Question

A parabolic mirror is kept along y² = 4x and two light rays parallel to its axis, are reflected along one straight line. If one of the incident light rays is at 3 units distance from the axis, then the distance of other incident ray from axis will be

• A. $\frac{5}{3}$
• B. $\frac{4}{3}$
• C. $\frac{7}{3}$
• D. None

Answer 1

Answer: (B)

$\frac{4}{3}$

Answer 2

Let incident rays are PA and QB. After

election both rays passes through focus (1, 0),

\ AB is focal chord.

Let point A is (at², 2at)

a = 1

\ 2t = 3 (given)

t = $\frac{3}{2}$

So point B will be $\left( \frac{1}{t^{2}},\frac{- 2}{t} \right)$

Distance of B from axis = $\frac{2}{t}$= $\frac{4}{3}$