Physics lenses: 2 identical diverging lenses are separated by 14cm. The focal length of each lens is -8.0 cm.?

Two identical diverging lenses are separated by 14 cm. The focal length of each lens is -8.0 cm. An object is located 4.0 cm to the left of the lens that is on the left. Determine the final image distance relative to the lens on the right._____?

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  • 1 decade ago
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    I'll tackle this using the method of vergences.

    Light leaves the object and diverges over a distance of 4 cm to reach the 1st lens, so the vergence of the wavefronts entering the first lens is

    V1 = - 1/(4 cm)

    The vergence exiting the first lens is the entrance vergence plus the power of the lens:

    V2 = V1+P1 = -1/(4cm) + 1/(-8cm) = -3/(8cm)

    Since the vergence is negative the light is diverging, so this lens creates a virtual image upstream (left of it) a distance 1/|V2| away, or 8/3 cm to the left of the first lens.

    That virtual image acts like an object for the 2nd lens. V3, the vergence of the wavefronts entering the 2nd lens is given by

    V3 = - 1/(14cm + 8/3 cm) since the wavefronts diverge from the location of the intermediate image formed by the first lens. Simplifying,

    V3 = -1/(16+2/3cm)

    The light exiting the 2nd lens has vergence

    V4 = V3 + P2 = -1/(16+2/3cm) + 1/(-8cm) = -0.185/cm.

    This means that the light exiting the second lens looks like it originates a distance of magnitude 1/|V4| upstream (to the left) of the 2nd lens. So the image is virtual and a distance of 1/0.185cm^-1 = 5.405 cm to the left of the right hand lens.

    [If you email me I'll send you a more detailed writeup of the vergence method I've used in some of my classes.]

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