Medical Physics and Ultrasound
Ionising and non-ionising radiation are used in medicine.
X rays, high energy EM waves, get absorbed by dense material, used for medical imaging. Ioninsing so there is a health risk.
CAT scans use intense x rays to produce an image of a 2D slice through the body. Can be used to image tissue also. Strongly ionising - large health risk.
Endoscopes reflect visible light along optical fibres to see inside the body without invasive surgery. Can even be keyhole surgery. Non ionising - no health risk.
Ultrasound, sound waves above human hearing (20kHz), is non ionising. For medical imaging. Kidney stones - ultrasound beam concentrates high energy waves at stone -turns to sandlike particles that come out in urine. No surgey and almost painless.
Prenatal foetus scanning - waves reflected off tissue boundaries and the times and distribution of the echoes are processed by a pc to form an image on screen.
Can measure the speed of blood flow as it works at real time so it can show things changing/moving. Can identify blockages in veins/arteries.
Radiation is energy that originates from a source.
Intensity of radiation depends on distance from source and what medium its passing through. - Unless passing through a vaccuum some radiation will be absorbed by the medium. The more dense the medium, the more radiation absorbed, decreasing radiation intensity e.g. sunglasses - dark - absorb light - decrease intensity
Intensity is the power of incident radiation per unit area
more intense, the more energy carried per second/higher power. the higher the power, the more energy that gets transferred per second when it hits an object
Surface area of the object affects the amount of radiation that hits it - big gets more
intensity of radiation = power of radiation / area it falls on
Refraction is a change in direction caused by a change in speed
Changes speed as it enters a different medium with different density. When waves slow down they bend towards the norm. if a wave hits a boundary at 90° it will slow down but not chsange direction. When light hits a different medium some will pass through but some will be reflected - all depends on angle of incidence. (angle of reflection same as angle of incidence).
Different lenses produce different kinds of image. Converging and diverging have opposite effects on light rays.
Converging lens - convex. Cause parallel rays of light to converge (move together) to focus at the lens's focal point.
Diverging lens - concave. Cause parallel rays of light to diverge (spread out). the focal point of diverging lenses is where the rays, when they come out the other side, appear to come from... Where they all meet from tracing the diagonal back :D
The position of the object affect the image
Object Position At 2F Between F - 2F Nearer than F
Real or virtual? Real Real Virtual
Image orientation Inverted Inverted Right way up
Image size Same Bigger Bigger (than object)
Imge position at 2F beyond 2F Same side of lens as object
Lenses cont. 2
An image produced by a diverging lens is always virtual, smaller than the object + on the same side.
Power and the Lens equation
Focal length is proportional to power. The more powerful, the stronger/tighter it converges the rays so the focal length is short.
Lens power (D - dioptres) = 1 / focal length
Power of converging lens = positive. Power of diverging lens = negative
To make a more powerful lens from a certain material e.g. glass you just have to give it more strongly curved surfaces. Power depends on shape.
Object distance 'U' - distance between object and centre of lens (m)
Image distance 'V' - distance between image and centre of lens (m)
v is positive if real image, negative if virtual image
lens equation for converging lens:
Iris - coloured part. Made up of muscles control pupil size so amount of light that enters.
Cornea - Transparent 'window'. Convex. Does most of the focusing.
Light focused on retina by lens + cornea together. Cornea has fixed power, but lens can change focusing power by changing shape. Ciliary muscle control lens- contract: lens becomes fat and more spherical can see short range, relax: thin, flatter, can see long range.
Retina covered in light sensitive cells (rods+cones - cones see color) detect light+send signals to brain via optic nerve. Brain then forms image and puts it the right way up.
Far point: furthest point the eye can comfortably focus. For normal sighted people, point is infinite - can focus on all distant objects. Near point: closest distance the eye can focus on, average adult 25cm
Short sighted far point closer than infinite. Images of distant objects focus in front of retina. Cornea+lens too powerful? Eyeball too long? Ciliary muscles cant contract enough?
Longsighted near point further away than normal. Images of near objects focus behind retina. Cornea+lens too weak? Eyeball too short? Ciliary muscles cant contract enough? Longsighted with age, lens stiffens/ciliary muscles weaken - eye loses some focusing power
Correcting vision defects
Short sight corrected by diverging lens as focuses far objects later, on retina, so can be seen. Diverging lens focal point at eyes faulty far point in order to work.
Long sight corrected by converging lens as focuses near objects quicker, on retina, so can be seen. Converging lens produces a virtual image of objects 25cm away at eyes near point.
Contact lenses sit on top of the cornea if it is the wrong shape (too weak or too powerful) and compensates for the fault. Contact lenses converging or diverging.
Laser eye surgery vaporises tissues, changing cornea shape. Slimming it makes it less powerful and improves short sight. Making it more powerful will improve long sight. Surgeon can control how much tissue taken off, improving sight perfectly.
Short and long sight corrected by glasses, contacts, or laser eye surgery
Glasses cheapest. Can be heavy, uncomfortable, unattractive. Contact lenses are convenient, quite cheap, lightweight, nearly invisible. Can be uncomfortble, fall out, can cause eye infections if misused. Laser eye surgery - permanent solution. But expensive, complication risk, might not work. Over time your lenses change shape and lose focus ability. Glasses + contacts easy to adjust. Laser eye surgery doesnt stop vision deterioating.
Snells Law and Total Internal Reflection
Every transparent material has a refractive index, which dictates how much light is refracted.
Speed of light in air about the same as in a vaccum, so refractive index of air is 1 (2d.p)
Snells law says... When an incident ray passes into a material, the angle of incidence 'i', angle of refraction 'r' and refractive index 'n' are linked/related by this formula:
Total internal reflection (TIR) depends on the angle of incidence
When light leaves a material with a higher refractive index and enters one with a lower refractive index, it speeds up and bends away from the normal e.g. glass to air. If you keep increasing the angle of incidence, the angle of refraction gets closer to 90°. Eventually the angle of incidence reaches a critical angle, and angle of refraction becomes 90°, and the light is refracted along the boundary. Once past the critical angle, there is no internal refraction, and no light leaves the medium.
Uses of TIR
You can find critical angles using Snells law.
When the incident angle 'i' is equal to the critical angle 'c', the angle of refraction is 90°.
sinC = i / r
Optical fibres totally internally reflect visible light off the plastic/glass inner core. The wave enters one end of fibre and reflects repeatedly until it emerges. The fibre is very narrow and cant bend too sharply to keep the angles above the critical angle so no light leaves the medium. Used in medical diagnosis and communications.
Endoscope - a thin tube containing optical fibres that let surgeons examine inside the body. It consists of two bundles of optical fibres - one to carry light to the area of interest, one to carry image back so it can be viewed. Image can be seen through eyepeice of displayed as a full colour moving image on a screen, Surgeons can now perform keyhole surgery.