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PS124 PHYSICS FOR NURSING A non-mathematical introduction to modern science and technology and how it relates to the human body. Dr Eamonn Cunningham School of Physical Sciences Office N103 Tel. 5297 eamonn.cunningham@dcu.ie http://www.physics.dcu.ie/~ecu/nursing Module 1 semester 5 credit module 24 lectures (8 weeks) End of semester exam (May) 100% Essay (40%) & 3 questions (20% each) Essay topics given in advance! PS124 Nursing 1 SYLLABUS HEARING & SIGHT Sound waves, Human ear & deafness. Properties of light & lenses. Infrared and ultraviolet light. The eye and eyesight problems. Laser light, CDs & DVDs MEDICAL IMAGING & RADIATION X-rays and CAT scans. MRI imaging and Ultrasound techniques. Dangers of ionising radiation. Radioactivity & its uses. Natural & artificial radiation in the environment. ELECTRICITY & RADIO Battery and mains electricity. Dangers of electricity and safety. Radio waves and broadcasting. Microwave oven & mobile phones. Dangers of radio waves. PS124 Nursing 2 Books Lecture notes These can be downloaded from my webpage. http://www.physics.dcu.ie/~ecu/nursing Useful book Louis A Bloomfield. How Things Work: The Physics of Everyday Life. 2nd edition Wiley and Sons 2001 WWW Everything is there, hidden in the rubbish! http://www.lsw.com/htw/ http://www.howstuffworks.com/ PS124 Nursing 3 HEARING & SIGHT SOUND WAVES. Sound waves are pressure variations in the air, which transfer energy from the source of sound to the ear. They can travel through solids or liquids but mostly through the air. No sound in a vacuum. Atmospheric pressure at sea level is about 1013 hPa (hectopascal or mbar) This is equal to a weight of 10 tonnes per square metre! Changes by 20-30 hPa from day to day. The pressure variations are ~1 000 000 000 times smaller than atmospheric pressure but occur very quickly. PS124 Nursing 4 Sound travels at a speed of 330 m/s (1200 kph). Bit faster than a jet airliner. Speed depends on air temperature but not pressure. Sound wave High Low High pressure molecule sound The air molecules vibrate in the same direction as the wave moves but don’t move along with wave. This is called a longitudinal wave. Most waves are transverse, vibration at 90o to motion. PS124 Nursing 5 Wavelength & frequency Sound wave 1.5 1 0.5 0 -0.5 -1 -1.5 Pressure The peaks in pressure move along with the wave. The air molecules just vibrate back & forth. Wavelength = peak to peak distance Frequency = No of peaks pass /sec. Speed = Frequency x Wavelength This is true for all waves, not just sound. PS124 Nursing 6 Frequency range The human hearing range is conventionally given as 20 Hz to 20 kHz. (Optimistic!) [ 1 Hz or Hertz = 1 cycle per second] Elephants & whales hear lower frequencies (infrasonic), while small animals like bats can hear at much higher frequencies (ultrasonic). Wavelengths Corresponding wavelengths are: 20 Hz 20 kHz 16.5 m 16.5 mm This means that low frequencies can’t travel in small rooms and that normal sized objects diffract sound waves easily. PS124 Nursing 7 Loudness. The sound intensity in Watts/m2 drops with the inverse square of the distance. Same sound energy spread over larger area. But the loudness is measured in decibels, which is related to the intensity in W/m2. This is a logarithmic scale, which corresponds better to the way our ears detect loudness. sound Threshold soft loud W/m2 10-12 10-7 10-3 dB 0 50 90 PS124 Nursing 8 The threshold of hearing is arbitrarily set at 10-12 W/m2, which is below the limit of any human ear. [10-12 = 0.000 000 000 001] Above 90-100 dB we get hearing damage, either temporary or permanent. Note 50 dB + 50 dB = 53 dB! Because it is a logarithmic scale, 3 dB means 2x energy. Loudness in dBs drops off with distance, but more slowly than 1/d2. Perceived loudness also depends slightly on frequency. This is not significant except at the low and high frequency limits. PS124 Nursing 9 Human voice mouth tongue vocal cords air flow trachea (windpipe) We produce sounds by blowing air past our vocal cords. These vibrate and the sound resonates in our windpipe. Alter tension to change frequency. Further modified by mouth and tongue. Bass voice Soprano Piano 80 – 400 Hz 200 – 1200 Hz 20 – 4000 Hz Larger windpipe  lower frequency PS124 Nursing 10 We also produce harmonics of the basic frequency, 2x, 3x, etc. These add to the quality of the sound. Telephones only transmit up to 4 kHz, but we recognise speech. Sound quality poor for music. THE HUMAN EAR inner ear outer ear middle Pinna (outside bit) collects sound and gives sense of direction Auditory canal, resonates at ~ 3-4 kHz. Eardrum (red) vibrates with sound PS124 Nursing 11 Eustachian tube relieves pressure to prevent eardrum bursting. Middle ear contains 3 bones to transmit sound from eardrum to inner ear. Inner ear - Cochlea Spiral filled with liquid, like snail, detects sound. high f low f This vibrates at different parts for different sound frequencies. Detected by movement of the hairs, send signals to brain. High frequency sounds are detected at the narrow end, low at the wide end. PS124 Nursing 12 Sensitivity to loudness loudness 100 dB normal ageing damaged 60 dB 0 dB freq. 100 1k 10 k Normal hearing is most sensitive at ~ 4 kHz and is less at low and high frequencies. As we age, or hearing is damaged, the range and sensitivity of hearing is lost.  Need louder sound to be heard  Loss of frequency range, mainly at high f PS124 Nursing 13 A loss of 40 – 60 dB can be tolerated, as there is a lot of dynamic range left. Loss of high frequencies only matters when it drops to ~4 kHz and effect speech. Lose ~ 1 kHz for every 10 yrs older! Many older people use other clues, such as lip-reading, to compensate for hearing loss. Common in people who have been exposed to loud noise for long periods. DEAFNESS Damage to middle or inner ear caused by:  Ageing  Injury  Disease  Genetic effects  Loud noise PS124 Nursing 14 Total deafness is rare, usually due to damage to nerves in ear. Most people have some hearing, which can be assisted. Hearing aid A simple hearing aid amplifies all frequencies equally and is mainly useful for mild deafness. More advanced hearing aids amplify differently at different frequencies. This allows compensation for high f hearing loss without amplifying the low f too much. PS124 Nursing 15 Cochlear implants wires attached to cochlea hearing aid This is used to bypass the ear where there is little or no hearing. In an operation, about 30 wires are attached to nerves in the cochlea and go to a loop under the skin. The external hearing aid transmits the electrical signal through the skin. The small currents stimulate the nerves and mimics sound. PS124 Nursing 16 Frequency discrimination is poor. 30 wires replace 30,000 hairs in the cochlea. Adults Used in adults who have lost most/all hearing through illness or injury. Get distorted sound but can use it as they are used to hearing sounds. Children Children born with little or no hearing. Operation must be done by 2-3 yrs old, as otherwise child won’t learn to interpret sound. This is a controversial topic as some deaf people say it is wrong to turn deaf children into handicapped hearing children. “Genocide of the Deaf” PS124 Nursing 17 PROPERTIES OF LIGHT Light is a type of electromagnetic wave, which has a number of unique properties.  Travels at 300,000 km/s  Can travel through a vacuum  Transverse wave (not unique)  Can be polarised  very high frequency/short wavelength Visible light is only a small part of the spectrum, but can be detected by the eye. low frequency high IR long red green wavelength blue UV short PS124 Nursing 18 Visible light, detected by the eye, is in the wavelength range 400 nm (blue) to 700 nm (red). [ 1 nm = 0.000 000 001 m ] Light at less than 400 nm wavelength is invisible ultraviolet (UV). Beyond 700 nm is invisible infrared light (IR). Most sunlight is in the visible range with a bit of IR and UV. As the wavelength is so small, diffraction effects are not important, so light travels in straight lines. Shorter wavelength (bluer) light has more energy and ultraviolet light can damage molecules as it is ionising. e.g. Books go yellow & people go red in sunlight. PS124 Nursing 19 REFRACTION & LENSES. Light slows down when it goes through a transparent material such as glass. If it hits the glass at an angle it will be bent (refracted). This is most useful in a lens, which can alter the direction of the light and focus it to a point image. focal length * focus The flat wavefronts of light are bent by the lens and brought together at one point, the focus. PS124 Nursing 20 There are only 2 types of lenses: Convex lenses (fatter in middle) These converge light to a point (focus) and are the basis of all optical instruments. Concave lenses (thinner in middle) These diverge light away from a point. Used in combination with convex lenses to make them weaker. PS124 Nursing 21 Lens power Focal length is distance from lens to focus. Short focal length = powerful lens Long focal length = weak lens Diverging light will focus at a greater distance from the lens. Converging light will focus at a shorter distance from the lens. Dioptre is the unit used to measure lens strength by opticians. dioptre = 1/focal length e.g. 5 dioptre = 0.2m focal