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**How many photons???**

**1) A typical incandescent light bulb emits 3x10^18 visible-light photons per second. Your eye, when it's fully dark adapted, can barely see the light from an incandescent light bulb 10 km away. How many photons per second are incident at the image point on your retina? The diameter of a dark-adapted pupil is 6mm.**

Some relevant formulas I found from the textbook are E=hf, c=(lambda)(frequency), lambda=h/(mv)

But none of the above formulas seem to apply by any means to this weird question. Why does the distance 10km matter? Why does the diameter of an eye matter? How can I even use them?

I am really lost and confused. I hope that someone would be nice enough to help me out. Thank you so much!