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Homework Help: Light Intensity through glass blocks

  1. Nov 29, 2005 #1
    Hey dudes and dudettes, loving the forum here. Will be around quite a bit for the next year or so hopefully..

    Just a quick question, any help would be appreciated so much..

    I'm doing my A2 main coursework, that's the one you do when you're 18 for all the American types, and was wondering if any of ya could kindly help a struggling physicist..

    Basically, my coursework is shining various lights (lasers, lamp, UV, IF, etc) through various medians and seeing how it effects light intensity.

    Doing my preliminaries, I was playing around with a lamp and some glass blocks. After adding 5 blocks, it was all going smoothly. The voltage reading was increasing, meaning resistance was dropping, meaning the LDR was recieving less light, meaning Fosock was a damn happy camper right now. Nice little pattern going on there as well.. why can it never stay fine?! :frown:

    I added a sixth block, and the reading in volts went down.. meaning more light was making it to the LDR. Added a seventh glass block, yet more light was making it..

    So I was wondering.. what do you reckon this is down to? Can TIF be something to do with it, as I've only heard of it in optical fibre so far.. Can it be something to do with the light being in and out of phase where its sometimes reflected by the glass blocks edges? Or would you just blame the quality of the glass blocks, maybe imperfections deflect the light?

    I'm leaning towards it being in and out of phase.. Check this out.

    Light :block: :block: :block: :block: :block: LDR

    Some light is reflected at each block, some makes it through. This gives a different time gap for each wave of light, rather than a constant flow, yeah?

    Light :block: :block: :block: :block: :block: :block: LDR


    Is it possible that this has simply delayed the light that little bit more to make it start going back into phase again?

    Any help would be recieved lovingly, pretty much my entire A2 mark depends on nailing this quickly.. I'm gonna go on to tougher stuff in this coursework, yet i'm struggling over Prelims right now..

    I love you all, thanks for any help..

  2. jcsd
  3. Nov 29, 2005 #2
    Im doing my A2 at the moment aswell. Mine was about a pendulum, but never mind. Are you sure that there were no other light sources that could have affected the result. That would be quite a serious problem results wise. btw, if you can't figure out how the error was caused, and why they are a bit off, either ignore them and say they were caused by random error in the equipment, or don't include them in your results, that tends to work, lol:biggrin:.
  4. Nov 29, 2005 #3
    Yeah, one of my class mates is doing a compound pendulum, PM me or something if ya need any advise that I can ask him about.

    As for your answer, cheers for taking the time my friend. I'm pretty sure there was no other light source, I have a big black curtain draped over a desk in the room I'm working in, and with the light off it's pretty much pitch black.

    I reckon I've worked out the answer though, and damn it was an easy one.. My lamp is raised about two centimetres with the stand included, while the LDR is right down on the plank i'm working on. That means the light is going down at an angle, meaning some bounces off the wood of the plank and back up to the lamp, meaning more light from different paths enters the LDR.. so basic, i'm an idiot I tell ya.

    About to hit up the whole 'Milk drops in water tank' thing.. If anyone has any advise for the project in general (Pretty much just about light intensity) then post it here, I will appreciate it SO much. Also, to add, I don't know how much depth I'm gonna get in this experiment..

    I have shined this lamp through glass blocks, Im gonna shine it through a tank of water, adding milk drop by drop, and I'm gonna use a gertrude block I think its called. Also thinking about shining it through a bi-focal lens, one of the ones that turns a darked shade in sunlight? That could be pretty interesting, I'll definately do that as part of this experiment. But yeah, I'm worried.. will that be enough for a decent grade you reckon? I'm sat on a high C grade at AS, I really wanna get a B or better to get into a respectable Uni.

    Much love,

    Last edited: Nov 30, 2005
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