A source of electromagnetic waves radiates power uniformly in all directions at a single frequency. At a distance of 5.50 km from the source, a detector measures the intensity of the wave to be 39.0 μW/m2.
a. What is the peak value of the magnetic field at the detector due to the wave?
b. What is the average power radiated by the source?
c. Now the detector is replaced with a perfectly absorbing sheet normal to the incident flux, with surface area 2.30 m2. What is the force on the sheet due to the wave?
I need help with the third question only. I already answered the first two. I need the procedure because I dont know how to do it. Thank you!!!!
a. What is the peak value of the magnetic field at the detector due to the wave?
b. What is the average power radiated by the source?
c. Now the detector is replaced with a perfectly absorbing sheet normal to the incident flux, with surface area 2.30 m2. What is the force on the sheet due to the wave?
I need help with the third question only. I already answered the first two. I need the procedure because I dont know how to do it. Thank you!!!!

Part (c) is about 'radiation pressure'.
For an absorbing surface:
Radiation pressure [in pascals] = (flux density [W/m²]) / ( c [ 2.99 *10^8 m/s])
= (39.0 * 10^6) / (2.99 *10^8)
= 1.30 * 10^19 Pa
Pressure = [force] / [area] rearranges to
force = pressure * area
force = 1.30 * 10^19 Pa * 2.30 m²
force = 2.99 * 10^19 N
Hope that assists.
For an absorbing surface:
Radiation pressure [in pascals] = (flux density [W/m²]) / ( c [ 2.99 *10^8 m/s])
= (39.0 * 10^6) / (2.99 *10^8)
= 1.30 * 10^19 Pa
Pressure = [force] / [area] rearranges to
force = pressure * area
force = 1.30 * 10^19 Pa * 2.30 m²
force = 2.99 * 10^19 N
Hope that assists.