Questions tagged [light-emitting-diodes]

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68
votes
5answers
7k views

What challenges needed to be overcome to create (blue) LEDs?

In light of today's announcement of the 2014 Nobel laureates, and because of a discussion among colleagues about the physical significance of these devices, let me ask: What is the physical ...
21
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4answers
3k views

LED conversion efficiency exceed 100%

I have read this article, Which says that conversion efficiency of the LED have exceeded 100%. The results are published in Physical Review Letters In their experiments, the researchers reduced the ...
19
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2answers
6k views

Why fast eye movement splits a white LED into the RGB components?

I have a "white" LED as the notification light on my phone. When I look at it straight it looks whitish, but when I move my eyes around (or shake my head) I suddenly see glimpses of red and green (and ...
17
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5answers
5k views

Why can't single LEDs produce white light directly?

Why does production of white light using a LED require combining a short wavelength LED such as blue or UV, and a yellow phosphor coating? Why can't a single LED produce pure white light?
15
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6answers
2k views

Can someone explain the science behind MIT's 230% efficient LEDs?

I was reading Gizmodo the other day and I didn't quite understand the Physics behind this. Could anybody shed some light on how this effect actually works?
13
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1answer
1k views

What's the origin of the flattened X that appears when of a small (but powerfull enough) light is reflected in our non-powered TV screen?

The reflection of a small light (but with enough power) on our TV shows four emerging rays. The rays vary periodically in space and have a repeating pattern of the colors of the rainbow. The further ...
13
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3answers
5k views

LED Thermal Modeling (How to solve heat equation with constant heat source)

I have a mechanical design with LEDs that generate heat. I want to estimate the temperature at the LED junction vs. time, but especially at steady state. Knowing the LED voltage drop and current, I ...
12
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3answers
398 views

Spatial coherence of SLED light (width of active region?)

As far as I know, super-luminescent diodes (a.k.a. SLEDs) have a very similar structure to laser diodes, except that the front faces of the active region are tilted to prevent the sort of reflections ...
11
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8answers
23k views

Why is a 1mW laser dangerous?

In our Physics lab we have a 1 milliwatt (0.001W) helium neon laser. Despite the low power, we were cautioned not to even look at reflections of the beam as it could cause permanent eye damage - why ...
11
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1answer
498 views

Ruby fluorescence activation spectra

You might have seen the absorption spectrum of ruby: it has distinct absorption peaks near 400nm and 550nm. Looking at its absorption graph, one could expect some fluorescence when excited from a ~...
10
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2answers
908 views

Light reflecting off rain from bike light

The other day when I was cycling back from work in the rain I noticed an effect from the light on my bike. A single raindrop, as it passed the light, appeared as multiple dots or dashed that followed ...
9
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1answer
2k views

Where is the energy coming from to light my Christmas tree lights? [closed]

Today my wife gave me a little box of solar-powered Christmas tree lights. She bought them last year 2019 but had forgotten she had them so it was only today (a year later) that I took them out of ...
9
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2answers
943 views

Could anyone help explain this current voltage graph for an LED in liquid nitrogen?

I've been doing my coursework investigating LEDs at various temperatures and I've come across an interesting phenomenon which nobody I've asked has been able to explain thoroughly - wheras at room ...
8
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2answers
33k views

Does staring at a bright LED light damage your eyes?

According to this article it seems that it is the UV part of the spectrum from the Sun that causes damage to the eye. Would it therefore be "safe" to observe directly an equivalent energy density LED ...
8
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3answers
300 views

Why are blue light-emitting diodes important enough to warrant a Nobel prize? [closed]

I am using a miniature blue light as night bed room light since my childhood. For me blue light-emitting diodes almost look like miniature blue lights. I don't see any fundamental scientific ...
7
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2answers
24k views

What is the best way to collimate light emitted by a LED?

