Why frequency does not change when light passes through the denser medium?
What determines color -- wavelength or frequency?
I've got some lenses of brand X, but I've just got a new camera of brand Y. How can I tell if it's possible to use my lenses on my camera? I accept that I may lose a lot of automatic functionality like autofocus, aperture control and metering when using an adapter to connect the two. What's a way to implement a flexible buff/debuff system? How to check if a macro value is empty or will not create text with plain TeX conditionals? I have a macro which holds some text value, e.g. \macro{some text}. If it is empty, I would like for nothing to appear in the document, but it has some content of any kind which would produce text displayed in the document, I would like a message to appear, "Text: some text".   \macro{} would be considered empty. \def\somedata{}, \macro{\somedata} would be considered empty. \macro{0} would be considered not empty.   I have tried making a TeX conditional which can check if the value is not empty, but nothing I tried works, e.g.:  \ifx #1 {} \else     Text: #1 \fi  \ifx#1{} \else     Text: #1 \fi  \ifx #1 \nil \else     Text: #1 \fi  \ifx#1!="" \else     Text: #1 \fi   What is the correct syntax for creating a conditional in plain TeX with checks if a value is not empty? How does gravity escape a black hole? My understanding is that light can not escape from within a black hole (within the event horizon).  I've also heard that information cannot propagate faster than the speed of light.  It would seem to me that the gravitational attraction caused by a black hole carries information about the amount of mass within the black hole.  So, how does this information escape?  Looking at it from a particle point of view: do the gravitons (should they exist) travel faster than the photons? Acces VBA code combination not working, issue with exit sub? I'm very new at VBA and I can't seem to solve this problem. It seems like it should be an easy problem to fix, I just don't know how.   So what the code does: In the main form is a record. This record will be copied to another table bij pressing a button on the form. Before the copying begins, the code checks first if the 'OMnummer' has been filled in in the subform.  If not, a message box appears saying that the user should fill in the subform and the code stops running.   Secondly the code checks if the record is allready present in the other table. If so, a messagebox appears and the record will not be copied. If not, the record will be copied to the other table.   Both pieces of code work ok seperately. However, when I try to include both pieces in the same private sub, only the first one works. It may be something with the 'Exit Sub' code or I am not using the If - Then right.   Hope you can help me!  Monika   Private Sub KnopProjectVersturen_Click() On Error GoTo ErrProc  If IsNull(Me!Subform_OMnummers.Form!Omnr) Then  If MsgBox("Vul het OMnummer in. Je kan het project niet exporteren zonder OMnummer.")     Then       Exit Sub      DoCmd.OpenQuery "Qry_Depo_ControleAanwezig"  If DCount("Deponering.projectnummer", "Qry_Depo_ControleAanwezig") = 0 Then         DoCmd.SetWarnings False         DoCmd.OpenQuery "Qry_projectnaarDepot"          DoCmd.OpenQuery "Qry_ToevoegProjectDepot"         DoCmd.OpenForm "Depot_uitvoer", , , "[Projectnummer] = '" &amp; Me![Projectnummer] &amp; "' And [subID]=[subID]"     Me.Status = 8     DoCmd.Close acQuery, ("Qry_Depo_ControleAanwezig")     DoCmd.SetWarnings True  Else         MsgBox "Dit project bestaat al in de Depot_Uitvoer, verander de status in het projectformulier", vbInformation, "Example"         DoCmd.Close acQuery, ("Qry_Depo_ControleAanwezig")     End If  End If  End If    'Als er fouten zijn laat deze code een messagebox zien met het nummer en de melding.  ExitProc:  Exit Sub  ErrProc:     Select Case Err.Number     Case Else         MsgBox Err.Number &amp; "--" &amp; Err.Description         Resume ExitProc         End Select End Sub glob() - sort array of files by last modified datetime stamp I'm trying to display an array of files in order of date (last modified).  I have done this buy looping through the array and sorting it into another array, but is there an easier (more efficient) way to do this? Hotspot and Internet access together — why not? If I try to set up a hotspot in GNOME, I get the following message:  If you have a connection to the Internet other than wireless, you can use it to share your internet connection with others. Switching on the wireless hotspot will disconnect you from &lt;network name&gt;. It is not possible to access the internet through your wireless while the hotspot is active.  What are the technical reasons why I can't set up an Hotspot and be connected to the Internet at the same time? Could not load 'vboxdrv' after upgrade to Ubuntu 16.04 (and I want to keep secure boot) Some sort of battle school (no, not Ender's Game), where teens are being trained as military commanders.   The control system for their army hinges around some sort of system that requires extremely fine motor control -- being able to "push" around digital representations of military units using a special interface that interprets minute finger movements.  The boy protagonist eventually meets a girl in the battle school, and is astonished to learn that most people do not live as well as he does -- for instance, he gets a fresh jumpsuit outfit every day and disposes of the old one; this is a luxury.  One scene I recall is them kissing for a moment in some sort of underground passageway; the boy protagonist notes the exact length of the kiss, as he has been trained to have heightened time perception.  I don't recall much about other commander characters; I think they have different special abilities, and one other major character might have physical deformities, possibly because of postwar-radiation-related birth defects.  