General Physics L19_EMwaves Chapter 24 Light This lecture takes lots of demonstration apparatus: Transmission overhead projector with Diffraction grating after focusing lens Slit mask Square mask CMY filters RGB filters RGB filter wheels Vertical-slit lamp and variac Class set of diffraction gratings R, G, B projectors Chapter 24

General Physics L19_EMwaves Objectives Describe the transverse “medium” of electromagnetic waves.

Maxwell’s pivotal insight Last Piece of EM Theory Maxwell’s pivotal insight A changing electric field acts as a current Thought experiment: charging parallel plates E B + – I Charges accumulate on the plates The E field between the plates increases The changing E field creates a virtual current A changing E field creates a B field!

Electromagnetic Fields Faraday’s law: a changing magnetic field creates an electric field Virtual Current: a changing electric field creates a magnetic field DB DE

AC Creates Oscillating B Field General Physics L19_EMwaves AC Creates Oscillating B Field Observe here

General Physics L19_EMwaves The End Result Electric and magnetic fields are perpendicular: to each other, and to the direction of propagation.

General Physics L19_EMwaves Question Are electromagnetic waves transverse, longitudinal, or a combination of both? Transverse. Longitudinal. A combination of transverse and longitudinal.

General Physics L19_EMwaves Speed of Light (in vacuum) c = 2.9979  108 m/s

Electromagnetic Wave Energy General Physics L19_EMwaves Electromagnetic Wave Energy Non-classical result: E = hf E = energy f = frequency h = Planck constant = 6.621  10-34 J s

Electromagnetic Spectrum General Physics L19_EMwaves Electromagnetic Spectrum Demonstrate Overhead projector with grating over focusing lens. Light from slit is diffracted into separate colors in both horizontal directions.

General Physics L19_EMwaves

Temp Influences Spectrum General Physics L19_EMwaves Temp Influences Spectrum Higher T  greater power P/A = sT4 s = 5.6710–8 W m–2 K–4 Higher T  higher peak frequency lmax = b/T b = 2.898 106 nm K Demonstrate Vertical slit with variac, students each have diffraction grating. Show that light is dimmer and redder at lower temperatures. Source: M. A. Seeds, Exploring the Universe

General Physics L19_EMwaves Poll Question How do “warm” colors (red, orange, yellow) and “cool” colors (green, blue) relate to temperature? At higher temperatures, more of the light emitted is “warm”-colored. At higher temperatures, more of the light emitted is “cool”-colored.

General Physics L19_EMwaves Something to Ponder Why do hotter objects emit a greater fraction of their energy at short wavelengths (high frequencies)?

Next Time Color How we see color Why things are colored