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26 votes
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Why does light bend after travelling half of the lens?

It changes on both E and the exit point of the beam (see image below). source My understanding is that refraction occurs at any boundary that there is a change of the wave propagation velocity. The ...
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20 votes
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Why is it more reliable to use the land/pit transition in a CD-ROM?

To elaborate on ratchet freak's answer; CD-ROMs work by sensing the intensity of the light reflected from the CD as it is spinning. More light being reflected stands for a 1 and less light being ...
18 votes
Accepted

Can a very bright light attract an incoming laser guided weapon?

No. It might blind it but it would not attract it. Laser guided weapons aren't just laser versions of heat-seeking weapons. You would modulate the targeting laser so that it could be differentiated ...
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15 votes
Accepted

Will more electricity be generated by using a lens to focus sunlight onto solar cells?

Your equation is partly correct. You've calculated the energy per photon ($\hbar \nu$), but you've neglected the number of photons. That's why the units don't match (power is energy per unit time, ...
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12 votes
Accepted

How does a microbolometer (IR camera) work?

There are a lot of questions in your question, and it should probably be broken up into multiple different questions. I don't want to wait until that happens though so I'll address the ones that I ...
12 votes

Why aren't screens round instead of rectangular?

The earliest CRT displays were in fact round. However, that was more due to manufacturing tradeoffs of making large glass envelopes with vacuum inside than a desire for the display area to be round. ...
  • 11.3k
11 votes

Must rifle scopes have revealing reflections?

The lenses which make up the scope will always reflect some light (~0.5%). All optical materials will reflect some amount of light. This is due to the fact that light travels slower inside of ...
11 votes

Why is it more reliable to use the land/pit transition in a CD-ROM?

Because of optical interference. When the laser is fully in a pit or fully on land then it gets reflected cleanly and will be picked up by the sensor. However, on the transition, half the laser is ...
11 votes
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Why are lasers used for concentrated light applications instead of incoherent light sources?

There are many reasons why highly monochromatic light, such as that emitted by a laser, is useful for delivering a large amount of power to a small spot. First of all, incoherent light sources such ...
10 votes

Will more electricity be generated by using a lens to focus sunlight onto solar cells?

Yes, increasing the illumination on a solar cell by using lenses or mirrors increases the electric power output. However, there are limiting factors. The efficiency of a solar cell goes down with ...
  • 11.3k
8 votes

Why does light bend after travelling half of the lens?

You are right, rays refract at both the entry and the exit points. That drawing is just a simplified diagram, frequently used in textbooks about geometrical optics. It helps to illustrate concepts ...
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7 votes

How can I measure the brightness of a LED?

You can easily measure the brightness of any light source in the visible with a silicon (Si) or indium gallium arsenide (InGaAs) photodiode. Both can be picked up at relatively low cost especially if ...
7 votes
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Alternatives to laser line?

A few points that don't fit into a comment but could help you: Get a better laser source than a 4 \$ laser diode. "Better" means a laser with higher beam quality. When you look at a laser's ...
6 votes

Would it be hard to create a camera that could see the whole electro-magnetic spectrum?

Look at it this way: A simple compass will detect from zero to about 0.1 Hz your AM radio detects around a MHz your cellphone around a few GHz or so Radar systems anywhere from 100kHz to 1 THz ...
6 votes

How can I build a micrometer diameter laser beam from a common laser diode?

Why this is hard One micron is on the border of what is theoretically possible. You cannot create a parallel ray at this scale. One micron is almost as small as the wavelength of the light you are ...
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5 votes
Accepted

Focusing light from a laser shot into one point

Briefly: The way to distinguish your signal from any others (and incidentally get a massive gain in signal to noise ratio) is to use modulation. The simplest modulation scheme just turns the laser on ...
  • 466
5 votes

How does an ultrasound device determine where the reflection is coming from?

There are several types of ultrasound images. The most familiar for medical imaging is probably the B-mode type. This type of device uses a linear array of transducers to generate sound waves and to ...
5 votes

Do contiguous laser curtains exists? and if not are there any lenses that could spread a concentrated beam into a "planar" beam

This could be done with a laser (and I'll give you one idea of how), but honestly you will probably be better off with an array of infrared LEDs instead of a laser. The infrared LEDs can be treated ...
5 votes

How can I measure the brightness of a LED?

If you have a DMM with a uA current range, then a photodiode is the simplest detector. I get about 15 uA of current in my lab (PD area is 44 mm^2) An LED held up close can give more than 1 mA of ...
5 votes
Accepted

Can cameras+display screens with window/mirror-like optical behaviour exist?

The technology to do what you want has existed for decades, and it is called holography. The problem with common photographic sensors and displays is that they only record/reproduce amplitude ...
5 votes

Transparent, hard material that lets infrared through

Acrylic and glass do let 800-900 nm IR radiation through quite well (see, e.g., here). However, your problem is probably that part of the light gets reflected at each of the surfaces. This is ...
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5 votes
Accepted

Periscope camera with a mobile phone, how to make walls not visible on the camera/alternative solution?

Ok let me draw some pictures you right now have the following situation: If you add a convex lens to this setup you get: Which does work. However you may want to add a concave lens after the convex ...
  • 3,455
4 votes

How can I make a rudimentary sunspotter?

I you look up sunspotter on the internet you get many things that look like this. While this appears to do precisely what you say you don't want, i.e. imaging the sun whith a bit of calibration it ...
  • 1,025
4 votes
Accepted

Difference between continuous wave laser and a pulsed laser

Pulsed lasers release their energy in very short pulses which can have incredibly high peak powers. A run-of-the-mill nanosecond laser will have a peak power in the multi-kilowatt range while a ...
4 votes

How to see through an opaque mask?

Just a lot of pinholes (without lenses) will be enough. With a high enough density Your eyes and brain will be able to compensate the lack of complete vision pretty dang well. It'll be darker and you ...
4 votes
Accepted

How can I detect black spots?

The simplest method will be a multi-step approach. The first step will be a defective product detection, that gathers general data about the product stream and quarantines defective products. On a ...
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4 votes

Would connecting 100's of small telescopes into one huge telescope make for a significantly better telescope?

There are two issues here, noise and resolution. Combining the optical images from multiple telescopes would reduce the overall noise. This is because each image is independent of the others, so all ...
  • 11.3k
4 votes

How is a parabolic mirror made by hand?

Large telescope mirrors are made by spinning a disc of molten glass so its surface assumes a parabolic shape, and then slowly cooling it while spinning so it retains that shape after it solidifies. ...

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