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