Medical Applications of Physics

• Vacuum - 3x10^8 m/s
• EM waves travel slower through matter
• Absorbed causing heating, cancerous changes & damage to cells & tissue
• Create an alternating current with same freguency as the radiation itself
• Electromagnetic spectrum: Radio, Micro, Infra-red, Visible light, Ultraviolet, Xrays & Gamma
• Gamma: Short wavelenght, High frequency, High energy - dangerous, detect malfunction in organs, kill body tissue
• X-rays: dangerous, pass through flesh but absorbed by bone
• UV: Fluorescent lights, sunburn & skin cancer, darker skin aborbs more so less reaches deeper body tissue 
• Visibe light: Wavelenght - 300nm and 650nm, Communication - fibre optic cables, photography 
• IR: cause heating when absorbed by object, night time photography 
• Microwaves: heating in water, burns to body tissue, global communications - satellite, pass through Earths atmosphere, narrow beams
• Radio : Low frequency, Low energy, Long wave length, diffracted, broadcasting

• Short wavelength - diameter of an atom
• Pass through skin & soft tissue 
• Absorbed by bone or metal 
• Photography of bone to check for damage
• Dental problems
• Industry - metal components & welds for cracks
• Ionising - cancerous, precautions are taken in hospitals to limit dose
• Treat cancer

CT Scanners

• Computerised tomography 
• Create 2d-pictures of the inside body 
• Some sophistcated software can reassemble them into 3d image 

Charge-coupled Devices

• Form electronic images of X-rays

• Sound-like wave 
• Frequency higher than human hearing range (20KHz)
• Partially reflected when they meet a boundary between two different materials
• Distance (m) = speed (m/s) x time (s)
• Non-ionising, safer than X-rays
• Unborn babies and shattering kidney stones

Creating Ultrasound Images

• Oscilloscope
• Time x-axis & Amplitude y-axis
• Example: 

Each square on the x-axis is 0.02ms (0.00002s) we can calculate the time it takes for the sent pulse to return

4 squares = 4x 0.00002 = 0.00008s Therefore it takes 0.00008s for signal to return. If the speed of the ultrasound in the tissue is 1500m/s

Distance = speed x time = 1500 x 0.00008 = 0.12m

As this is there and back, divide by 2 to get the distance on way = 0.06 = 6cm 

• Change of direction of light as it passes from one transparent material to another
• Speed of waves will change when they cross a boundary - change direction
• Air to glass - wave slow down & angle of refraction smaller than angle of incidence
• Glass to air - wave speed up & angle of refraction greater than angle of incidence
• Cause a vritual image to appear (an image that isn't really there)

• Sin i/Sin r = n (refractive index)
• Glass - 1.6, Water - 1.33, Diamond - 2.4
• n is always greater than 1
• i < c; r < 90 : i is less than the critical angle, most of light refracted, small amount reflected back into denser medium
• i = c; r = 90 : i is equal to the critical angle, angle of refraction is 90, slightly more light reflected back into denser medium 
• i > c; TIR : i is greater tahn the critical angle, all light reflected back into denser medium, boundary behaves like a mirror  
• Sin i/Sin r = 1/n
• If i = c and r = 90, sin 90 =1 therefore Sin c = 1/n or n = 1/Sin c 
• The critical angle is the angle of incidence for which the angle of refraction is 90
• Total internal reflection occurs when angle of incidence is greater than the critical angle & all light is reflected back into denser medium
• TIR - sending visible light along optical fibres
• Endoscopes - see inside body without cutting open

• Curved piece glass - form an image
• Two types Covex - converging & Concave - diverging 
• Only convex can form real image

Diverging (concave) lens: 

• Parallel rays get refracted away from a point called priniciple focus - always virtual 

• Parallel rays of light brought to a focus at the principal focus (focal point)
• Focal length - distance from the lens to the principle focus 
• Strong lens - short focal length 
• Light refracts on both sides as it passes through the lens 

Converging (convex) lenses

• Ray diagrams 
• Object in front of lens & image opposite side of lens
• Real or virtual, upright or inverted
• Object is placed at different distances from the lens different image will be formed
• Object placed at a distance less than the focal length, magnified image will form, upright, bigger & virtual
• Magnification = image height/ object height 

• Light enters through cornea
• Cornea & eye lens focus light into retina
• Iris adjusts the size of pupil, controlling how much light enters
• Ciliary muscles alter thickness, to control focusing, attached by the suspensory ligaments
• Retina equivalent to film in camera or CCD in digital camera

• Near point of 25cm & far point of infinity
• Long-sightedness - eyeball being too short, unable to focus, can't see near objects clearly 
• Short-sightedness - eyeball being too long, unable to focus, can't see distant objects clearly 
•  Long-sightedness - converging lens, Short-sightedness - diverging lens 

• Power of lens - P = 1/f, f = focal length (m)
• lens power measured in Dioptre
• Converging len - positive power
• Diverging lens - negative power
• Focal lenght is determined by refractive index of material from which the lens is made & curvature of the two surfaces of the lens
• The greater the refractive index, the flatter the lens - lens can be manufactured thinner