The average intensity of sunlight on Earth's surface is about 700 W/m². 

(a) Calculate the amount of energy that falls on a solar collector having an area of 0.500 m² in 4.00 h.

(b) What intensity would such sunlight have if concentrated by a magnifying glass onto an area 200 times smaller than its own?

Strategy a

Because power is energy per unit time or P = E/t, the definition of intensity can be written as I =P/A = E/t/A, and this equation can be solved for E with the given information.

Strategy b

Taking a ratio of new intensity to old intensity and using primes for the new quantities, we will find that it depends on the ratio of the areas. All other quantities will cancel.