A spherically spreading EM wave comes from a 1400-W source. Assume that the wave travels in free space. What is the intensity at a distance of 4.2 m ? Express your answer using two significant figures.

Given Information:  

Power of EM source = W = 1400 Watts

distance = d = 4.2 m  

Required Information:  

Intensity of Radiation = I = ?  

Intensity of Radiation = 6.31 W/m²

Explanation:  

The intensity of the electromagnetic radiation is the energy delivered by the photons.

Electromagnetic radiation are of many types including Gamma rays, X-rays, Ultraviolet rays, Visible light, Infrared, Microwave, and Radio waves.

The intensity of a spherically spreading electromagnetic radiation is given by  

I = W/4πd²  

Where W is the power of the EM source and d is the distance from the center of the EM source.

So the Intensity of radiation is

I = W/(4πd²)  

I = 1400/(4π(4.2)²)

I = 6.31 W/m²

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