Let There Be Lights Trivia Quiz

Answer: A device with exactly two connectors (di = two)

Originally, -ode names were used to identify various types of vacuum tubes. A diode had two terminals, anode and cathode. A triode had three (adding a gate). There were also tetrodes (4 terminals) and pentodes (5) of various designs. The function of a diode tube was to rectify an alternating current, because, by design, it can pass current only in one direction. Today, diodes still have this primary function, so the name is used for the semiconductor element as well.

The other names have mostly fallen into disuse - the semiconductor equivalent of a triode is a transistor.

Answer: Gallium

All LEDs emitting visible light are made of a semiconducting alloy and with one single exception (zinc selenide), all of these alloys contain gallium combined with one or more group 13 and/or 15 elements (boron, nitrogen, aluminum, phosphorus, arsenic and indium). The interaction between the electrons of these trivalent elements creates the necessary conditions for the light-emitting process.

None of the other three options is a semiconductor, although all of them have been crucial components of various kinds of light sources.

Answer: Electrons and holes

Semiconducting materials derive their electrical properties from free electrons (those not tightly bound to atoms) and holes (locations in the atomic lattice that are missing an electron). The more electrons and holes exist at any one time, the larger a current can flow, but this will not release any significant energy by itself. Only when an electron fills a hole will energy be released in the form of a photon - a light particle - as that electron is then more tightly bound. Of course to keep this process going, new free electrons and holes need to be created; that is achieved by the electric energy flowing into the LED.

Answer: A significant percentage of the light can never leave the semiconductor crystal.

No, antiphotons do not exist. I made that one up. The main cause of energy loss in LEDs is simply that the light never makes it out of the crystal because it is reflected internally. If you have ever seen a polished wafer of a semiconducting material, you may have noticed that it makes an excellent mirror - and the same applies from the inside: Photons that are not emitted almost exactly perpendicularly to the surface will get reflected over and over until they finally hit an atom and get absorbed. LED makers try to counteract this by either trying for a spherical shape (which is theoretically ideal but very hard to achieve in practice) or creating an irregular surface with many different angles so that more photons can escape.

Answer: 30 to 40 percent

While LEDs have matured from being low power devices useful only for indicators to one of the most efficient electric lighting source available in little more than a decade, they still convert less than half the available energy into visible light. Nonetheless, they effectively replaced the far less efficient variants of the venerable incandescent bulb as the standard light source within a decade, reaching similar brightness and other vital parameters in a comparably small package with less than a fifth of the energy consumption.

Answer: Pink

The light emission spectrum of an LED is a very narrow band - a nearly pure color. However, unlike a laser, an LED is not entirely monochromatic as there is some minor variance in the wavelengths emitted. However that still limits LEDs to spectral colors - pink and white LEDs are either made up of several separate chips in the same case (producing different wavelengths that add up to the desired color) or need to convert some of the emitted light to a different wavelength.

Answer: Blue

Phosphors can convert light colors very efficiently, but for physical reasons, the emitted light will always be of a longer wavelength (and lower energy) than the color absorbed. Thus, a yellow or infrared base would never be able to create a full white spectrum. An ultraviolet LED could work, however this would incur a loss as some of the light would actually escape in the ultraviolet spectrum. Using a blue LED with either a yellow-emitting phosphor (for simple and high power applications not requiring accurate color reproduction, for example some kinds of street lights or industrial lighting) or two phosphors, emitting red and green respectively, for a full color spectrum, yields a highly efficient white light source.

Answer: The one on alternating current will be half as bright as the one on direct current.

LEDs, like any diode, function as rectifiers, thus they let current pass in only one direction. If voltage is applied in the opposite direction, they are effectively insulators and thus break the circuit. The LED on AC will thus be on half of the time and off the other half, effectively providing half the brightness it could deliver on a direct current of the proper polarity. If the frequency of the alternating current is low enough, you may notice some flickering as well.

Providing a pulsed voltage is the standard way of dimming LEDs, although this will usually not be realized with alternating current but rather with a pulsed direct current.

Answer: Warm white LEDs are subjectively seen as warmer light that cold white ones.

The "warm" and "cold" designation reference the subjective perception of the emitted light. While this is measured in color temperature, this measurement is seemingly backwards: a high color temperature indicates a more bluish, cold white while a low one indicates a "warm" spectrum similar to that of an incandescent bulb or a fire with dominant red and orange elements.

On the engineering side, warm and cold white LEDs typically only differ in the types and amounts of phosphors used; the underlying semiconductor and thus the operational parameters are the same. For room illumination, there are now LED lights available with an adjustable color temperature; these either contain both warm white and cold white LEDs in the same package or directly use RGB (red/green/blue) LEDs to mix various shades of white. In either case, the additive effect of properly dimmed individual components will result in the desired light color.

Answer: The cold tree outside will shine brighter and the lights will likely last longer

In what seems a paradoxical outcome, LEDs operated at lower temperatures not only yield more light (but at the cost of also consuming more power - the resistance of the circuit is lower) but also last longer. Like almost all electronic components, heat will significantly shorten an LED's lifespan. For this reason, most higher power LEDs are equipped with intricately designed heat sinks similar to those used in computers to carry away the heat from the small active component. Using your LEDs in well-ventilated, cool places thus prolongs their life and, as a side effect, will also make them a little bit brighter.