X-rays
X-rays are a part of the electromagnetic spectrum.
They are before Gamma rays and after ultraviolet in terms of wavelength, frequency and energy.
Properties:
Short wavelength
Ionising (causes electrons in atoms to be removed)
They are absorbed by metal and bones.
They are transmitted by tissues (passed through)
They affect photographic film like how visible light does
These properties make X-rays useful for medical treatments.
Photographic film
X-rays can be used to show images of bone fractures in medical imaging.
They are directed at the areas where there is a bone fracture or problem.
X Rays will pass through tissues and absorbed by bones.
The X Rays that pass through the tissues will cause the photographic film behind it to darken.
The X rays also pass through fractures so they darken on the photographic film.
The image is a negative image of the bones where bones are more white and tissues are more black.
Fracture will be where there are dark areas on the bone.
Teeth imaging acts the same way. Decayed teeth will absorb less X rays and appear darker.
CCDs
Charged coupled device.
A modern alternative to photographic films
The images are formed electronically which allows them to be stored electronically which is easier than photographic film,.
CT Scans
Computerised Tomography.
This uses multiple 2D images scanned around the body in different angles and a computer is used to build a 3D image of the body. This allows the doctors to see in detail structures in the body at different angles. This allows doctors to get a greater insight of the patient.
Cancer treatment
X-rays are ionising and can damage DNA in cells, this causes cancer.
Low doses cause cancer and high doses may be used to kill cancerous cells.
High energy X-rays can be directed at tumours which causes damage to cancerous cells so they die.
This treatment is called radiotherapy
Patients are limited to the number of scans to reduce risk of cancer as their bodies wont be exposed to too much radiation.
X-ray machines produce low energy X-rays which reduce the risk of damaging the human tissue.
Staff are shielded by a lead wall which protect them from radiation getting to them by absorbing the radiation so their body wont be exposed to dangerous ionising radiation.
Radiographers use x-ray machines. They also wear lead aprons which protects them by absorbing radiation so their bodies won't be exposed to the radiation.
Ultrasound
Very high frequency sound waves (above 20,000 Hz) above the range of human hearing
A range of human hearing is 20 Hz- 20,000 Hz.
Ultrasound is heard by some other animals such as bats.
Ultrasound are sound waves so they follow the law of waves
Reflection
Ultrasound can be used to check internal cracks in buildings and other objects.
Cracks acts as a boundary between two media so they cause a partial reflection back. Remaining ultrasound waves continue to pass through. When the Ultrasound hit the back edge of the object it also causes a partial reflection as it is a boundary between two media.
A detector is placed at the source of the ultrasound wave so it can detect reflected waves.
This process is used to check for defects and damage on manufactured objects e.g railways.
s= v *t
distance = speed * time
(m) (m/s) (s)
As a reflection is like an echo you need to divide the time by 2 to find the distance from the detector to the crack and not from there back again.
Medical imaging
Ultrasound can also be used in medical imaging,
the wave is partially reflected at different tissues.
This allows to find how a foetus is developing and we can take measurements of the growth.
Computers produce an image by combining the many ultrasound reflection readings.
Ultrasound can also be used to remove kidney stones.
They are solid crystals found in urine and they may sometimes build up inside the kidney causing a blockage which causes pain.
High frequency ultrasound can be used to cause the kidney stones to vibrate, breaking it into smaller pieces which can be flushed out by urine.
Ultrasound compared with Xray
X-rays have a much shorter wavelength so it produces much more clearer and accurate images in higher quality, this is vital for detecting fractures and small abnormalities.
X-rays can be used to produce and even higher quality image through CT scans which allows us to see at different angles and different levels so it can see obstructed images done using a simple x-ray scan.
Problem of x-rays is that it is ionising and damages cells.
It can lead to cancer if it damages the DNA of cells causing rapid division.
X-rays are particularly damaging to foetus as they are still growing.
Ultrasound are not ionising so they are always safe to use but they are not as high detail.
Lenses.
Lenses rely on refraction of light.
Refraction is the process where light changes direction as it goes from one medium to another but not at 90 degrees.
When it goes into a denser medium it moves towards the normal
When it goes into a less dense medium it moves away from the normal
Refractive index is a constant for a material how much it refracts depending on the angle as different material have different densities. It shows how much the material slows the speed of light.
Refractive index = sin i / sin r
i = angle of incidence
r = angle of reflection.
Convex = converging
Concave = diverging
The amount of refraction depends on how curved the lenses are and the refractive index of the material it is made from.
In order for them to be refracted,Rays from an object travels to the lens parallel to each other .
They are converged by a convex lens and diverged by a concave lens
The principal focus is where parallel rays are refracted and come together. (F)
Focal length is the distance from the principal focus to the centre of the lens.
