Questions tagged [light-emitting-diodes]
250 questions
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5 answers
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 significance of blue LEDs, which challenges had to be…
Martin
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21
votes
4 answers
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 LED’s input power to just 30 picowatts and…
hsinghal
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19
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2 answers
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 probably blue) lights coming out of there. Why?
Sparkler
- 3,324
18
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5 answers
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?
violet kiwi
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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 the flame is from the TV the greater the distance…
Deschele Schilder
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15
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6 answers
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?
Kian
- 253
13
votes
1 answer
Do all diodes emit light?
My understanding of an LED is that when electrons and holes recombine inside a pn junction, they produce a photon that leaves the diode. The energy they release is equal to the band gap of the semiconductor, as long as this is in the visible range,…
Jack
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13
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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 can estimate the power dissipated via heat (some…
Patrick
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12
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8 answers
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 is such a low power level hazardous? I have a 3W…
Thomas O
- 3,267
12
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2 answers
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 that would benefit lasing. However, I know that a…
Wylde
- 193
11
votes
2 answers
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 ~450nm source - not as good as from a 400nm source,…
BarsMonster
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10
votes
2 answers
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 the expected trajectory of the raindrop. I…
edward
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9
votes
2 answers
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 temperature, the LED gives a standard exponential…
Tom H
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9
votes
1 answer
Why do individual light spots appear when shaking an LED flashlight?
I have a hand-cranked LED flashlight, and when I shake it, the light projected onto the ground appears as individual light spots (light balls) rather than a continuous light strip. Can someone explain the principle behind this phenomenon? Is it…
ztdepyahoo
- 99
9
votes
1 answer
Where is the energy coming from to light my Christmas tree lights?
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 the box. These lights consist of a small solar…
Richard Hammond
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