor” for that purpose. Detergents have fluorescent dyes or “brighteners” added because it helps to make fabrics appear whiter. Aging fabric appears yellowish because it absorbs some blue light. To replace the missing blue light, the brighteners absorb invisible ultraviolet and use its energy to emit blue light.

Is it better to use warm or cold air to defrost your windshield?

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Is it better to use warm or cold air to defrost your windshield?

If you can’t alter the air’s humidity, warm air will definitely heat up your window faster and defrost it faster than cold air. The only problem with using hot air is that rapid heating can cause stresses on the window and its frame because the temperature will rise somewhat unevenly and lead to uneven thermal expansion. Such thermal stress can actually break the window, as a reader informed me recently: “On one of the coldest days of this Boston winter, I turned up the heat full blast to defrost the windshield. The outside of the window was still covered with ice, which I figured would melt from the heat. After about 10 minutes of heating, the windshield “popped” and a fracture about 8 inches long developed. The windshield replacement company said I would have to wait a day for service, since this happened to so many people over the cold evening that they were completely booked.” If you’re nervous about breaking the windshield, use cooler air.

About the humidity caveat: if you can blow dry air across your windshield, that will defrost it faster than just about anything else, even if that air is cold. The water molecules on your windshield are constantly shifting back and forth between the solid phase (ice) and the gaseous phase (steam or water vapor). Heating the ice will help more water molecules leave the ice for the water vapor, but dropping the density of the water vapor will reduce the number of water molecules leaving the water vapor for the ice. Either way, the ice decreases and the water vapor increases. Since you car’s air condition begins drying the air much soon after you start the car than its heater begins warming the air, many modern cars concentrate first on drying the air rather than on heating it.

When a device uses two batteries, why do they have to be place positive to negat…

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When a device uses two batteries, why do they have to be place positive to negative? Are there any exceptions? – MS

Batteries are “pumps” for electric charge. A battery takes an electric current (moving charge) entering its negative terminal and pumps that current to its positive terminal. In the process, the battery adds energy to the current and raises its voltage (voltage is the measure of energy per unit of electric charge). A typical battery adds 1.5 volts to the current passing through it. As it pumps current, the battery consumes its store of chemical potential energy so that it eventually runs out and “dies.”

If you send a current backward through a battery, the battery extracts energy from the current and lowers its voltage. As it takes energy from the current, the battery adds to its store of chemical potential energy so that it recharges. Battery charges do exactly that: they push current backward through the batteries to recharge them. This recharging only works well on batteries that are designed to be recharged since many common batteries undergo structural damage as their energy is consumed and this damage can’t be undone during recharging.

When you use a chain of batteries to power an electric device, you must arrange them so that each one pumps charge the same direction. Otherwise, one will pump and add energy to the current while the other extracts energy from the current. If all the batteries are aligned positive terminal to negative terminal, then they all pump the same direction and the current experiences a 1.5 volt (typically) voltage rise in passing through each battery. After passing through 2 batteries, its voltage is up by 3 volts, after passing through 3 batteries, its voltage is up by 4.5 volts, and so on.

How does a parabolic sound collecting dish work? – C

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How does a parabolic sound collecting dish work? – C

A parabolic dish microphone is essentially a mirror telescope for sound. A parabolic surface has the interesting property that all sound waves that propagate parallel its central axis travel the same distance to get to its focus. That means that when you aim the dish at a distant sound source, all of the sound from that object bounces off the dish and converges toward the focus in phase—with its pressure peaks and troughs synchronized so that they work together to make the loudest possible sound vibrations. The sound is thus enhanced at the focus, but only if it originated from the source you’re aiming at. Sound from other sources misses the focus. If you put a sensitive microphone in the parabolic dish’s focus, you’ll hear the sound from the distant object loud and clear.

How do the automatic doors at a supermarket know when to open and close? How do …

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How do the automatic doors at a supermarket know when to open and close? How do they work? — KL

Devices that sense your presence are either bouncing some wave off you or they are passively detecting waves that you emit or reflect. The wave-bouncing detectors emit high frequency (ultrasonic) sound waves or radio waves and then look for reflections. If they detect changes in the intensity or frequency pattern of the reflected waves, they know that something has moved nearby and open the door. The passive detectors look for changes in the infrared or visible light patterns reaching a detector and open the door when they detect such changes.

I have a digital camera and when I put an IR remote control in front of the lens…

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I have a digital camera and when I put an IR remote control in front of the lens and press a button, a bluish white light is visible on the camera’s monitor. Why is that? — MC

What a neat observation! Digital cameras based on CCD imaging chips are sensitive to infrared light. Even though you can’t see the infrared light streaming out of the remote control when you push its buttons, the camera’s chip can. This behavior is typical of semiconductor light sensors such as photodiodes and phototransistors: they often detect near infrared light even better than visible light. In fact, a semiconductor infrared sensor is exactly what your television set uses to collect instructions from the remote control.

The color filters that the camera employs to obtain color information misbehave when they’re dealing with infrared light and so the camera is fooled into thinking that it’s viewing white light. That’s why your camera shows a white spot where the remote’s infrared source is located.

I just tried taking some pictures through infrared filters, glass plates that block visible light completely, and my digital camera worked just fine. The images were as sharp and clear as usual, although the colors were odd. I had to use incandescent illumination because fluorescent light doesn’t contain enough infrared. It would be easy to take pictures in complete darkness if you just illuminated a scene with bright infrared sources. No doubt there are “spy” cameras that do exactly that.

I work in a company shop that uses a 600-watt laser with a wavelength of 1064 nm…

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I work in a company shop that uses a 600-watt laser with a wavelength of 1064 nm. How safe is this machine? What is the radiation hazard, if any? I’ve noticed that my eyes feel strange after working with it for 4-5 hours. It also has an uncomfortable smell. — EC

The laser you’re using is a neodymium-YAG laser. It uses a crystal of YAG (yttrium aluminum garnet), a synthetic gem that was once sold as an imitation diamond, that has been treated with neodymium atoms to give it a purple color. When placed in a laser cavity and exposed to intense visible light, this crystal gives off the infrared light you describe. You can’t see this light but, at up to 600 watts, it is actually incredibly bright. You don’t want to look at it or even at its reflection from a surface that you’re machining. That’s because the lens of your eye focuses it onto your retina and even though your retina won’t see any light, it will experience the heat. It’s possible to injure your eyes by looking at this light, particularly if you catch a direct reflection of the laser beam in your eye.

In all likelihood, the manufacturer of this unit has shielded all the light so that none of it reaches your eyes. If that’s not the case, you should wear laser safety glasses that block 1064 nm light. But it’s also possible that the irritation you’re experiencing is coming from the burned material that you are machining. Better ventilation should help. High voltage power supplies, which may be present in the laser, could also produce ozone. Ozone has a spicy fresh smell, like the smell after a lightning storm, and it is quite irritating to eyes and nose.