length PS124 Nursing 22 INFRARED & ULTRAVIOLET Infrared light is emitted by warm bodies at room temperature and also by LEDs, light emitting diodes, and infrared lasers. The Earth loses heat by emitting IR, which balances the heat input from the sun. Burglar alarms can work by detecting the extra IR from hot bodies. Only a few degrees hotter produces lots more IR. TV remote controls flash infrared light to switch the TV. IR lasers are used in CD players and optical fibre communications. PS124 Nursing 23 Ultraviolet light can cause chemical changes, breaking bonds between atoms. It is used for sterilising medical instruments as it kills all bacteria. Most of the UV from the sun is stopped by the ozone layer in the atmosphere and only a little UV, near the visible, gets through. This is enough to cause chemical changes in the skin, ranging from a nice tan to skin cancer. Damage to the ozone layer due to pollution means that more UV is getting through. This has led to increased skin cancer etc. This problem is under control but it will take 50 years to recover. PS124 Nursing 24 THE HUMAN EYE lens retina optic nerve ~ 25 mm diameter sphere The lens, in the front of the eye, focuses light onto the light sensitive retina. The light sensitive cells are closest together at the centre of the retina, giving the best vision there. fovea centralis Image is upside down on retina! The optic nerve connects the retina to the brain. Blind spot where optic nerve is PS124 Nursing 25 attached. Off centre and in a different place for each eye. The lens muscles cornea flexible lens iris cornea window on front of eye, also does some focusing iris coloured part of eye, controls amount of light into eye. pupil dark opening in iris lens 3-8 mm flexible convex lens, can change focal length and remain useful muscles PS124 Nursing used to alter lens shape 26 Accommodation Need stronger (shorter focal length) lens to focus light from nearby objects. far nearby For far objects, the muscles are relaxed and ligaments pull the lens into a thin shape. Light from nearby objects is expanding and needs stronger lens. Muscles contract, ligaments loosen and lens becomes rounder and a stronger lens. Babies can focus from 10 cm – infinity Accommodation range reduces with age. PS124 Nursing 27 EYESIGHT PROBLEMS Short sight (Myopia) Lens is too powerful, focuses light from distant object in front of retina. This is the most common problem. Can focus light from nearby objects correctly, because light is diverging. Short sight retina Shortsighted person can focus from few cm to few m instead of 20-30 cm to very far away. May be due to slightly misshaped eye. PS124 Nursing 28 Putting a diverging (concave) lens in front compensates for this. Corrected short sight retina The corrective lens fools eye into thinking everything is closer. This can be:  glasses – in front of eye  contact lens – sitting on cornea  reshaped cornea - using UV laser to change curvature of surface PS124 Nursing 29 Long sight Hyperopia/Presbyopia There are two types of long-sight. Hyperopia opposite to myopia. Lens bit too weak, use convex lens to help focus light. This is rare. Presbyopia age related long sight. Most people in 50s lose accommodation and need reading glasses to focus on nearby objects. Lens stiffer and/or muscles weaker, so lose ability to change focusing. Person with good eyesight only needs glasses for reading. Shortsighted person may not need reading glasses, only for long distances. PS124 Nursing 30 Corrected long sight retina Usually corrected by glasses only. Bifocals Bottom half of glasses is stronger for reading. long range reading If sight is otherwise normal, only need bottom half of glasses. Shortsighted person needs weaker correction for reading. Varifocals gradually change power across lens. No telltale line across lens! PS124 Nursing 31 Other sight problems Astigmatism Lens is thicker (stronger) in some areas than others. Can’t focus perfectly. Solution is lens with opposite characteristics. Glasses, contact lenses or laser treatment. Cataracts Either lens or cornea becomes translucent due to age or injury. Can be surgically removed and replaced by plastic lens or transplanted cornea. Detached/bleeding retina Focus laser light on back of retina. Spot welds retina in place or closes off blood vessels. PS124 Nursing 32 Colour Vision There are 2 types of light sensitive cells in the retina. Rods Most of the light sensitive cells, which only detect brightness. The nerve impulse lasts for ~0.1 s, which is why we see movies/TV as moving pictures. Rods work in dim light. Cones Mostly in the centre of the retina, can detect colour but need reasonable brightness. 