CATAGORY: Light
 QUESTION: The focal length of a telescope is the distance between the
mirrot and the point where the light rays are focused by the mirror. The
focal ratio is found by dividing the focal length by the diameter of teh
mirror. For example, if a telescope has a focal length of 48 inches and a
mirrot diameter of 6 inches, then the focal ratio is written f/8.

   a.)Find the foacl ratio of a telescope with a focal length of 154
inches
and a mirror diameter of 14 inches.

   b.)The 200 inch Hale telescope on Mount Palomar in California is so
large that the observer sits in a basket at the focal point inside the
telescope. The focal ratio of the Hale telescope is f/3.3. How far from
the
mirror does the observer sit? Give your answer in inches, and then in
feet.

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                        focal length 
(1)      focal ratio = ---------------
                          diameter
                        
                           154 in
                     =    -------- 
                            14 in
                            
                     =      11
                     
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                  *                      *        
For part (a)      * the focal ratio = 11 *
                  *                      *
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                        focal length 
         focal ratio = ---------------
                          diameter
                          
                          
substitute 3.3 for focal ratio and 200 in for diameter.


                        focal length 
(2)             3.3 = ---------------
                          200 in
                          
Solving for focal length

(3)            focal length = 3.3(200in)

                            = 660 in
                            
                            = 660 in x 1ft/12in
                            
                            = 55 ft

                          
For part (b)

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               *                        *
               * focal length = 660 in  *
               *              = 55 ft   *
               *                        *
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