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Convex Lens Question

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    We were going through a question sheet set by our teacher today, and usually he likes to set a few 'outside of the box' ones to think about. However, he didn't go through this in class today and I wanna know the answer!

    "A student takes a convex lens and a piece of white card to a window. She moves the lens until an image of a distant building is seen on the card. Another student takes a second piece of card and gradually covers the lens, starting at the bottom."

    Consider the following possible consequences:

    1) The top of the image will disappear
    2) The bottom of the image will disappear
    3) The image will become less bright
    4) Nothing will happen until just before the entire lens is covered

    Select the appropriate option
    A
    1 and 3
    B
    2 and 3
    C
    1 only
    D
    2 only
    E
    3 only
    F
    Some other option or combination

    Anyone know?

    Thanks
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    I suspect the top of the image will slowly disappear because a convex lens produces an image that is upside down.

    Thus, the top of the building will be at the bottom of the card and the bottom of the building at the top of the card.

    Click image for larger version. 

Name:	550px-Lens3.svg.png 
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ID:	134311
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    (Original post by LukeySherlock)
    We were going through a question sheet set by our teacher today, and usually he likes to set a few 'outside of the box' ones to think about. However, he didn't go through this in class today and I wanna know the answer!

    "A student takes a convex lens and a piece of white card to a window. She moves the lens until an image of a distant building is seen on the card. Another student takes a second piece of card and gradually covers the lens, starting at the bottom."

    Consider the following possible consequences:

    1) The top of the image will disappear
    2) The bottom of the image will disappear
    3) The image will become less bright
    4) Nothing will happen until just before the entire lens is covered

    Select the appropriate option
    A
    1 and 3
    B
    2 and 3
    C
    1 only
    D
    2 only
    E
    3 only
    F
    Some other option or combination

    Anyone know?

    Thanks
    The easiest way to find out the answer is to actually do this.

    Use a pair of spectacles.
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    (Original post by Ari Ben Canaan)
    I suspect the top of the image will slowly disappear because a convex lens produces an image that is upside down.

    Thus, the top of the building will be at the bottom of the card and the bottom of the building at the top of the card.

    Click image for larger version. 

Name:	550px-Lens3.svg.png 
Views:	58 
Size:	14.3 KB 
ID:	134311
    Look at the picture you have attached and see the various routes light can take to each point on the image. You might come to a different conclusion.
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    It just gets fainter.

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