A source emits yellow light (λ = 589nm) uniformly in all directions and has a power of 1W. Calculate the total number of photons emitted per second and the frequency with which they impinge on a surface area of (10^−19 m2) at a distance of 1m

First, to break down this sort of question we should read each physical unit (W(atts) in this case) as its equivalent statement. ie 1W=1Js^-1, if this is not intuitive, think of the basic equation P=E/t. (Further aid in student's understanding of units and dimension calculations if required). Then think what that joule could be coming from, here it is implying an energy unit of 1 Joule is being emitted per second. We can then think of the equation E=hf which gives the energy of one photon. We can then find the number of photons being emitted by dividing one joule by the energy of one photon, this agrees with the desired units. (2.96x10^18s^-1)Now for the frequency they hit an area 1metre away we want to find the number of photons hitting per second . From "Uniformly distributed" we can think of the light extending inside the surface of a sphere, in this case with a radius of 1 metre (draw diagram). We can construct a fraction which will scale the number of photons emitted at the source- by dividing the surface area in the question by the surface area of a sphere with radius 1 metre, and multiplying this fraction by the number of photons actually emitted per second. (ans: 0.0236s^-1). As an addition, if we wanted to find the time between each photon striking, we can just invert this value (ie 1/0.0236) to give approx. 42 seconds. Key of this question is interpretation of units.