3 types, sensitive to red, green and blue light. Approx. wavelengths red green blue PS124 Nursing 700-550 nm 600-480 nm 480-400 nm 33 Cone cell sensitivity 700 600 500 400 Any colour in between can stimulate 2 or 3 cones and so we distinguish many more than 3 colours. Paintings, movies and TV represent different colours by adding various proportions of red, blue and green. This fools our eyes but looks different to a spectrometer. PS124 Nursing 34 Colour Blindness In dim light, only the rods work and we are all colour blind. In normal light ~ 10% of men and ~1% of women are colour blind to some extent. This means reduced or no sensitivity in at least 1 type of cone. A very few people have no colour vision at all. Most common type is red/green colour blindness. One cone type doesn’t work and the entire region 700-480 nm looks the same colour. Great for traffic lights! Reason is genetic. Men have 1 X chromosome and ~10% have the colour blindness defect. Women have 2 X chromosomes and are only colour blind if both are faulty. PS124 Nursing 35 LASER LIGHT Laser - Light Amplification by Stimulated Emission of Radiation Laser light is:  Monochromatic only 1 wavelength (colour)  Very high intensity – brightness  Coherent– all waves in phase/step  very directional – small angle of spread Common types are: HeNe Helium Neon gas laser Semiconductor laser often infrared Ruby laser (artificial ruby) – original PS124 Nursing 36 HeNe laser uses an electrical discharge in a mixture of Helium and Neon gas. The gas atoms store the electrical energy for a short time and then spontaneously emit a small flash of red light ~630 nm wavelength. mirror mirror As this happens some light is reflected back by the mirror and this can set off flashes from other atoms – stimulated emission. This will be in step (phase) and in the same direction. Builds up a strong light beam between the mirrors. PS124 Nursing 37 Without the mirrors, the light would be incoherent, like normal light. Incoherent light has a large range of wavelengths and spreads in all directions. One mirror is only partly reflecting and lets part of the laser beam out. HeNe 632.8 +/- 0.000001 nm Other types of laser have different wavelengths, e.g. 780nm for some semiconductor lasers. These are used for optical fibre communications and in CD players Most lasers produce red or infrared light. Blue and green lasers are expensive and unreliable. PS124 Nursing 38 Laser uses Lasers are used where we need high intensity light, of one wavelength to go in a particular direction or focus on a small spot. communications - optical fibres high precision cutting surgery, eyes & removing birthmarks surveying information storage – CDs & DVDs precise measurements barcode reading The smallest spot size that laser light can be focused to is about the same as its wavelength. For red/infrared light this is ~ 700 nm or 0.7 millionths of a metre. PS124 Nursing 39 COMPACT DISCS A compact disc can contain a lot of information in digital form. Either 74 min of music or the equivalent amount of data. CD 120 mm diameter 1.2 mm thick label on top clear plastic on bottom The digital information is coded on a spiral path out from the centre. This track is 0.5 μm wide and up to 5.6 km long. [μm, micrometer, 0.000 001 m] Each track is 1.6 μm apart. PS124 Nursing 40 The track has a series of ridges, which encode the digital information. Ridges are 110 nm high 500 nm wide 833-3560nm long [nm, nanometre = 0.001 μm] Structure of CD label metal layer clear plastic ridge valley Laser The ridges are read by an infrared laser, which has 780nm wavelength in air. PS124 Nursing 41 The laser reads the ridges and valleys through the plastic as 1s and 0s. These lasers were easily available in 1980. In the plastic, the wavelength is reduced to 500nm, as light is slowed down. The ridges are 1/4 wavelength high. The amount of information on the disc is limited by the laser wavelength. Anything smaller than 500nm can’t be reliably seen with this wavelength. 