A paraxial ray parallel to x− axis is incident at a point P on a parabolic reflecting surface, and the reflected ray becomes parallel to y− axis as shown in the figure. If the equation of the parabola is given by y2=2x, then the coordinates of P are
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Solution
Since the reflected ray becomes perpendicular to the incident ray, we can conclude that the rays make angle 45∘ each with normal at the point P.
Equation of parabola is given as y2=2x
Differentiating both side w.r.t x, 2ydydx=2 ⇒1y=(dydx)p=tanθ=tan45∘=1 ⇒y=1
Put y=1 in y2=2x⇒x=12
Comparing above equation with y2=4ax, we get a=12
So, focus of given parabola will be at (a,0)=(12,0).
And we know that reflected ray always passes through its focus and in this case, reflected ray is parallel to y− axis.
So, P(x,y)=P(12,y).
Since, point P lies on the parabola, so it will satisfy the equation of parabola. ∴y2=2×12=1 ⇒y=−1or+1
For this case, y=1 ∴P(x,y)=(12,1).