A normal human eye can detect yellow light if more than 10 photons enter into it per second. A star is generating as much power as the Sun and is emitting predominantly yellow light `(lambda = 6000 Å)`. How far is the star if our eye isbarely able to see it? It is given that intensity of solar light on surface of the earth is `I = 1400 Wm^(– 2)` and the distance of the Sun from the Earth is `r = 1.5 × 10^(11) m`. The diameter of pupil of our eye is `d = 6 mm`.

Answer

`1.64 xx 10^(19) m`