PhysicsLAB Amusing Problems
The Perfect Pew

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Sunday morning finds Dr. J sitting in his normal pew at church. Everything seems to be going as usual. Tripod is leaning against the outside of the building singing along with the choir. (It is a rather ruff version of Bark of Ages.) Dr. J is more tired than usual and finds it difficult to keep his eyelids from drooping. During the sermon, the drooping becomes particularly bad, and Dr. J begins to notice rays of blue light above the back row of the choir. Opening his eyes wide to investigate the source of these strange rays, he is surprised to see them disappear. After a little thought, he realizes that his eyelashes were diffracting the blue light coming through the stained-glass window high above the choir. Dr. J apologizes to the minister for his drowsiness but he eagerly awaits the following Sunday. He begins immediately to plan his personal blue-light special (Tripod thinks they are going to K-Mart.) Regular members are surprised to see Dr. J sitting in a different pew at the next service. He has correctly estimated the distance between each choir member to be 70 cm, and he has chosen the perfect pew so that a ray of blue light appears directly over each choir member's head. The pew happens to be exactly 70 meters from the back row of the choir.
 
If the wavelength of the blue light is 4.5 x 10-7 m, how many eyelashes does Dr. J have per centimeter?

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