A real image is produced when light rays are focused and this can be projected onto a screen.
Concave lens spread out parallel rays of light.
They form a virtual image which cannot be projected onto a screen.
The rays can only be seen by the eye and appears to have came from a different point to where the object is.
The point where rays appear to be is called the principal focus (F)
The focal length is the distance from the principal focus to the centre of the lens.
A produced image can be described as.
Magnified or diminished
Upright or inverted
real or virtual
Magnification of an object is worked out by this:
Magnification = image height / object height
When the magnification is:
Above 1 = image is larger
Below 1 = image is smaller
1 = image is the same size
Concave lens always produce images that are virtual, upright and diminished.
The image appears to come from the same side of the lens as the object.
As the image is virtual, rays leaving the lens are traced back in straight lines till they reach a point where they cross. This is where the virtual image will be
Eye
The eye is a sense organ.
Some people have vision defects which results them being unable to focus light from objects into the retina
Lenses are used to correct this by having them in front of the eye.
Cornea - Transparent covering over front eye. It is convex so it refracts light as it enters the eye (fixed)
Iris is the coloured part of the eye and it contains muscles that adjust the size of the pupil. It controls the amount of light entering the eye.
Pupil is where light pass through as it enters the eye.
Lens is a transparent flexible disk behind the iris that acts like a convex lens. It refracts lights to focus onto the retina. The amount is adjusted by altering the curvature of the lens.
The Ciliary muscles are muscles that connect to the lens by suspensory ligaments. It adjusts the shape of the lens to make it change curvature which changes the amount of refraction.
The retina is a lining at the back of the eye which contains two types of receptor cells.
Rods are sensitive to dim light and black and white.
Cones are sensitive to colour.
The retina sends electrical impulse to the brain.
The optical nerve connects to the brain.
Ciliary muscles contract when objects are near, the suspensory ligaments loosen and the muscle tension on the lens are low. The lens is fatter and more curved.
Ciliary muscles relax to distant objects, the suspensory ligaments stretch and the lens are in high tension. the lens are thinner and less curved.
Near point is the closet an object can be without the object being blurred. It is normally 25 cm.
Long range people have this further than 25cm as they cannot focus properly on objects.
The Farpoint is the furthest an object can be without it being blurred. This is infinite as light rays arrive parallel to each other.
Short sighted people have this closer than infinity.
The distance between the far and near point is the range of vision.
Camera
A camera acts the same way as the eye.
The imaged produced is diminished, inverted and real.
Lens- focus light to a photographic film or CCD - lens in eye
Focusing screw adjusts the focus for near and further objects -ciliary muscles
Aperture- Adjusts the amount of light entering the camera -iris
Shutter- Adjusts the length of time light enters the camera, controls the amount of light the photosensitive material is exposed to -eyelid
Photosensitive surface- Detects and records light, photographic film or CCD - Retina
Vision defects
Short sightedness is caused by the eyeball being too elongated or the lens is too thick and curved. The light are focused in front of the retina. The farpoint is closer than infinity. So it cannot focus on distant objects.
A diverging lens is able to correct the defect as it diverges and the light focuses at the retina.
Long sightedness is caused by the eyeball being too short or a loss of elasticity in the lens which means it cannot become fat enough to focus. this means the light rays focus behind the retina.
Their near point is further than 25cm. They cannot focus properly at near objects.
Converging lens are able to converge the light rays so they focus at the retina.
Power of lens
P= 1/f
Lens power = 1/ focal length
(D) (m)
The focal length is determined by the refractive index and the thickness of the lens.
Concave have negative power. Concave has positive.
With a greater refractive index you can have thinner lens which are lighter.
Lasers
A laser produces a narrow beam of light, It has low divergence.
It can be used to:
Cut material
Burning
Cauterising.
Cauterising means to destroy damaged tissues or to stop bleeding used in medicine.
It can be used in eye surgery to repair damaged retinas or to cut the cornea to correct a person's vision.
Total internal reflection
Light speeds up when going from a more dense to a less dense medium e.g glass to air.
The light ray bends away from the normal.
After the critical angle, waves reflect back into the glass and no refraction occur.
Optical fibres are thin rods of high quality glass.When light gets in it under goes TIR even when the wire is bent.
Optical fibres are used for high speed communications e.g. TV and broadband.
It is also used in medicine. Endoscopes.
Endoscopes are a bundle of optical fibres which can be inserted into the body. Light is carried into the body and light is reflected back out. Doctors can see the insides of the body.
Critical angle
The angle above which Total internal reflection occur.
This depends on the refractive index of the material.
The lower the refractive index, the higher the critical angle.
Refractive index = 1/sin c
c = critical angle in degrees.
No comments:
Post a Comment