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Understanding Electromagnetic Radiation: Photons, Flux, and Intensity

This article explores the dual nature of electromagnetic (EM) radiation as both particles and waves, focusing on photons — discrete energy packets. Key concepts include Planck's constant, the relationship between energy and frequency, and the significance of flux and intensity in radiation analysis. We'll discuss how monochromatic radiation interacts with surfaces, measuring incident flux in watts per square meter, and the use of spherical coordinates to describe intensity. Additionally, the interaction of light with water, including reflection, refraction, and absorption, is examined.

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Understanding Electromagnetic Radiation: Photons, Flux, and Intensity

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Presentation Transcript

  1. METR 415/715 Monday 2/4/2013

  2. EM Radiation can be considered as particles as well as waves • Small “packets” ot EM radiation that behave like particles • Called “photons” • E= Energy carried by photon • h= Planck’s constant = 6.626 X 10-34J s • ν = Frequency

  3. Photon’s are “indivisible” • As text states on page 31 “very low intensity light deposits discrete packets of energy on a surface in a manner analogous to the occasional random splashes of fat raindrops on your windshield” • If monochromatic radiation of wavelegthλ deposits F Watts per unit area on a surface, then this corresponds to F/hν photons per unit area per unit time

  4. Flux (F) • Flux is the rate at which radiation is incident on, or passes through a flat surface • Expressed in units of Wm-2 for “broadband” radiation (radiation of many wavelengths) • The flat surface may be oriented in any direction • Monochromatic flux (Fλ) has units of Wm-2 per unit wavelength, Wm-2μm-1

  5. Broadband Flux • Encompasses a range of wavelengths. Defined as

  6. Intensity • Tells you in detail both the strength and the direction of various sources contributing to the incident flux on a surface • Spherical coordinates are useful in describing intensity • Coordinates are Φ (azimuth) and θ (elevation)

  7. Solid angle • Measured in steradians (sr) • Steradian – (ratio of area in a solid angle compared to the area of a sphere of unit radius) 4π r2, where r=1 • In differential form, a steradian would be:

  8. Reflection refraction scattering

  9. Refraction of light in a raindrop

  10. Scattering

  11. Reflectivity of water

  12. Absorption • “Imaginary” part of the index of refraction • Gives you information on how much absorption will take place

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