Tag: incandescent light bulbs

How does the wattage of a candle compare to the wattage of a light bulb?

A 60 watt light bulb emits about 6 watts of visible light while wasting the remaining 54 watts of electric power as other forms of thermal energy. A candle probably also consumes about 60 watts of chemical energy (the paraffin wax) but emits much less than 3 watts of visible light. The light bulb is clearly not very efficient at converting electric power into visible light but the candle is even less efficient. That’s because the candle flame operates at a lower temperature (about 1700° C) than the filament of the light bulb (about 2500° C) and the spectrum of light emitted by a hot object depends strongly on its temperature. The cooler flame emits relatively more infrared light and less visible light (particularly blue light) than the hotter filament.

What is the efficiency of a 60-watt bulb to convert electricity to light?

Since only about 80% of the heat a 60-watt bulb releases is thermal radiation and only about 12% of that thermal radiation is visible light, the bulb emits about 6 watts of visible light. A halogen bulb is a little more efficient than this and a long-life bulb is a little less efficient than this.

Does everything (all matter) emit radiation? What about if something is at absolute zero? What about if it’s inside a black hole? Does a black hole emit radiation? Are Hawking particles emitted by the black hole or are they spontaneously created? If a black hole causes particles to be created, is that the same as the black hole emitting them?

To begin with, matter always emits radiation. That’s because, at any temperature above absolute zero, the electrically charge particles in matter are in thermal motion and they accelerate frequently. Any time an electrically charged particle accelerates, it emits electromagnetic radiation. If you could cool matter to absolute zero, the thermal motion would vanish and the matter wouldn’t emit radiation. However, absolute zero is an unreachable destination—it can’t be achieved—so everything experiences thermal motion and emits radiation.

The issue of radiation emitted by a black hole is another story. For decades, people thought of a black hole as perfectly black—it absorbed radiation perfectly but emitted none itself. However, Stephen Hawking showed that a black hole does emit radiation and that it behaves like a normal blackbody: an object that emits thermal radiation characteristic of its temperature. The temperature of a black hole is inversely proportional to its mass. For black holes of any reasonable size, this temperature is so extraordinarily low that the black hole emits very little Hawking radiation.

This radiation originates in the vicinity of the event horizon, the surface inside which the black hole’s gravity finally becomes strong enough to prevent even light from escaping. At that surface, quantum fluctuations in which particles are temporarily created and destroyed can occasionally lead to the creation of a particle that escapes the black hole forever. In effect, two particles are created simultaneously, one of which falls into the black hole and is lost and the other of which escapes forever. The particle that falls into the black hole actually decreases the mass of the black hole, and the missing mass escapes with the other particle. As for whether the black hole causes this emission or is actually doing the emission, there is no difference. The only feature that the black hole has (other than electric charge and angular momentum) is its event horizon (actually a characteristic of its mass). If the event horizon is causing the particles to be created, then the black hole itself is at work creating those particles.

How can I differentiate between daylight and incandescent light?

Actually daylight is a form of incandescent light. Incandescent light is the thermal radiation emitted by a hot object such as the filament of a light bulb or the surface of the sun. But the spectrum of incandescent light emitted by an object depends on its temperature. Since the filament of an incandescent light bulb has a temperature of only about 2500° C, its light is much redder than the light emitted by the 6000° C sun. That’s why photographs taken indoors with incandescent lighting turn out so orange—the light just isn’t white, it’s orange-red. So you can differentiate between sunlight and the light from an incandescent bulb by comparing the spectrums. Look for the relative intensities of red, green, and blue lights. Sunlight will have much more blue in it than light from an incandescent bulb.

Why aren’t you supposed to touch halogen bulbs with your bare hands?

When they’re operating, halogen bulbs become extremely hot, so you certainly wouldn’t want to touch them then. But even when a bulb is cool, touching it would deposit greases and salts from your skin onto its surface. The aluminosilicate glass used in the lamp’s envelope would be weakened when these salts are baked into the glass during the lamp’s operation and the greases would scorch and darken the bulb’s surface.

Why do regular light bulbs have different effects on plants than fluorescent lights?

Regular (incandescent) light bulbs create light with a hot filament. This light is relatively reddish and contains very little blue, violet, or ultraviolet light. Since it comes from a hot, thermal source, this light covers all the wavelengths from infrared to the green and blue range of the spectrum continuously and smoothly, although its intensity peaks in the red and orange range of the spectrum. Fluorescent lights, on the other hand, create light through the fluorescence of atoms, molecules, and solids. The light is not created by hot materials so it contains certain regions of the spectrum, often including blue and violet light. Depending on the exact make-up of the fluorescent lamp, this light may include wavelengths that are particularly important to a plant’s metabolic processes.

Why does a refrigerator light last so long?

The life of an incandescent bulb depends almost exclusively on how many hours its filament has been hot. Since the bulb in a refrigerator is only on for a few minutes a day, it lasts for many years.

At what point is it more efficient to leave a light on when leaving and the returning to a room?

Since turning an incandescent bulb on and off doesn’t shorten the life of its filament significantly, you do well to turn it off whenever possible. The same isn’t true of a fluorescent tube—turning it on ages its filaments significantly (due to sputtering processes) so you shouldn’t turn a fluorescent lamp off if you plan to restart it in less than about 1 minute.

Can I produce light without using electric power?

Since light carries energy with it, something must provide that energy. However, the energy doesn’t have to come from electric power. Since objects emit visible thermal radiation when they have temperatures above about 500 C, anything that heats an object to high temperatures will make light. But light can also be made without heat. There are many ways to convert electric energy into light without making anything hot (for example, a neon sign or a light-emitting diode). But you ask about making light with electricity. The next best choice is light-emitting chemical reactions, such as those used in light sticks (liquid-filled plastic sticks that you bend to activate and which then glow bright green for about 12 hours). However, such reactions don’t produce all that much light and they consume the chemicals fairly quickly. If you are trying to produce large amounts of light without electric power, I’m afraid that you’ll have to burn sometime. That’s what people did before 1879 and the electric lamp.

How does a heat-seeking missile and a radar-homing missile work?

A heat-seeking missile studies the infrared light coming toward it from the sky in front of it. It uses a lens to form a real image of that light on an array of infrared sensors. If there is a hot object in front of the missile, that object will emit more infrared light than its surroundings and the missile’s lens will form a bright image of the hot object on one of the infrared sensors. If the bright image falls on the central sensor, the missile will do nothing—it will flight straight ahead. But if the bright image falls on one of the side sensors, the missile will turn. It will turn by deflecting its rocket exhaust so that the missile begins to rotate in flight. As the missile rotates, the image of the hot object will move from one sensor to the next and it will eventually fall on the central sensor. At that point, the missile will stop turning and will flight straight ahead. Since the missile automatically turns to head toward the hot object, it will eventually fly right into the hot object and explode. A radar-seeking missile will do that same things, except that it will look for an object that is emitting lots of microwaves (radar), rather than lots of infrared light. A radar-guided missile is much more complicated, since it must first emit a burst of microwaves and then analyze the reflected microwaves to look for something to fly toward. Many laser-guided missiles are just like heat-seeking missiles except that they look for an object that is reflecting a laser beam. The people who fire the missile simply illuminate the target with a bright laser beam and the missile flies directly toward the laser spot on the target.