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Homework Help: Converging lens and Concave mirror

  1. Jun 13, 2004 #1
    A converging lens (convex) and a concave mirror, both of focal length 5.0 cm, are arranged with their axes in line. The lens and the mirror are 10.0 cm apart and a candle is placed 3 cm infront of the mirror.

    By using a ray diagram or some other metho, find the location of all the images formed.

    If the candle and images were photographed, how many candles would appear in the picture?

    i tried the problem (roughly done on computer) and i found two images, but the teacher said there was 3. my mistake may be in the reflection of the mirror. help would be appreciated

    Attached Files:

  2. jcsd
  3. Jun 15, 2004 #2

    Chi Meson

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    First of all, I don't like the way the "photographing" of the images was put in the question. A photograph would not pick up any of the images unless pieces of paper were put at the location the real images. Without the paper the camera would not pick up these images.

    The "third" image would be created from the mirror image which would act as a new object and therefore create another image on the other side of the lens (the lens makes an image of the object and the mirror's image).

    If the camera was looking into the lens, it would only see the two images that the lens produced.
  4. Jun 15, 2004 #3
    Chi about the real part, in physics class it is often images which are real are often refered to as something you could photograph.
  5. Jun 15, 2004 #4
  6. Jun 15, 2004 #5

    Chi Meson

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    In my opinion this is a very bad way to define a "real image." A real image can be projected on a screen. You can photograph a virtual image. Any image you can see with your eyeball can be photographed. Virtual images are usually defined as those that can NOT be projected on a screen.

    You can not define a virtual images as one that can only be seen with your eye since you can also see real images with your eye.

    Bottom line, if you can't see it with your eye, you can't photograph it; if you can see it with your eye, you can photograph it (assuming were talking about visible light).

    Oh, I can't tell from your sketch which is the lens, mirror, object, or images.
    Last edited: Jun 15, 2004
  7. Jun 15, 2004 #6
    save the image its a lot bigger
    there are labesl lens
    and mirror
    the candel in the middle is the object
    the two lines to the outsides point to the two images.
  8. Jun 15, 2004 #7


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    If this is not already clear by now, the 3 images are :

    1) A virtual, upright, magnified image behind the mirror,
    2) A real, inverted, magnified image behind the lens,
    3) This is an image of the virtual image (1) from the mirror and is real, inverted, magnified and formed behind the lens, but closer than (2).
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