Physics - Tripple Flashcards
How are images formed by using X rays?
Images can be formed electronically by using a charge - coupled device (CCD)
Silicon chips are divided up into grids of millions of identical pixels these detect X rays and produce electronic signals which are used to form high resolution images
What can a X ray be absorbed by?
Bone and metal
They pass through healthy tissue
How does a CT scanner work?
- Patient is put inside a cylindrical scanner
- X rat beam fired through the body from an Xray tube and picked up by detectors on opposite side
- X ray tube and detectors are rotated in the scan
- Computer intercepts signal and forms a 2D images
- Lots of scans can be put together to form a 3D image
How are X rays used to treat cancer?
- X ray is focused on tumour using a wide beam
- Beam is rotated round the patient with the tumour at the centre
- This minimises the exposure of normal cells to radiation and reduces the risk of damaging the body
How do you minimise the radiation dosage?
Lead apron Lead screen Leave room Lead is issued to shield other parts of body Film badge
How does ultrasound work?
When a wave passes from one medium to another some is reflected, the ultrasound measure the time it takes for the reflections to reach a detector and measures how far an object is away
How do you measure the distance travelled by the pulse?
Speed of ultrasound waves x transit time
How do you work out the depth of the boundary below the surface?
0.5 x speed of ultrasound waves x transit time
How are ultrasounds used to break down kidney stones?
The ultrasounds concentrate high energy waves at the kidney stone turning it into sand like particles that pass out the body into the urine
How do ultrasounds scan a pre-natal foetus?
The ultrasound pass through the body until it reaches the boundary between the womb fluids and the skin of the baby some of the wave is reflected back and detected on a computer, the exact timing and distribution of the echoes are processed into a computer producing an image
What are the unsafe risks of waves? ( ultrasounds, x rays, CT scans?
Ultrasounds are non ionising so are safe
X rays are ionising
CT scans are even more ionising these will cause cells to mutate
What are the images like of various waves?
Ultrasound - fuzzy
X rays - clear images of bone
CT scans - detailed images with high resolution
What is total internal reflection?
When a substance travels through a dense substance towards a less dense substance
If the angle of incidence is bigger than the critical angle ….
Most of the light passes out but a small amount is internally reflected
If the angle of incidence is equal to the critical angle …
Goes along the surface but there is a lot of internal reflection
If the angle of incidence is greater than the critical angle ….
All of it is internally reflected
How does an endoscope work?
Consist of 2 bundles of optical fibres
One carries light towards and the other carries and image back
Image is seen through an eye piece or TV
Perform key hole surgery
What is a converging lens?
Also known as a convex lens
Its a lens that makes light rays parallel to the principal axis to converse at the principal focus
The principal focus of a converging lens is where rays hitting the lens parallel to the axis all meet
What is a diverging lens?
Also know as a concave lens
Its a lens that makes light rays parallel to the axis diverge as if from a single point
The principal focus of a diverging lens is the point where rays hitting the lens parallel to the axis appear to come from
What is a principal focus?
The point where light rays parallel to the principal axis of a lens are focused or appear to diverge from
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 of the lens carries on in the same direction
What affects the image?
Distance affects the image
The closer the object the bigger the image
If the image is closer than F it will make a virtual image the right way up and bigger than the object on the same side
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 is a real image?
Light from an object comes together to form an image on screen
What is a virtual image?
An image seen in a lens or mirror from which light rays appear to come from after being refracted by the lens or reflected by the mirror
How do you describe an image?
How big it is in comparison to the object
Upright or inverted
Real or virtual
What kind of image does a diverging lens produce?
A virtual image
Right way up
Smaller than object
On same side of lens as the object
What does an object have to be to be magnified?
Closer to the lens that the focal length
Virtual image
Cannot be projected on to a screen
In what way is focal length related to power?
The more powerful the lens the more strongly it converges rays of light
Is a converging lens negative or positive in power?
Positive
Diverging is negative
What is focal length determined by?
Refractive index of lens material
Curvature of the 2 surfaces of the lens
Name the components of the eye
Iris Cornea Pupil Ciliary muscle Eye muscle Lens Suspensory ligaments Retina Blind spot Optic nerve
What does the retina do?
The retina is where images are formed covered in light sensitive cells which detect light and send signals to the brain to be interrupted
What does the lens do?
The lens changes shape to focus light from varying distances.
Its connected to the ciliary muscles and suspensory ligaments.
When the ciliary muscles contract this builds up tension and makes the lens go flat and spherical.
When the ciliary muscles relax tension the lens becomes thinner and flatter
What does the ciliary muscle do?
Changes the thickens of the lens
What does the iris do?
Its the coloured part of the eye that controls the light intensity and the size of the pupil
What does the cornea do?
Its the transparent window and has a convex shape and a high refractive index this focuses the eye
How does the eye focus on closer subjects?
As the eye focuses the power of the lens increases and the lens changes shape and the focal length decreases but the distance between the lens and object stay the same
How does a camera work?
For a distance object the distance from the lens to the film must be equal to the focal length if the lens.
The nearer and object is to the lens the greater the distance from the lens to the film
What kinda image does a camera form?
Real inverted image thats smaller than the object due to the object being further away than the focal length same thing happens in our eye
What is short sight cause by?
Its when the eye cannot focus on distance objects this is caused by the eyeball being too long or the cornea and lens being too powerful so the light rays form in front of the retina instead
How do you fix short sight?
Short sight is corrected with a diverging lens, the diverging lens has negative power so reduces the power and diverges the light before it enters the eye focusing it
What is long sight caused by?
Its when the eye cannot focus on near objects.
Caused by the eye being too short or the lens and cornea being too weak this mean images are formed behind the retina
How is long sight corrected?
Corrected by putting a converging lens in front this has positive power which increases the amount of power in the eye. The converging lens refracts the light before it enters the eye focusing it on the retina.
How does laser eye surgery work?
The laser vaporises the cornea to change the shape and therefore change the focusing of the retina.
Laser eye surgery can increase or decrease power.
