Old 12-12-2015, 04:10 PM #17
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Default Re: Question about wavelength and power

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Originally Posted by FUBeaches View Post
i'm not a pro at this but i remember shorter wavelength have higher frequency.(frequency is a rate which energy is transfer over the period time). Violet have frequency of 8 x10 to the power of 14 while Red have 4 x 10 to the power of 14 which means a 405nm laser will deliver twice as much photons compare to 660nm at any given time.

For a constant amount of total energy the number of photons decreases as the frequency increases.



Last edited by Encap; 12-12-2015 at 07:06 PM.
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Old 12-12-2015, 05:07 PM #18
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Default Re: Question about wavelength and power

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Originally Posted by Encap View Post
Yes---some X-ray photons carry enough energy to ionize atoms and disrupt molecular bonds. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers. X-rays with photon energies above 5–10 keV (below 0.2–0.1 nm wavelength) are called hard X-rays, while those with lower energy are called soft X-rays. soft X-rays x-rays with E<1keV cannot penetrate a sheet of paper

X-rays don't penetrate everything---there is a whole range of x-ray wavelengths of different photon energies--the penetration depth varies with several orders of magnitude over the X-ray spectrum.
Xrays can be reflected --here is one way: X-ray Mirror and there is a whole field called x-ray optics see: https://en.wikipedia.org/wiki/X-ray_optics ---see also:CXRO X-Ray Interactions With Matter

and can actually be seen by the human eye, reportedly. See below
"While generally considered invisible to the human eye, in special circumstances X-rays can be visible. Brandes, in an experiment a short time after Röntgen's landmark 1895 paper, reported after dark adaptation and placing his eye close to an X-ray tube, seeing a faint "blue-gray" glow which seemed to originate within the eye itself. Upon hearing this, Röntgen reviewed his record books and found he too had seen the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen had noted the same blue glow, seeming to emanate from the eye itself, but thought his observations to be spurious because he only saw the effect when he used one type of tube. Later he realized that the tube which had created the effect was the only one powerful enough to make the glow plainly visible and the experiment was thereafter readily repeatable. The knowledge that X-rays are actually faintly visible to the dark-adapted naked eye has largely been forgotten today; this is probably due to the desire not to repeat what would now be seen as a recklessly dangerous and potentially harmful experiment with ionizing radiation. It is not known what exact mechanism in the eye produces the visibility: it could be due to conventional detection (excitation of rhodopsin molecules in the retina), direct excitation of retinal nerve cells, or secondary detection via, for instance, X-ray induction of phosphorescence in the eyeball with conventional retinal detection of the secondarily produced visible light." from htthttps://en.wikipedia.org/wiki/X-ray
Just wanted to add this here since it expands on the last part of your post about X-rays being able to be seen by the human eye:


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Old 12-12-2015, 06:47 PM #19
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Default Re: Question about wavelength and power

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Originally Posted by Encap View Post
For a constant amount of total energy, say 50mW...
mW is a unit of power, not energy.

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Originally Posted by FUBeaches View Post
Violet have frequency of 8 x10 to the power of 14 while Red have 4 x 10 to the power of 14
You're rounding in the wrong direction. It's 7.41*10^14 and 4.62*10^14 for 405nm and 650nm respectively.

Quote:
Originally Posted by FUBeaches View Post
a 405nm laser will deliver twice as much photons compare to 660nm at any given time.
No. The opposite is true.

Using the formula E=hc/λ where hc is a Planck constant of 1.986*10^-25, we find the energy of all 405nm photons is 4.90*10^-16 joules. The energy of all 650nm photons is 3.06*10^-16 joules.

Power = Energy / Time. 1 watt equals 1 Joule per 1 second.

Therefore, a 1 watt CW laser of ANY wavelength will emit 1 joule per second. That 1 joule of energy per second will be divided evenly between the photons. How many photons per second does it take to make 1 joule per second? That depends on the photon's wavelength, as we established above.

For 1W 405nm: 1/(4.90*10^-16) = 2.04 * 10^15 photons per second.
For 1W 650nm: 1/(3.06*10^-16) = 3.27 * 10^15 photons per second.
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Last edited by Cyparagon; 12-12-2015 at 06:51 PM.
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Old 12-12-2015, 09:32 PM #20
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Default Re: Question about wavelength and power

Quote:
Originally Posted by FUBeaches View Post
i'm not a pro at this but i remember shorter wavelength have higher frequency.(frequency is a rate which energy is transfer over the period time). Violet have frequency of 8 x10 to the power of 14 while Red have 4 x 10 to the power of 14 which means a 405nm laser will deliver twice as much photons compare to 660nm at any given time.
For any given power, 405nm will delivery less less photons per unit time than 660nm would. 405nm photons have more energy, so you need less of them to provide the same amount of power (Power is just energy divided by time [P=E/t]). It may not be twice as many, I think someone mentioned that it was around 30% more for 650nm.

The calculation to work out the energy of a photon for a given wavelength/frequency is simple enough, calculating what the percentage is wouldn't be difficult.

Edit: I see Cyparagon has done the calculation.
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Last edited by diachi; 12-12-2015 at 09:34 PM.
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Old 12-12-2015, 10:03 PM #21
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Default Re: Question about wavelength and power

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mW is a unit of power, not energy.
Oops---corrected it---you are 100% right
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Old 12-13-2015, 12:16 AM #22
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Default Re: Question about wavelength and power

wow stuff i learn't at school years ago seem useless now!!.. they never go into details thanks for the clarification guys. i think this thread should be stickied.....
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