I'm new to this forum. This is half a question, half a challenge. And it's more engineering then physics but I thought I might get good insights from a physics forum. I would like to cure a UV ...
7
votes
1answer
182 views

Has the Nobel committee mixed up this years prizes for Physics and Chemistry? [closed]

The title of the question is tongue-in-cheek but the question remains: How does the Nobel committee delineate the fields when awarding work which is of such an inter-disciplinary nature. The chemistry ...
6
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1answer
4k views

Diffraction Pattern on LCD Screen

I was up late working with my red LED headlamp on and when I was looking at the black part of my LCD (Apple Retina Macbook Pro) screen I noticed this interesting diffraction pattern. I'm confused ...
6
votes
1answer
5k views

How to determine divergence of a LED source from a single biconvex lens

I'm trying to determine the divergence angle of light from a single lens that is completely illuminated by a high power LED. Most optics textbooks only deal with imaging optics and I'm having a hard ...
6
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2answers
1k views

White LEDs and colour perception

I’m very interested in LED lighting and the different spread of wavelengths that are produced compared to other types (like tungsten). Given that a White LED bulb actually produces 3 different peaks ...
5
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4answers
11k views

Can light emitting diodes be used to generate UV wavelengths?

I don't know much about light-emitting diodes, but I imaging if you had a panel of RGB diodes you could produce any wavelength of color within the visible light spectrum. However, if I also wanted to ...
5
votes
1answer
850 views

LED heat radiation

Consider two bodies in space exchanging heat solely by radiation. Let's call them a star and a planet. If the star contains a heat source and the planet doesn't, it can be shown that temperature of ...
5
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2answers
327 views

Is there something equivalent to a diode for light?

In electronics a diode is a component allowing current passing in only one direction, and blocking the other side. I'm wondering if something similar exists for visible light or other EM waves, like ...
5
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2answers
134 views

Opposing Laser Beams Turns One Laser Off

When I send two separate laser beams directly into each other one of the beams goes completely out. These are inexpensive 5mv red diode lasers. What’s happening inside the laser to make it “turn off”? ...
4
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1answer
2k views

Why did LED lit up while soldering? [closed]

I was soldering an LED when accidentally the soldering wire touched the LED's other Pin while the soldering iron was touching the other and the LED lit up, not bright but bright enough to be seen. I ...
4
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2answers
5k views

Can a UV LED cause air ionization?

I am thinking of the high power LEDs at around 250nm wavelength and an output power of some 30mW optical, in continuous mode either divergent beam or focused.
4
votes
1answer
132 views

Placement of an LED in a circuit

When an LED is connected in a circuit it needs a resistor so that excess current does not pass through it, but when a resistor is placed after the LED, it works the same as when the resistor is placed ...
4
votes
4answers
8k views

What determines the forward voltage drop for a diode?

I have always had the idea that the forward voltage drop in a semiconductor diode was related in a simple way to the bandgap energies in the semiconductor. However this is apparently not the case: ...
4
votes
1answer
51 views

Colour Temperature and the thermal temperature [duplicate]

I understand that Colour Temperature correlates with the object's thermal temperature, such as a red-hot piece of iron being ~360c and the incandescent bulb filament is about 2500c. Why are sources ...
4
votes
1answer
424 views

How to make a green LED as visually bright as a 0 magnitude star?

I'm trying to estimate the distance and power I'd need for a green LED to appear visually roughy as bright as a relatively bright star - say a visual magnitude of zero. Here is what I have so far. Be ...
4
votes
1answer
1k views

Where does a LED use energy other than emitting light?

I have a quantum formula describing what kind of photon should be emitted by an LED depending on its voltage. Of course the colour is depending on the material, but every type of LED also needs its ...
4
votes
1answer
88 views

Can white light have two spectral peaks?

I was looking at a graph on the receptive ranges of the 3 types of color sensitive photoreceptors in us humans, and I had the idea that if we place only two peaks at the right places, we'll be able to ...
4
votes
2answers
330 views

Is it possible to use fresnel lens(es) to focus an LED light on a headlamp

I was hoping that someone would be willing to offer any insight or guidance they have on a project that I am working on. We are trying to use a lens mounted on a headlamp to focus a plain white light ...
4
votes
1answer
116 views

If the energy of photons at the bottom of the visible spectrum (~720 nm) is 1.72 eV, why does a red LED light up at only 1.48 V?

