P3.1 Medical Applications of Physics Flashcards
How do X-ray’s affect photographic film?
The same way as light (they turn it black)
X-rays are —- by healthy tissue and —- by denser material e.g teeth and bones.
1) transmitted
2) absorbed
Name 2 uses of X-rays.
- Diagnoses bone fractures
- Diagnoses dental problems.
What do CCDs do?
Form X-rays electronically.
How do CCDs form X-rays electronically?
Small silicon chips divided into millions of identical pixels in a grid detect the X-Ray waves and produce electronic signals used to form high resolution images (same technology used camera)
What are CT scans used for?
They use X-rays to produce high resolution images of soft and hard tissue.
How do CT scans take place?
- The patient is put inside a cylindrical scanner and an X-Ray beam is fired through the body from an X-day tube and picked up by detectors on the opposite side.
- X-ray tube and detector are rotated during the scan.
- Computer interprets scan to form a 3D image from 2D slices.
How are X-rays used to kill cancer cells?
- X-rays focused on rumour using wide beam.
- Beam is rotated around the patient with the tumour at the centre.
- This minimises normal cell exposure to radiation.
- X-rays are at just the right dosage to kill all cancer cells and do minimal damage to normal cells
Name 3 properties that X-rays have.
- Electromagnetic transverse wave.
- Short wavelength (diameter of an atom)
- High frequency.
What precautions do radiographers take to stay safe in X-rays?
- Wear lead aprons
- Stand behind a lead screen
- Leave room while scans take place
What precautions are taken for a patient having an X-ray to be as safe as possible?
- Lead is used to shield areas of the patients body not being scanned
- Minimum exposure time
What are ultrasound waves?
Higher frequency waves than the range of human hearing (+ 20,000 Hz)
How does ultrasound work?
- When the ultrasound wave passes from one medium to another, some of the waves are reflected off the boundary and some are refracted.
- The amount of time it takes for the reflections to reach a detector = how far away the boundary is.
Name 2 uses of ultrasound.
- Breaking down of kidney stones
- Pre-natal foetus scanning
Give the one advantage and one disadvantage of ultrasound.
Advantage = non ionising, safe Disadvantage = images usually fuzzy so it's hard to diagnose illness.
Give the one advantage and two disadvantages of X-rays.
Advantage = clear images of bone and metal Disadvantage = Ionising, causes cancer, not safe for babies Disadvantage = doesn't produce images except of bone and metal
Give the two advantages and one disadvantage of CT scans.
Advantage = produces detailed images with high resolution for easy diagnosis. Advantage = produces high quality 3D images useful for planning complicated surgery. Disadvantage = more x-radiation than standard X-ray photographs so more ionisation.
What is refraction?
When waves change direction after entering a new medium.
How does refraction happen?
- When light slows down it bends towards the normal
- When light enters a new medium density changes so light slows down
When does a light wave not refract?
If it hits a 90 degree boundary it will slow down but not change direction.
What do convex lenses do?
- Converge light
- Bulge outwards
- For longsightedness
What type of image do convex lenses produce between F - 2F?
- Real
- Inverted
- Magnified
What type of image do convex lenses produce when the object is closer than F?
- Magnified
- Virtual
- Upright
- Same side of lens
What type of image do concave lens produce?
- Smaller
- Virtual
- Same side of lens
- Upright
What do concave lens do?
- Diverge light
- Cave inwards
- Nearsightedness
What is a virtual image?
One that cannot be projected on a screen.
How are glasses made thinner?
- The greater the R.I, the flatter the lens will be.
- Powerful lenses can be made thinner using high R.I material
What is the function of the cornea?
Protection and initial focus
What is the function of the retina?
Where images are formed (covered in light sensitive cells which detect light and send signals to the brain)
What way do waves move when they slow down?
They bend towards the normal
What happens if light hits a boundary at 90 degrees?
It will not change direction but slow down
What happens when light reaches a new medium?
Some light will pass through the new medium but some is reflected back, depending on the angle of incidence.
What is the ‘refractive index’?
The ratio of speed of light in a vacuum to the speed of light in the medium
What does the angle of refraction of a ray depend on?
The refractive index of the material it travels through
What type of lens diverges light?
A concave len which caves inwards
What type of lens converges light?
