Lesson: Radiative Power of Electromagnetic Radiation Physics • 9th Grade

In this lesson, we will learn how to determine the radiant power received per unit area at a distance from a point source of electromagnetic radiation.

Lesson Plan

Students will be able to

relate the irradiance, , of electromagnetic radiation at a point that is at a distance from a point source of electromagnetic radiation to the radiative power, , of the source, using the equation ,
recognize that if the irradiances, and , of light from two sources are equal at a point at distances and from the sources, then it must be the case that ,
recall that spectral irradiance is the ratio of the radiative power received at a point to the product of the unit area and unit wavelength.

Lesson Worksheet

Q1:

A point source of electromagnetic radiation has a radiant power of 225 W. The radiation emitted by the source is all visible light, with wavelengths between 400 nm and 700 nm. Each wavelength emitted has a 1 nm difference in length to its most similar emitted wavelength. All the wavelengths are emitted with the same energy. How much radiant power is emitted at each wavelength?

Q2:

Three point sources of electromagnetic radiation are at different points on the surface of a sphere centered on the point , as shown in the diagram. The radius of the sphere is 2.5 m. What is the irradiance at the point ?

Q3:

An object that emits electromagnetic radiation uniformly is at the position , as shown in the diagram. Which of the following statements most correctly describes how the irradiance produced by the radiating object at the points , , and on the surface of a sphere centered at compare to each other?