They have been training for an upcoming, prearranged final battle between all the existing national governments; the military units are very possibly robotic or entirely digital in nature, so as to avoid wasting materiel / human life.    They ultimately win the battle, just barely; I think the girl protagonist gives them an edge in the competition by hacking the timing / control system in favor of their team, causing a discrepancy which only the boy protagonist notices due to his heightened perceptions.  It was agreed that the winning government gets dominion over one of the few islands of land left that was not irradiated in some previous war; the protagonists get to live there as a reward for their victory.  I believe the boy protagonist later realizes he's gradually losing his ability to exactly quantify the passage of time.  Published no later than 2004, likely somewhere between 1980-2000, probably sometime in the '90s. Probably a novice question, but how do I correctly diagnose from a photo for what reason that camera wasn't able to focus properly?  It can be any 1 of the following   Unsteady hand Lens issue Metering Re-Calibration required in camera or ...   Is there a set of unsaid protocols that needs to be followed by a photographer?  Goal is to get a sharp and crisp image prior to editing.  [PS: I own a Nikon D5200.] Completion handlers and return values What is "cloning a hard drive" and how does one go about doing it? What does cloning a hard drive mean? Is it just simply copying over all the "CONTENT" of the hard drive EXCLUDING the operating system, drivers, all the installed programs? or is it copying the ENTIRE hard drive BIT for BIT? Is the cloned hard drive bootable?  I have an external USB hard drive and trying to make a exact copy of my computer system by cloning, so that I can connect the cloned hard drive to another machine and boot into it, or install the cloned disk on another computer system and boot into it. Is this possible? Or am I going about this totally wrong?  What is ghosting a hard drive? How to Send Byte array To Server How to handle trigger recursion I have two objects A(parent) and B(child) related by a lookup relationship. I  need to update records in object A using data loader. There is an after update trigger on object A which updates some fields on related records of object B. On object B as well there is an after update trigger which updates a field on parent object A. How can i handle recursion. No sound (speakers/jack) + microphone's dead after spilling water Sadly, yesterday I've spilled some water on my MacBook Pro (El Capitan). Before you yell at me - it was closed, turned off and it wasn't a lot of water, the laptop was wet only on the top and under - not inside.  Today I've turned him on. At first battery wasn't recharging for some reason, but SMC reset fixed the problem.  At the first booting speaker was like talking to me, hard to explain but it was weird. Also only right speaker was "talking", despite the fact that only left side of the laptop was watered. Then there was no sound at all, not from the speakers, nor from the headphones. Also microphone's not working. USB ports are okay. The water was spilled from the left side of the laptop. I know this is hardware issue, and something's broken, but I want to be 100% sure. No it's not muted. I've also tried NVRAM reset a few times, but it doesn't fix the problem :(  There might be one factor, before spilling, my laptop "died" for the first time (battery energy ran off), but I doubt it's the case. (but hey, every info matters, right?)  It wouldn't be weird for me, if only left speaker wouldn't work, but built-in microphone not working is kinda weird in this case, isn't it?  I just want to ask you guys, what else can I check, before I'll be 100% sure it's dead inside?  I've plugged in external microphone - it's not working too. tag:* links incorrectly when using Save And Copy Profile To All Stack Exchange Accounts The  tag in my Stack Exchange profile links to different sites depending on where it's being viewed:  Profile on StackOverflow:  [tag:CSS] =&gt; https://stackoverflow.com/questions/tagged/css   Profile on StackExchange:  [tag:CSS] =&gt; https://stackexchange.com/questions/tagged/css   Profile on CodeReview:  [tag:CSS] =&gt; http://codereview.stackexchange.com/questions/tagged/css    I'd like to have these all link to https://stackoverflow.com/questions/tagged/css. You can ; why not add the ability to specify the site with tags? &lt;!-- tag: stackoverflow --&gt;[tag:css] &lt;!--tag: meta --&gt;[tag:feature-request]  The current behavior also cause a bug on my :    All tags send me to find a Panda aka 404 Page Not Found. Adding boolean yes/no field in ArcGIS Desktop? How does one add a field to an ArcGIS feature class with a boolean data type? That is an attribute where the allowed value is only one of a pair such as 1/0, Yes/No, On/Off, Present/Not-present, etc.   A boolean data type is not available in the Data Type pick-list. My iPad mini retina is out of warranty,  I forgot to plug in my iPad after the critical battery warning and it went some minutes following the warning. I was not worried because same thing had happened a few times before and iPad was perfectly okay after charging it. However, this time I remember seeing a flash of blue screen immediately after it went black. Then, I plugged it in an waited for a while and tried starting it. Ever since, it fails to start hanging on the apple logo with white background screen.   I tried to connect to my Mac but nothing showed up in iTunes. I am worried now. Can I troubleshoot this or is it broken and needing repair? Origin/reason for the expression "on the bus" instead of "in the bus" This is sort of a follow up to my question .  I was told a while ago that the reason why we use "on the bus" instead of "in the bus" is because back in the day buses were open, that is, they didn't have a roof.   Is this story correct or is there another reason why the correct expression is "on the bus", in spite of the fact that when one gets on the bus, he is actually inside the bus? How to XZ a directory with TAR using maximum compression?