Question
Question: The diameter of a parabolic reflector is 12cm and its depth is 4cm. The position where the bulb on t...
The diameter of a parabolic reflector is 12cm and its depth is 4cm. The position where the bulb on the axis is to be placed at a distance from vertex is equal to
A
3cm
B
2cm
C
3.5cm
D
2.25cm
Answer
2.25cm
Explanation
Solution
From the diagram (4, 6) is point on y2 = 4ax
⇒ a = 2.25
By the reflection property of parabola, any ray incident on the parabolic mirror which is parallel to the axis will reflect through the focus.
∴ The position where the bulb placed = The position of the focus.
∴ Required distance = a = 2.25