Wikipedia:Reference desk/Archives/Science/2018 December 6

= December 6 =

What's the mean free path of a 400nm photon in sea level air?
Sagittarian Milky Way (talk) 02:44, 6 December 2018 (UTC)
 * Courtesy link: Mean free path. Loraof (talk) 17:50, 6 December 2018 (UTC)
 * That's within the visibilityle spectrum, and air is effectively transparent to visible light, so the mean free path is effectively infinite. See opacity (optics). 202.155.85.18 (talk) 05:31, 6 December 2018 (UTC)
 * Please stop posting incorrect answers to the reference desk. Anyone who watches a sunset can clearly see that when visible light travels through enough air (as it does during sunset) colors on the blue end of the spectrum are significantly attenuated. If the earth's atmosphere actually was "effectively transparent to visible light, so the mean free path is effectively infinite" we would be able to see the stars during the daytime, as we can when standing in lunar daylight. --Guy Macon (talk) 23:17, 6 December 2018 (UTC)
 * The answer depends a great deal on the weather conditions, see visibility. Dr Dima (talk) 06:17, 6 December 2018 (UTC) . Specifically for 400 nm (violet light) -- shorter wavelengths in the visible range tend to be scattered more in the atmosphere (that's why the clear sky is blue), so the elastic scattering cross-sections would be larger; see Rayleigh_scattering and the links therein. Dr Dima (talk) 06:28, 6 December 2018 (UTC)
 * 1/e=(1-(5.1*10**-31*2.5*10**25*(532/400)**4))**~25065 and oxygen covalent radius is slightly smaller so about 25 kilometers? Sagittarian Milky Way (talk) 00:21, 7 December 2018 (UTC)

monitoring patient's fluid levels during surgery
How does a surgeon know if a patient requires saline during surgery? After the usual 8 hours without eating or drinking before having anesthetic, you are going to be a bit dehydrated, so how would the surgeon or anesthetist detect that when you were unconscious? 49.197.52.224 (talk) 23:52, 6 December 2018 (UTC)
 * Fluid status is detected clinically—probably by an anaesthetist rather than the surgeon—based on factors such as assessment of the blood volume (blood pressure, pulse rate, CVP (central venous pressure, measured via central catheterizion), urine production, evidence of peripheral vasoconstriction based on colour and temperature) and knowledge of the pathophysiology of the pertinent disease process. Most patients are hydrated before surgery begins (when they are not eating or drinking) so they don't start out dehydrated. IV lines would be present in any case as a means of introducing medication as well as fluids. - Nunh-huh 00:02, 7 December 2018 (UTC)

(OP) Thanks for that answer, I recently had a minor operation (skin cancer removal), and was feeling very dehydrated when I was anesthetised, then woke up feeling full of fluid (peeing often for about 6 hours). Would they just have given me a lot of saline as a matter of course, or do they have a simple way of determining that I needed it? I only had an IV, no other catheters.49.197.144.237 (talk) 02:16, 7 December 2018 (UTC)
 * The need is to replace the insensible loss (loss of fluid via evaporation from the skin and lungs) as well as any fluid lost by other means (urine output (an issue only if you are catheterized or peed during surgery), blood loss, etc.) It's not an exact science.  A bit of overhydration ensures that you'll pee, which is often a requirement to permit you to leave the recovery room. (Also, you probably didn't get actual normal saline, but a more dilute fluid (D5&frac12;NS, or something similar). That said, it's also unlikely that you'd need a lot of fluid during a skin operation. - Nunh-huh 07:01, 7 December 2018 (UTC)

Next time you schedule surgery, talk to your doctor about "the usual 8 hours without eating or drinking". The American Society of Anesthesiologists and the Association of Anaesthestists of Great Britain and Ireland recommended no food for 6 hours before surgery and no drink for 2 hours before surgery. See Preoperative fasting. That's before the time you get anathematized for the surgery, not when they ask you to show up for prep; you will typically be put on an IV an hour before they put you under and start cutting. 2 hours is actually safer for you than 8 hours. The danger they are trying to avoid is you vomiting while under anesthesia and your breathing sucking into your lungs. This is less likely if you drink fluid up to 2 hours before surgery. --Guy Macon (talk) 07:49, 7 December 2018 (UTC)