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64
votes
5answers
6k 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 ...
17
votes
4answers
2k 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?
13
votes
2answers
794 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 ...
10
votes
2answers
4k 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 ...
9
votes
8answers
9k 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 ...
9
votes
2answers
508 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
votes
2answers
123 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 ...
7
votes
1answer
153 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 ...
5
votes
4answers
6k 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
3k 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 ...
4
votes
1answer
612 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 ...
4
votes
2answers
145 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 ...
3
votes
2answers
8k 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 ...
3
votes
1answer
1k 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.
3
votes
1answer
204 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
213 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
votes
2answers
103 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
1answer
695 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
votes
1answer
68 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 ...
3
votes
0answers
195 views

Where does an 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 ...
3
votes
0answers
298 views

Nobel prize for (dangerous) LED lamps? [closed]

There are many scientific articles about the danger of LED lamps (see below from the LED magazine, from scientific american, from a governmental journal and from Harvard health newsletter): Light ...
3
votes
0answers
596 views

Increase in Wavelength of an LED immersed in liquid nitrogen

"Hi, I've been working on an experiment where I dip LEDs into liquid nitrogen and note the color change. All online sources I've seen predict that the wavelength of the light emitted by the LED will ...
2
votes
3answers
193 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, ...
2
votes
2answers
149 views

Why do we call a white led with high color temperature “cool”?

one can buy LED bulbs with defined color temperature. why cool white = many kelvins (= high temperature?) why warm white = few kelvins (= low temperature?)
2
votes
1answer
68 views

Why there's any voltage on P-N junction in diodes?

If all electrons here have their counterparts in protons, on both, P and N sides, then why there's any voltage (built-in voltage, on the bottom of the image)? Both sides should be neutral, shouldn't ...
2
votes
1answer
418 views

Why does a LED light up in reverse direction at a DC generator worked by a falling object

I have done the following experiment: A DC generator is worked by an object falling and connected to the DC generator via string. The DC generator is coupled to a LED in reverse direction. The ...
2
votes
2answers
84 views

How is the backlight in a LCD monitor dispersed equally?

How is the backlight in a LCD monitor dispersed equally? I mean are there any layers that disperse the light?
2
votes
1answer
29 views

Why does this illumination pattern of an LED light going through a small pinhole not emulate the point spread function?

I have an optical setup where a component LED passes through a 50 um pinhole, and creates an illumination pattern on an image sensor approximately 5 cm away that looks like the following: Now, I'm ...
2
votes
0answers
51 views

Interpretation of the diode constant in a LED

For a real diode the current as function of the potential difference between its terminals (for big enough voltage) is given by: $$I=A e^{\frac{eV-E}{\eta k_B T}} \tag{1}$$ So in the case of LEDs ...
2
votes
0answers
43 views

What causes this pattern in the reflection of a laser on a screen? [duplicate]

When I shine a red laser behind me at a wall I see a strange reflection occurring on my laptop screen Why does this happen? The pattern on my screen occurs exactly where the laser is hitting the wall. ...
2
votes
0answers
123 views

Formulas for Haitz's law?

The Nobel Prize in Physics 2014 is for LED. There is a related Haitz's law: ...every decade, the cost per lumen (unit of useful light emitted) falls by a factor of 10, and the amount of light ...
1
vote
1answer
144 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 ...
1
vote
3answers
162 views

In what applications are lasers necessary vs. just convenient?

I’m trying to understand, given the wide array of current laser applications: Which applications actually require laser characteristics, as opposed to just use lasers because they are the most ...
1
vote
2answers
144 views

Why does the light instantly disappear when we switch off the source?

For example i take a box which is completely covered by the most perfect mirrors possible inside and inside that box i have a bulb whose bulb holder is also covered with the most perfect mirrors ...
1
vote
2answers
63 views

Why doesn't silicon emit light? [closed]

Why doesn't silicon re-emit light when light falls on it? Or if it emits light then why we can't see it?Hence it has valence electrons.
1
vote
1answer
875 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: ...
1
vote
2answers
6k 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 ...
1
vote
2answers
576 views

Building a LED lamp, which is the brightest? 3 LEDs of 2500 mcd or 1 LED of 6000 mcd?

I want to build the brightest lamp, for it to be seen at daylight, should I use 3 2500 mcd LEDs or 1 6000 mcd? And also Why?
1
vote
1answer
392 views

Does interchangeable wavelength LED exist in NIR?

Does exist a led material that allow to interchange the emitted wavelenght in near IR spectrum? Thanks
1
vote
2answers
122 views

Do photons emitted from a LED show bunching?

If photons are emitted from a thermal source, we get photon bunching. For coherent radiation, the detection probability doesn't change after detecting a photon. For single photon sources, we get ...
1
vote
2answers
341 views

Why would mode-hopping occur as the current is changed?

If some diode laser is given an input current, why should we see mode-hopping if the current is changed?
1
vote
3answers
758 views

Intensity distribution of a LED light source

The light source: 190 high power LEDs in NIR. I have the datasheet of the LED. I want to make a graph showing the spatial distribution, and to measure the intensity of the beam on the whole field of ...
1
vote
0answers
6 views

why are triplet excitons more tightly bound than singlet?

In organic light emitting diodes, the Jablonski diagrams always show he T1 states to be lower in energy to the S1 states. They are also declared to be more tightly bound as a result. I don't ...
1
vote
0answers
91 views

Doesn't this LED created by MIT violate the second law of thermodynamics?

Here is the article I am referring to: http://www.wired.co.uk/news/archive/2012-03/09/230-percent-efficient-leds In their own words (or at least the article writer's), "it appears to draw in heat ...
1
vote
0answers
31 views

Can you grow more Biomass in a brickhouse only powered by solar cells than in a greenhouse? Will we ever be able to? [closed]

Right now I read that 1m² of solar cells generates roughly 100Watts of energy. There are really efficient Grow LEDs that use only two spikes in the lightspectrum to make plants grow. I have seen some ...
1
vote
0answers
27 views

Luminous Efficacy problem for LED. Semiconductor Physics [closed]

You have a white light LED with 100 lumens/W luminous efficacy when biased at 100 mA  and 0.8V.  Assume the diode has ideal characteristics and that luminous flux is linear with  diode current (a ...
1
vote
0answers
17 views

What size aluminum foil capacitor would power an LED?

Inspired by this video where a $9 \mu F$ capacitor is made from aluminum foil, if I had students put this together in a classroom, one question I would have is what would it take to have this power an ...
1
vote
0answers
169 views

Variable wavelength LED

Does there exist a type of LED (or other type of light source) that can change it's wavelength depending on the current/voltage it receives on a second wire? If not, would something like this even be ...
1
vote
1answer
49 views

Can magnets destroy notification light on phone? [closed]

I'm posting this question here because it's about magnets and electronics. Please tell me if it's better suited to another site! I've been messing around with some reasonably strong magnets around my ...
1
vote
0answers
36 views

Where does the reverse current come from in a organic semiconducting diode?

Unlike traditional inorganic diodes, organic semiconducting materials hold few thermally exited free carriers under room temperature, so the reverse current should be small if pinholes of the films is ...