A reflecting telescope has diameter D and and length L . The eyepiece is set per

Question

A reflecting telescope has diameter D and and length L. The eyepiece is set perpendicular to the tube at a position that is 1/10 the tube length from the opening. Additionally, the eyepiece is set such that its focus point  fe resides on the edge of the tube.

Part A - Which expression gives the total magnification for the telescope?

M=9L/10feD/fe

  M=19L/10feD/2fe

M=9L/10feD/2fe

Part B - For a nearby object that is a distance x from the opening of the tube, how far must the eyepiece be moved to achieve focus for the telescope?

For this part, let the focal length for the objective lens be fo.

For this part, let the focal length for the objective lens be .

((x+L)fo)/(x+Lfo)

(xfo/(xfo))fo

((x+L)fo/(x+Lfo))fo

M=9L/10feD/fe

  M=19L/10feD/2fe

M=9L/10feD/2fe

Part B - For a nearby object that is a distance x from the opening of the tube, how far must the eyepiece be moved to achieve focus for the telescope?

For this part, let the focal length for the objective lens be fo.

For this part, let the focal length for the objective lens be .

((x+L)fo)/(x+Lfo)

(xfo/(xfo))fo

((x+L)fo/(x+Lfo))fo

Explanation / Answer

PartA: M=19L/10feD/2fe

Part B: ((x+L)fo)/(x+Lfo)

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