finding focal length.

Question

Please be aware that you need to use centimeters instead of meters for the answer in this question. This is different than the practice test.

If you build a telescope with a focal length of 6.33 m, what should the focal length in centimeters of your eyepiece be if you want a magnification of 60. Give your answer to two decimal places and include the unit cm for centimeter in your answer.

Answer 1

M = f_o/f_e

• M = magnification = 60
• f_o = focal length of the telescope = 6.33 m = 633 cm
• f_e = focal length of eyepiece

 

M = f_o/f_e

60 = 633/f_e

 

Solve for f_e.  The units will be cm.

Answer 2

magnification = (focal length of objective)/(focal length of ocular)

 

M = magnification = 60

 

focal length of objective = 6.33 m

 

focal length of ocular = focal length of eyepiece = x

 

60 = (6.33 m)/x ⇒

 

x = (6.33 m)/60 = 0.1055 m = 10.55 cm