1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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!
    a.
     
  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,
    a.
     
  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

    jtbell

    User Avatar

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Positioning of a concave mirror
  1. Concave Mirror (Replies: 3)

  2. Concave mirror (Replies: 1)

  3. Concave Mirrors (Replies: 4)

  4. Concave Mirrors (Replies: 9)

  5. Concave Mirror (Replies: 3)

Loading...