A convex lens which bulges outwards
What type of lens is used to treat short-sightedness?
A concave lens
What type of lens is used to treat long-sightedness?
A convex lens
Define ‘focal length’.
The distance from the principal focus to the centre of the lens
How do lenses produce images?
By refracting light and changing its direction
What are the three rules of refraction in a converging lens?
- An incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side
- An incident ray passing through the principal focus refracts through the lens and travels parallel to the axis
- An incident ray passing through the centre of the lens carries on in the same direction.
What are the three rules of refraction in a diverging lens?
- An incident ray parallel to the axis refracts through the lens and travels in line with the principal focus
- An incident ray passing through the lens towards the principal focus refracts through the lens and travels parallel to the axis.
- An incident ray passing through the centre of the lens carries on in the same direction.
What type of image does an object at 2F form at a convex lens?
- Real
- Inverted
- Same size as object
- At 2F
What type of image does an object nearer than F form at a convex lens?
- Virtual
- Right way up
- Bigger than the object
- Same side as the lens
What type of image always forms at a concave lens?
- Virtual
- Right way up
- Smaller than object
- Same size as the lens
What type of lens does a magnifying glass use?
A convex, converging lens
Where should the object be placed when using a magnifying glass?
Closer than F to produce a virtual, right way up, bigger than object, same side of the lens image
What is the relationship between power and the focal length and why?
The more powerful a lens, the shorter the focal length as the lens converges rays of light more strongly
The power of a converging lens is …
positive
The power of a diverging lens is …
negative
What 2 factors does focal length depend on?
- The refractive index of the glass
- The curvature of the lens’ two surfaces
How can glasses be made with thinner lenses?
- The greater the refractive index of a glass, the flatter the lens will be
- This means powerful lenses can be made thinner using glass with a high refractive index
What is the purpose of the iris?
Controls how much light enters the pupil.
What is the purpose of the pupil?
Allows light to pass through as it enters the eye.
What is the purpose of the lens?
Refracts light to focus it onto the retina.
What are the purpose of the ciliary muscles?
To adjust the shape of the lens to make it more or less curved, so as to increase or decrease the refraction of light.
What are the purpose of the suspensory ligaments?
To slacken or stretch as the ciliary muscles contract or relax, to adjust the thickness and curvature of the lens.
What does the retina correspond to in a camera?
CCD
What does the lens correspond to in a camera?
Lens
What does the iris correspond to in a camera?
Aperture
Define a ‘far point’
The furthest distance that the eye can focus on comfortably
Define a ‘near point’
The closest distance that the eye can focus on comfortably
How does the near point depend on age?
The older a person is, the further their near point
What are the 2 causes of short sightedness?
- Eyeball being too long
- Cornea and lens being too powerful
What are the 2 causes of long sightedness?
- Cornea and lens are too weak
- Eyeball too short
How can laser surgery be used to correct eye problems?
- It can be used to vaporise some of the cornea to change its shape which changes its focusing ability.
- This can increase or decrease the power of the cornea so the eye can focus images properly on the retina.
How can a lens be used to correct long sightedness?
- A converging lens with positive power can be put in front of the eye.
- The light is refracted and starts to converge before it enters the eye so the image can be focused on the retina where it belongs.
How can a lens be used to correct short sightedness?
- A diverging lens with negative power can be put in front of the eye.
- The light is refracted and starts to diverge before it enters the eye so the lens can focus it on the retina where it belongs.
What is the average near point for adults?
25cm
What is total internal reflection?
The complete reflection of a light ray reaching a less dense medium when the angle of incidence exceeds the critical angle.
What is the critical angle?
The angle which, if exceeded by the angle of incidence creates total internal refraction.
What is the relationship between total internal reflection and the refractive index?
The higher the refractive index, the lower the critical angle so the higher the amount of light that is internally refracted.
What is an endoscope?
A thin tube containing optical fibres that lets surgeons examine inside the body.
How do endoscopes use total internal refraction?
- There are two bundles of optical wires
- One carries light into the body, and the other carries light reflected off internal body surfaces back out.
- The light waves are bounced off the sides of the inner core of glass or plastic and are reflected repeatedly until they emerge at the other end.
Describe one use of endoscopes.
- Keyhole surgery - surgeons only need to cut small holes in people to perform operations
