In the medical imaging techniques that the students have investigated, they should have identified a number of different methods of detecting the state of the patient. Each method has its particular reason for being used, and has limitations on what it can be used for. The diagnostic power of each method is described below. The reasons for the differences can be traced to basic physical properties of different sorts of waves.
There are three major categories of emitted waves/particles/rays that the students have been exposed to in the previous lesson:

1. Compressional waves such as sound waves (sonar, echolocation, ultrasound). These types of waves require a medium for transmission and will not travel in a vacuum.
2. The second minor category (for this lesson) is matter radiation; in the case of the methods surveyed so far, these include beta rays of positrons in PET scans and electrons in Electron microscopy. These rays are beams of elementary particles such as electrons or positrons (anti-electrons). They, like EM radiation, can be considered as both waves and particles. Each particle has a characteristic wavelength.
3. The final category is electromagnetic (EM) radiation or rays. These are X-rays (in CAT scans and x-rays), visible and near visible light scattering (PLSS), nanocameras (similar to PLSS with heavy metal particles used for better resolution) infra and near infrared imaging (imaging spectroscopy), infrared imaging, Radar (radio waves), and MRI (radio waves emitted by cells experiencing strong, fluctuating magnetic fields). As you can see, detection methods utilize radiation types along a large portion of the spectrum.

Students will be taught the following topics:

• Photoelectric effect, including a historical consideration of Einstein’s paper(s)
• EM spectrum

And will further explore

• How each method uses its EM radiation, and how that determines the diagnostic capability of the method.

Finally, students will be asked to consider

• The effect of a specific EM radiation, UV, on human tissues in light of what they have learned to this point. (http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html)

Photoelectric effect:
A good conversational website: http://www.colorado.edu/physics/2000/quantumzone/photoelectric.html, this site gives a very basic explanation of what Einstein described. Students should already have considered the information about compression waves and matter radiation, and should realize that they will not be part of the ensuing discussion. They may go back to these in their diagnostic activity, but not while considering the photoelectric effect and the EM spectrum. This site allows some consideration of the dual particle/wave nature of EM rays without going into too much depth.

In summary: EM radiation falls along a spectrum, based on different wavelengths of light (so the particle under consideration is the photon, a massless, chargeless particle). This radiation interacts with matter based on its’ wavelength (which describes the energy of the radiation) and its’ intensity/amplitude (which describes the number of photons interacting per unit time). Einstein’s 1905 paper on the photoelectric effect (for which he won the Nobel Prize) clarified this by describing quantized light energy, or packets of light energy. What this meant is that radiation with a given wavelength would have photons of a certain energy, and increasing that wavelength would decrease the energy of the photons interacting with matter. Increasing the amplitude of these waves would increase the number of packets of energy (photons) that interacted with matter in a given period of time, but they would all have energies determined by the wavelength.

For large parts of the EM spectrum, the energy (wavelength) of the radiation is not high enough to have harmful effects in humans (radio waves), and in fact, humans are invisible to these waves.* As the energy of EM radiation increases, the interaction with matter increases. These interactions range from vibrating electrons (infrared) to changing energy states (near ultra violet to ultraviolet) to scattering electrons (UV ‡ smaller wavelengths). It is also important to consider the properties of the matter along with those of the radiation to get the full picture of the interaction. Materials hold on to their electrons to different degrees, causing the effects of a given radiation to vary due to the material in question.

The following are good sites for more information about the above topic.
http://en.wikipedia.org/wiki/Electromagnetic_radiation, http://en.wikipedia.org/wiki/Ionizing_radiation.

*ultra high intensity, low energy waves remain a subject of research in regard to human health.