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Q.

A parallel beam of light ray parallel to the x-axis is incident on a parabolic reflecting surface x=2by2 as shown in the figure. After reflecting it passes through focal point F. What is the focal length of the reflecting surface?

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a

14b

b

12b

c

18b

d

1b

answer is B.

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Detailed Solution

given:  x=2by2

general equation of a parabola: y2=4ax , where, (a,0) is focal point.

comparing these two equations, we get,

4a=12b

 ⇒a=18b

hence, focal length: f=18b

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