chapter 9: invisible light “black body” radiation
TRANSCRIPT
Chapter 9: Invisible Light“Black body” radiation
Using infrared to “see”Law enforcementFire and rescueWeather satellites
– Detects water vapor, precursor to clouds– Surface water temp -- el nino
Heat seeking weaponsRemote controlsHeat lampsHome improvementPit vipersMosquitoes
Ultraviolet lightCauses some materials to give off
visible light (fluoresce)Lots of it in fluorescent lights (from
mercury). Converted to visible by coating on inside of glass bulb.
Has enough energy to do chemical damage -- sunburn, skin cancer, but also kills germs
Absorbed by reasonable thickness of ordinary glass
UV and the ozone layer
UV breaks up O2 molecules high in the atmosphere
O atoms combine with O2 to make O3
(ozone), a strong UV absorber
CFCs persist; F and Cl atoms react
with O3, destroying it.Crystals that form over Antarctica
speed up this process.
Medical (and other fancy) imaging
X-rays and CAT scansMagnetic resonance imaging (MRI)Positron emission tomography (PET)ThermographyUltrasoundX-ray backscatter
CAT scan (of CT scan)
Computer Aided Tomography (old)Computerized Axial Tomography (new)Computer Tomography (short)Lots of x-ray “slices”Better detectors and sources make for
better resolution and contrastSometimes contrast agent (usually
barium compound) is used, especially to image soft tissue.
Magnetic Resonance ImagingProtons in hydrogen nuclei in water
molecules act like magnetic tops that wobble around magnetic field lines
Wobbling “tops” emit radio-frequency radiation; frequency depends on strength of local magnetic field
Strength of signal tells how many nuclei are present -- how much water
Image “slices” obtained by using spatially varying magnetic field strength and “listening” for only the frequency emitted by that slice
Herniated discs“Slices” through
the heart
Positron Emission TomographySome radioactive beta decay involve
positrons (anti-particle of electrons)When positron hits electron, two
gamma rays leave in opposite directions
Find chemical that binds with tissue you want to image
Add radioactive tracer that undergoes beta decay
Lung cancer Lymphoma (note spleen)
Thermography
Ultrasound imagingHigh frequency sound waves (MHz)
have wavelength around 1 mm or less in soft tissue
These waves reflect from boundaries (changes in tissue density)
Travel time to and from the boundary gives depth; phase also helps
X-ray backscatterPut X-ray source and detector on same
side (not usual “shadowgram”)Detect X-rays scattered back towards
the source