Physics a Flashcards
How can X-rays produce images of the inside of the human body ?
X-rays are transmitted through healthy tissue but absorbed by denser materials like bones. A photographic film changes colour when the X-rays touch it, so the areas where the waves can’t pass through like bone or metal don’t change colour. This creates a black and white image
What are X-rays ?
High frequency, short wavelength electromagnetic waves
What can X-Ray photographs be used for ?
To diagnose medical conditions such as bone fractures or dental problems
What is a new way in which X-ray photographs are formed ?
In the same way as digital cameras. Silicon chips detect rays and produce electronic signals which form high resolution images. They are known as CCDs
What does the CT in CT Scan stand for ?
Computerised axial tomography
How do CT scans work ?
The body part being scanned is put inside a cylinder in which one side emits x-rays and the other side detects it. As the cylinder rotates multiple images are taken, forming a 2D slice I the body part. Multiple slices can be put together to form a 3D image.
How can x-rays be used to treat cancer ?
X-rays cause ionisation - high doses can kill living cells. If focused at just the right dosage and area, cancerous cells can be killed without damaging normal cells.
What is ultrasound ?
Sound with a higher frequency than we can hear.
Above what frequency is ultrasound ?
Over 20,000 Hz
What frequency can humans hear ?
20-20,000 Hz
What is partial reflection ?
When a wave passes into a medium, some is reflected off the boundary and some refracts through that medium.
How does ultrasound imaging work ?
A detector measures the time it takes an ultrasound wave to reflect of a boundary between two mediums. The longer the time the further away the object.
How do you work out the distance between two boundaries ?
Speed x Time
How do ultrasound waves break down kidney stones ?
Kidney stones block the urinary tract, making it very painful to urinate. Ultra sound waves can be directed at the kidney stones to turn them into sand-like particles. These can then pass out of the body in urine.
Ultrasounds can monitor the development of a foetus, how is this done ?
The ultrasound waves reflect when they reach a different medium. They are detected and the exact time and distribution of echoes are recorded by a computer. This forms an image of the foetus.
Which is the safest way of creating image of the human body ?
Ultrasounds. They are non ionising which means they won’t kill healthy cells, unlike X-Ray or CT scans
Which is more dangerous - X-Rays or CT scans ?
CT scans use a lot more x-ray radiation so the patient is exposed to even more radiation.
Which images are the most detailed ?
CT scans produce extremely detailed images in high resolution where as ultrasound images are fuzzy which makes it harder to diagnose illnesses. X-ray images are somewhere in the middle
Why do waves refract ?
As they pass into a different medium the waves change speed
What is the normal ?
A line perpendicular (90 degrees) to the boundary
What happens when a line hits the boundary at 90 degrees (the normal) ?
It will not change direction
True or false - when light hits a medium some will pass through some will be reflected
True - it changes depending on the size of the angle of incidence
What is refractive index ?
The ratio of speed of light in a vacuum to the speed of light in different mediums
What is the formula to work out refractive index ?
Sin i divided by Sin r
i = angle of incidence r = angle of refraction
How do lenses form images ?
They refract light and change its direction.
What are the two main types of lenses ?
Converging and diverging
What does a converging lens do ?
It is convex which means it bulges outwards. It causes parallel rays of light to move together (converge) at the ‘principal focus’
What does a diverging lens do ?
It is concave which means it caves inwards. It causes parallel rays of light to spread out (diverge)
What is the ‘principal focus’ point ?
The point where the rays all meet
How do you find the principal focus of a diverging lens ?
Because they all spread out there is no point where they all meet on the other side of the lens. Instead, the point is measured where the lines meet before hitting the lens
What is a real image ?
When the light comes together (converges) to form an image on a film
What is the screen in the human eye ?
The retina - where the light is focused on
What is a virtual image ?
When the light rays diverge, the objects appear to be coming from a completely different place
What is the proper word that describes an image upside down ?
Inverted
Using a converging lens, what image will be created when it’s at 2F ?
A real, inverted image the same size as the object
If the image is brought closer - between 2F and F - what will be produced ?
A real, inverted image bigger than the object
How is a virtual image produced by a converging lens ?
When the object is brought closer than F to the lens. The image will also become larger but not inverted
What type of image does a diverging lens produce ?
Always a virtual image that is smaller than object
How do magnifying glasses work ?
A virtual image is produced by placing an object closer to the lens than the focal length. This increases the size of the image without inverting it
What can you not do with a virtual image ?
Project it onto a screen
What is the formula for magnification ?
Image height divided by object height
How are focal length and power related ?
The more powerful a lens, the more strongly it converges light. This creates a shorter focal length
What formula incorporates focal length and power ?
Power = 1 divided by focal length
What is the cornea ?
A transparent ‘window’ that has a convex shape. It does most of the eye’s focussing
What does the iris do ?
It is made up of muscles which controls the light intensity entering the eye.
How does the iris reduce light intensity entering the eye ?
When the muscles contract the pupil (gap between the iris) gets smaller, so less light is allowed in
What does the lens do ? How does it do this ?
It focus light from objects of varying distances by changing its size. When ciliary muscles are contracted the tension is released so the lens is fatter. When the muscles relax the lens tightens making it thinner
How does the retina work as a screen ?
The light hits the light sensitive cells which send messages to the brain to be interpreted.
What is the ‘far point’ ?
The furthest distance that the eye can see clearly. Vice verse for ‘near point’
What is the ‘far point’ of the human eye on average ?
Infinity
What is the approximate ‘near point’ distance of the human eye ?
25cm
How do cameras work ?
In the same way as the human eye. A real image is formed on the opposite side of the lens to the image an is focused onto light sensitive film or a CCD in a digital camera.
What is happening if you are short sighted ?
Their far point is closer to infinity so they can’t focus on objects far away. Images are focused in front of the retina rather than on the retina.
How is short sight corrected ?
A diverging lens is placed in front of the eye (in glasses). This diverges light which means the light is focused further back, on the retina
What is long sight ?
This is when you near point is further away than normal. The light I focused behind the retina rather than on it. Images therefore become blurry.
What type of lens is used to treat long sight ?
Converging lenses
What is a laser ?
A narrow, intense beam of light
Why are lasers sometimes used instead of scalpels to cut through tissue during surgery ?
Lasers cauterise (burn and seal shut) small blood vessels as the skin is cut. This reduces blood loss which presents against infection.
How can lasers treat skin conditions such as acne scars ?
They burn off the top layers of skin, revealing the unscarred layers below.
How are lasers used for laser eye surgery ?
They vaporise some of the cornea to change its shape. This enhances the focussing ability by increasing or decreasing the power of the cornea so that the eye can focus images onto the retina.
How do optical fibres carry visible light over long distances ?
They reflect light inside a glass or plastic tube. The wave is repeatedly reflected until it emerges at the other end. This is known as total internal reflection.
How can total internal reflection happen ?
For the wave to travel through a dense material such as glass towards a less dense Material like air
What does the angle of incidence have to be for total internal refraction ?
Greater than the critical angle
What is the critical angle ?
The value of the critical angle changes depending on the refractive index. A dense material with a high refractive index has a low critical angle.
What formula relates refractive index and critical angle ?
Refractive index = 1 divided by sin c
C = critical angle
What is an endoscope ?
A thin tube containing optical fibres. It consists of two bundles of fibre optic cables. One carries light to the area of interest and another carries an image bak to be monitored.
How are endoscopes used ?
The image can be seen through an eyepiece which produces a full-colour moving image on a TV screen. This is used in keyhole surgery which allows us to perform operations with only only cutting tiny holes.
If the wave slows down what happens ?
They bend towards the normal
