Chapter 9: Invisible Light “Black body” radiation

Using infrared to “see” Law enforcement Fire and rescue Weather satellites – Detects water vapor, precursor to clouds – Surface water temp -- el nino Heat seeking weapons Remote controls Heat lamps Home improvement Pit vipers Mosquitoes

Ultraviolet light Causes 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 O 2 molecules high in the atmosphere O atoms combine with O 2 to make O 3 (ozone), a strong UV absorber CFCs persist; F and Cl atoms react with O 3, destroying it. Crystals that form over Antarctica speed up this process.

Medical (and other fancy) imaging X-rays and CAT scans Magnetic resonance imaging (MRI) Positron emission tomography (PET) Thermography Ultrasound X-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 contrast Sometimes contrast agent (usually barium compound) is used, especially to image soft tissue.

Magnetic Resonance Imaging Protons 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 Tomography Some 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 imaging High 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 backscatter Put X-ray source and detector on same side (not usual “shadowgram”) Detect X-rays scattered back towards the source