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Homework Help: Positioning of a concave mirror

  1. Apr 20, 2008 #1
    lenses and mirrors??

    hi all,

    i was wondering what would happen if a convex lens and a concave mirror were set up one after another, the convex len being first to receive the light. how would this work in spot lights? i dont see howa spot light can have a tightly focused spot on a stage, i dont see how it gets there..

    when i originally tried to use the graphical method to help me figure this out i relised that i didn't know how the light would reflect off the mirror after being through the lens. does anyone have any theories?

    thank you
    Last edited: Apr 20, 2008
  2. jcsd
  3. Apr 20, 2008 #2
    If you check out the Wikipedia page on stage lighting instruments, you'll find a couple links to the primary classes of spotlights: ellipsoid reflectors and Fresnel lens spotlights. The ellipsoid reflectors are more common, and often have a barrel and gate providing for an iris that lets you control the shape of the beam.

    So, that tightly focused spot you see is a result of an ellipsoidal reflector and an iris. It is possible to obtain tightly focused spots with simpler reflectors. A parabolic reflector, for instance, used to be used in old spotlights and anti-aircraft lighting. If you just want to create a beam of light with parallel rays, you can also use a collimator (which is just a curved lens with a light at its focus). Unfortunately, I haven't posted enough to provide links, but Google should turn up a few leads.

    Hope this helps!
  4. Apr 20, 2008 #3
    Thank you aresnick it did help me out a bit.
    what if you had to use only concave and convex lenses and mirrors to create the tightly focused spot? when i drew a set up of a light, then a convex lens, then a concave mirror the mirror relfects the light back towards the lens, so would that mean i would have to tilt the mirror?
  5. Apr 20, 2008 #4
    Yep, you can use the concave mirror as a collector and the convex lens to focus the light. So if you take a look at the lensmaker's equation, you can apply that repeatedly to predict the focal length (which is where you want the stage to be if you're looking for a tightly controlled spot). You'll need to back out the radii of curvature from whatever lenses/mirrors you have available to you.

    Good luck,
  6. Apr 20, 2008 #5
    ahh i see thank you for your help, i appreciate it :)
  7. Apr 20, 2008 #6
  8. Apr 20, 2008 #7
    if the set up goes a light, a convex lens, then a concave mirror how would the light rays reflec off the concave mirror.. thats what i am trying to figgure out.
  9. Apr 20, 2008 #8


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    Staff: Mentor

    What do you want the combination of lens plus mirror to accomplish?
  10. Apr 20, 2008 #9
    to create a spot light, but only have convex lenses and concave mirrors (at varios focal lengths) as materials to do so.
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