The Old North Church: During the American Revolution, Paul Revere instructed a friend to signal him of the approach of the British army. His friend was to use lanterns in the North Church in Boston, Massachusetts: "one if by land, and two if by sea." Paul Revere would watch for the signal from Charlestown, across the river. How far apart did the lanterns have to be placed in order for Paul Revere to reliably distinguish one lantern from two? Suppose the Old North Church in Boston was about 3.3 km from Paul Revere's spot in Charlestown. Assume the diameter of Paul Revere's pupil was about 4.9 mm. You may treat the lanterns as point sources. Assume the effective wavelength of light from each lantern was approximately 644 nm, and the total power output of a lantern was 56 W.

The Old North Church: During the American Revolution, Paul Revere instructed a friend to signal him of the approach of the British army. His friend was to use lanterns in the North Church in Boston, Massachusetts: "one if by land, and two if by sea." Paul Revere would watch for the signal from Charlestown, across the river. How far apart did the lanterns have to be placed in order for Paul Revere to reliably distinguish one lantern from two? Suppose the Old North Church in Boston was about 3.3 km from Paul Revere's spot in Charlestown. Assume the diameter of Paul Revere's pupil was about 4.9 mm. You may treat the lanterns as point sources. Assume the effective wavelength of light from each lantern was approximately 644 nm, and the total power output of a lantern was 56 W.