I have a red LED (623 nm peak wavelength) which I am able to light up at 1.48 V. I thought the switch-on voltage should be determined by $hc/(e\lambda)$, but that would give 1.99 V for 623 nm. Even ...
3
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3answers
482 views

How to smooth the spectrum of a light source?

Could somebody please tell me if there's a reasonably cheap substance or device can I use to smooth the spectrum of a light source? For example, if the spectrum has spikes as in the blue graph below, ...
3
votes
3answers
417 views

Why can blue LEDs be used for generating white light, but red LEDs cannot

LEDs consist of pn-junctions, so why can blue LEDs be used for generating white light, but red LEDs cannot
3
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3answers
356 views

Difference in spectrum of green laser and green LED

In an experiment I conducted I used a spectrometer to find the spectrum of green laser and green led. this is what I found: LED spectrum: Laser spectrum: why is the spectral width of the LED is ...
3
votes
3answers
624 views

Is there any relation between turn on voltage of LED and the colour emitted?

It is evident from the above V-I graph that longer the wavelength of the light emitted lower is the turn on voltage. I know that turn on voltage is voltage required to have significant current that ...
3
votes
2answers
90 views

Why LED light changes color when dips into liquid nitrogen?

Setup is simple we jsut need an LED, a battery as source of power, wires for to complete the circuit and a cup of liquid nitrogen and of course gloves and goggles. When the lit led is submerged into ...
3
votes
1answer
2k views

Laser diode spectrum vs LED spectrum, why so much difference in broadness?

I've been reading about how light is produced in both laser diodes (the most common types of lasers) and LED's. The mechanism seems very similar if not exactly the same: one applies a potential ...
3
votes
2answers
143 views

Intuitive explanation: how does a LED work?

I'm looking for an intuitive explanation of how a LED works. I have a bachelor level understanding of math (I did take some physics courses as well, but nothing too advanced). I'll explain what I know ...
3
votes
1answer
5k views

How does infrared light behave through water?

I have a project involving the detection of the presence of water in a cylindrical transparent plastic container (diameter=25cm) And I cannot put any device inside, so I was thinking of using and IR ...
3
votes
2answers
2k views

Can I use fluorescent light bulbs to calculate Planck's constant?

We did an experiment today to calculate Planck's constant, $h$. We measured the knee voltages $V$ of different LEDs (red, green, orange, blue, violet) and plotted them against the frequencies $f$ of ...
3
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2answers
197 views

Can zinc nitride be used as light emitting material?

I cannot manage to find any journal papers about the applicability of zinc nitride as active layer of an light emitting diode (LED). But certain papers got mention that zinc nitride with a direct ...
3
votes
2answers
1k views

What is the mechanism behind the halo around LED lights?

Was at a musical event last night, outdoors in what might be called a "beer garden", and I noticed that many of the lights in the area were surrounded by a sort of "aura". The more distant lights (...
3
votes
2answers
352 views

Why is my measured LED activation voltage lower than theoretical?

I have done an experiment to measure Planck's constant. I measured the activation voltage of the LED for each wavelength used. I calculated theoretical activation voltage results using the energy of ...
3
votes
2answers
2k views

What are the wavelengths of the red, green, and blue lights used for making LED pixels?

LED screens in televisions and smartphones use pixels that use a combination of Red, Green, and Blue diodes that can produce any color when used in the correct proportion of intensity. My question is,...
3
votes
1answer
2k views

Is a 3 LED flashlight brighter than a single LED one? (same LEDs, same power through each LED)

Say I have one LED in a flashlight, it produces some light. Now, if I have 3 of those LEDs in the same flashlight, each LED receiving the same amount of energy than the single LED was receiving in ...
3
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0answers
28 views

Lab Construction of LED

If given a semiconductor with known bandgap, how might someone construct an experimental LED within a lab? It seems common to prepare solar cells using FTO glass plates with a paste of semiconductor ...
3
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0answers
320 views

How to interpret the UPS spectrum of quantum dots?

I have been using the ultraviolet photoemission spectroscopy (UPS) to look at the energy levels of quantum dots. My quantum dot has a core/shell structure, where the shell is supposed to have a larger ...

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