Physics 3a Flashcards
What are X rays?
High frequency, short wavelength electromagnetic waves
What is their wavelength roughly the same size as?
The diameter of an atom
How do X rays work?
Transmitted by (pass through) healthy tissue, but are absorbed by denser materials like bones and metal. They affect photographic film in the same way as light which means they can be used to take photographs.
What are x rays used for?
To diagnose many medical conditions such as bone fractures or dental problems
How are x ray images formed?
They can be formed electronically using charge-coupled devices, they’re small silicon chips divided up into a grid of millions of identical pixels, they detects x rays and produce electronic signals which are used to form high resolution images-same technology as taking photographs with digital cameras
How do you read an x ray photograph?
The brighter parts are where fewer x rays get through (eg bone), it’s a negative image, the plate starts off all white
How do CT scans work?
Use x rays to produce high quality images of soft and hard tissue. Patient is put into a cylindrical scanner and an x ray beam is fired through the body from an x ray tube and picked up by detectors on the opposite side. The x ray tube and detectors are rotated during the scan. Computer interprets the signals fro the detectors to form an image of a 2D slice through the body. Multiple 2D CT scans can be put together to make a 3D one of the inside of the body
What does CT stand for?
Computerised axial tomography
How can X-rays be used to treat cancer?
They can cause ionisation-high doses of x-rays will kill living cells so can be used to treat cancers, like with gamma radiation. The x-rays have to be carefully focused and at just the right dosage to kill the cancer cells without damaging too many normal cells
What is the process of treating illnesses with x rays?
X-rays are focused on where the cells that are causing the illness are using a wide beam, the beam is rotated around the patient with the bad cells in the centre. This minimises the exposure of normal cells to radiation and so reduces the chances of damaging the rest of the body
How do radiographers take precautions to minimise radiation dose?
By wearing lead aprons, standing behind a lead screen or leaving the room while the scans are being done. Lead is used to shield areas of the patient’s body that isn’t being scanned.Exposure time is also kept to a minimum
Why can’t we hear ultrasounds?
Because it is a sound with a higher frequency that we can hear
What happens when a wave passes from one medium into another?
Some of the wave is reflected off the boundary between the two media and some is transmitted and refracted. This is partial reflection
What does this mean?
You can point a pulse of ultrasound at an object and wherever there are boundaries between one substance and another, some of the ultrasound gets reflected back. The time it takes for the reflections to reach a detector can be used to measure how far away the boundary is, this is how ultrasound imaging works
What can you use to find the boundaries?
Oscilloscope traces
What is the formula for working out the distance between the boundaries?
Distance (metres) = Speed of sound in the medium (metres per second) X time ( seconds)
How can ultrasound beams be used to break down kidney stones?
An ultrasound beam concentrates high-energy waves at the kidney stone and turns it into sand-like particles. These particles can then pass out of the body in the urine. It’s a good method because the patient doesn’t need surgery and it’s relatively painless
How can ultrasound waves be used for pre-natal scanning of a foetus?
The waves can pass through the body but whenever they reach a boundary between two different media (like fluid in the womb and the sin of a foetus) some of the wave is reflected back and detected. The exact timing and distribution of these echoes are processed by a computer to produce a video image of the foetus
How safe are ultrasounds, x rays and ct scans?
Ultrasounds are non-ionising and safe. X rays are ionising and can cause cancer at high doses or exposure and aren’t safe to use on developing babies. CT scans use a lot more x ray radiation than normal x ray photographs, more exposure to the ionising radiation so they aren’t taken unless its really needed
What is the image quality like for ultrasounds, x rays and ct scans?
Ultrasounds are typically fuzzy which can make it harder to diagnose some conditions. x rays are clear images of bones and metal but nothing else. CTs can produce detailed images and an diagnose complicated illnesses as the high resolution can make it easier to work out the problem, and high quality 3d images can be used in the planning of complicated surgery
What is refraction?
When waves change direction as they enter a different medium. This is caused by the change in density from one medium to the other, which changes the speed of the waves
What happens when waves slow down?
They bend towards the normal
What happens when light enters glass or plastic?
It slows down to about 2/3 of its speed in air
What happens if a wave hits a boundary at 90º (along the normal)?
It will slow down but wont change direction
What happens when light hits a different medium like glass or plastic?
Some of the light will pass through the new medium but some will be reflected, it depends on the angle of incidence (the angle it its the medium)
What is the refractive index of a medium?
The ratio of speed of light in a vacuum to speed of light in that medium
What is the formula to work out the refractive index?
Refractive index (n) = Sin i ÷ sin r
How do lenses work?
They form images by reflecting light and changing its direction
What are the two main types of lens?
Converging and diverging
What are converging lenses?
It’s convex, it causes parallel rays of light to converge (move out) at the principal focus
What are diverging lenses?
It’s concave, it causes parallel ays of light to diverge (spread out)
What is the axis of a lens?
A line passing through the middle of the lens
What is the principal focus of a converging lens?
Where rays hitting the lens parallel to the axis all meet
What is the principal focus of a diverging lens?
The point where rays hitting the lens parallel to the axis appear to all come from. You can trace them back until they all appear to meet up at a point behind the lens
What is the focal length?
There is a principal focus on each side of the lens, the distance from the centre of the lens to the principal focus is called the focal length
What are the three rules for 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 though the centre of the lens carries on in the same direction
What are the three rules for refraction in a diverging lens?
An incident ray parallel to the axis refracts through the lens and travels in line with the principal focus so it appears to have come from the principal focus
An incident ray passing though 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 is a real image?
where the light from an object comes together to form an image on a screen, like the image formed on an eyes retina (the screen at the back of an eye)
What is a virtual image?
When the rays are diverging so the light from the object appears to be coming from a completely different place
What sort of image do you see in the mirror?
A virtual image of yourself because the object (you) appears to be behind the mirror
What sort of image do you see in a magnifying lens?
A virtual image, it looks bigger than the object actually is
What three things do you need to talk about to describe an image?
How big it is compared to the object, whether it’s upright or inverted relative to the object and whether it’s real or virtual
