Suppose there is point source at the focal point of parabolic mirror. The observer's eyes are on the axis of the parabola. What does the observer see as he starts to move away from the light source along the axis? The amount of light (in terms of energy) would be same if he's 1 meter or 1 light year away, ignoring dissipation. What would be the difference in the eyes of the observer?
Problem: The mirror is perfectly parabolic with 1 meter in diameter. The light source is a homogeneous spheric bulb with 1 cm in diameter that produces 100 watt light. The center of the bulb is exactly located at the focal point of the mirror. What is the maximum distance from the mirror (on its axis) that the light is visible by naked eye? What are the relevant assumptions and equations for solving this problem?