74 min of sound corresponds to Beethoven 9th Symphony. Blue lasers would allow the ridges 2x as close and 4x as much data in the same space. DVDs use visible red lasers to pack the data closer. PS124 Nursing 42 Reproducing sound To reproduce sound we need something to change in step with the sound. Examples Electric current in a phone wire Magnetism along a magnetic tape Surface of a vinyl record This is analogue sound. The changes in the current etc. correspond to the sound. Louder = bigger current Higher frequency = faster changes Analogue reproduction is simple but you can lose detail each time the sound is copied or transferred from one type to another. PS124 Nursing 43 Producing a vinyl record The original recording is done on high quality analogue magnetic tapes. This is copied and a master disc produced. This then is used to produce the vinyl discs. Some detail is lost at each stage and vinyl records are easily damaged. Old recordings can be issued as CDs by going back to the original magnetic tapes. These are digital recordings where the information is stored as numbers so there is no loss in quality when it is copied. Vinyl records and cassette tapes don’t have the same range of frequency and loudness as CDs or other digital methods. PS124 Nursing 44 Digital sound As 20 kHz is the highest frequency anyone can hear we need to measure the loudness at least twice as fast to record sound. loudness time The sound loudness is measured 44,100 times /sec, which is more than 2 x maximum audible frequency. The loudness is measured from 0 to 65,536 and there are 2 channels of sound. PS124 Nursing 45 This totals 783 Mbytes data or the equivalent of 74 minutes of sound. Note 1 byte 1 kbyte 1 Mbyte Errors Scratches and marks on the CD plastic surface don’t matter as they are out of focus. Small errors are corrected by error correction codes (repeats of the data). More important for CD-ROMs than for music. ~ 1 letter ~ 1 page of text ~ 1 small picture PS124 Nursing 46 Playing speed. The data track starts at the centre of the CD and spirals out. It is 5.6 km long and 0.5 m wide. [CDs can be smaller in size] The laser scans at a constant speed, so the CD is spun at between 200 and 500 RPM, depending on which track is being read. CD players Have to do the following:  spin the disc at the right speed, to keep track speed constant  follow the track with a laser, to +/- 500 nm. Tracks 1600 nm apart.  read the ridges and valleys  process the digital data to music or alternative format PS124 Nursing 47 CD Player laser lens laser beam CD The laser is focused on the bumps, so that light is reflected to a detector. When it passes over a valley, 110nm (~1/4 wavelength) lower, the light is defocused and no signal received by the detector. Laser beam is kept on track and in focus by complicated feedback mechanisms. Spot size has to be ~1000 nm across so it only reads 1 track. PS124 Nursing 48 Recordable CDs Basic CD players just play pre-recorded discs. To make your own CD you need a more powerful laser and a recordable CD. Label Dye Plastic Laser The laser “writes” the information on to the CD making the dye become opaque. It can be read using a less powerful laser even in a normal CD player. Some CDs can be erased and used again but they are usually write once, read often. PS124 Nursing 49 Video Recording There is nothing to stop you using a CD to record video except that you would only get 20 s on a standard CD. 1 s of sound = 0.18 Mbytes of data 1 s of video = 37.5 Mbytes A TV picture contains ~ 1/2 million dots (pixels) x 3 colours x 25 frames/s. This is fairly poor resolution compared to film. The CD method of digitising means you copy all the data, which isn’t necessary. More sophisticated methods allow you to compress the data up to 40 times. This is used for MP3 files on a computer. For video this allows 15 mins on a CD. PS124 Nursing 50 DVD DIGITAL VIDEO DISC Also called Digital Versatile Disc. Similar to a CD but contains more data. A single layer/sided DVD contains 7x as much data as a CD  uses 640nm red laser rather than 780 nm, tracks 2x closer, 4x as much data (also put tracks closer together)  more area used for data (small label)  better error correction – less repetition  can be double sided  can have 2 layers on each side. Laser reads through bottom layer, which is partly transparent. Can focus on either layer. PS124 Nursing 51 A DVD contains the equivalent of 8 hours of music or 5 Gbytes (5000 Mbyte) of data. DVD players can also read normal CDs. Red laser spot is smaller but can read “big type”. Video on DVD The main reason for developing the DVD is to record video. You can store a 2 hour video on a DVD, while a CD would contain less than 15 minutes. Even so, there is no room to contain all the data. Without compression, a 2 hr video contains ~180 Gbytes of data. (40 DVDs) PS124 Nursing 52 Data compression allows this to be stored in 40 times less space. This is mainly done by or not repeating things which don’t change. There is a limit to how much more of this is possible. Recording high definition TV would need much more storage and isn’t practical with the existing system. DVDs also have much higher quality stereo sound, like CDs, and can have captions or dubbing in other languages. PS124 Nursing 53