GCSE P3 Chapter 1

• X-rays are part of the electromagnetic spectrum and have very short wavelengths and very high frequencies

• They are affected by photographic film in the same way as light
• They are absorbed by bone and metal
• They are transmitted by healthy tissue

• X-rays can be used to form images of bones on photographic film to check for fractures and dental problems
• Charge-coupled devices (CCDs) can be used to produce electronic images of X-rays

• CT scanners can be used to produce digital images of cross-sections through the body.
• Some soft tissued organs can be filled with a contrast medium that absorbs X-rays such as barium so that they can be seen on an X-ray image

• X-rays cause ionisation and can therefore damage living tissue when they pass through therefore precautions must be taken when using them. Workers should wear film badges and shield themselves with lead screens
• However they can also be used in therapy to treat/kill cancerous tumours at or near the body surface

• Ultrasound is sound waves with a frequency of above 20000Hz so cannot be heard by the human ear

• Electronic systems can be used to produce ultrasound waves
• When ultrasound waves meet a boundary between two different materials part of the wave is reflected and travels back through the material to the detector
• The speed of the wave and the time taken to reach the detector can be used to calculate the distance to the boundary
• Distance(m) = speed(m/s) X time(s) - time taken is halved

• Ultrasound can also be used in medicine for:
  • Scanning - scanning unborn babies and soft tissues such as the eye, non ionising so safer than X-rays
  • Therapy - Shatter kidney stones into small pieces

• Refraction is the change in direction of light rays as they pass from one transparent substance into another
• Refraction takes place because waves change speeds when cross a boundary which causes a change in direction unless the ray is travelling along the normal
• A light rays will refract towards the normal when entering a denser substance and away from the normal when entering a less dense substance

• The refractive index of a substance is a measure of how much it can refract a light ray

• Refractive index(n) = sin i(sine of angle of incidence) / sin r(sine of angle of reflection)

• A light ray will refract away form the normal from glass into air
• A partially reflected ray is also seen
• As the angle of incidence increases the angle of refraction also increases
• It will increase until the refracted ray emerges along the boundary of the glass box
• The angle of incidence which produces this refracted ray is called the critical angle
• If the angle of incidence is increased beyond the critical angle then the ray undergoes total internal reflection
• When this occurs the angle of reflection is equal to the angle of incidence

•  n = 1 / sinC
• SinC = 1 / n

• An endoscope is used to look inside a patient's body without cutting it open or when performing keyhole surgery
• The endoscope is a bundle of optical fibres which are very thin glass fibres. Visible light can be sent down these fibres by total internal reflection

• Laser light can be used as the energy source in an enedoscope to carry out some surgical procedures such as cutting, cauterising and burning
• The colour of the laser light is matched to the type of tissue to produce maximum absorbtion

• Eye surgery on the retina in the eye can be carried out by using laser light that passes straight through the cornea at the front of the eye but is absorbed by the retina at the back

• Converging lenses cause parallel rays of light, that pass through it, to converge to a point
• The point at which the light rays meet is called the principal focus(focal point)
• The distance between the lens and the focal point is called the focal length
• The shorter the focal length the more powerful the lens
• Because light rays can pass in either direction through the lens their is principal focus on both sides of the lens

• If the object is closer to the lens than the principal focus is then the image produced will be virtual, upright, magnified and will be produced in front of the lens
• Magnifying glasses are used in this way
• Magnification = image height / object height

• If the object is further away from the lens than the principal focus is then the image will be real, inverted, diminished and will be produced behind the lens.
• The closer the object to the lens the larger the image produced
• Camera lenses use converging lenses in this way

• Diverging lenses cause parallel rays of light that pass through it to refract so that they diverge away from a point
• This point is called the principal focus(focal point)
• The distance from the centre of the lens to the principal focus is the focal length
• Because light rays can pass in either direction there is a principal focus on either side of the lens

• The image produced by a diverging lens is always virtual

• Ray diagrams can be drawn to find the image that different lenses produced with objects in different positions
• The line passing through the centre of the lens and perpendicular to it is called the principal axis
• Ray diagrams include 3 construction lines to form the poistion of the image:
  • From top of the object through the centre of the lens no refraction
  • From top of the object horizontal till the lens where it is refracted through principal focus
  • From top of the object through the first principal focus to the lens then horizontal
• Where the points meet is where the image is formed

• If the rays of light will never meet then a virtual image is produced infront of the lens

• Power of lens(dioptres) = 1 / focal length(m)

• Cornea - transparent layer that protects the eye. Light enters the eye through the cornea and helps to focus the light onto the retina
• Lens - Focuses light onto the retina
• Pupil - Central hole formed by the iris where light enters the eye
• Iris - Coloured ring of muscle that controls the diameter of the pupil and therefore controls how much light enters the eye
• Suspensory ligaments and cilliary muscles - control the thickness of the lens by contracting and relaxing
• Retina - The light sensiteve cells aound the inside of the eye
• Blind spot - The region of the retina where no light cells are present
• Optic nerve - Carries impulses from the retina to the brain

• The normal human eye has a near point of 25cm and a far point of infinity so its range of vision is 25cm to infinity

A person who is short sighted:

• Can see close objects clearly but distant objects are blurred
• This is beacuse either their lens is too powerful or their eyeball is too long
• This causes the light rays to come into focus in front of the retina
• Their vision can be fixed by wearing glasses with a diverging lens
• Rays spread out before passing through eye lens bringing rays to focus on the retina

A person who is long sighted:

• Can see distant objects clearly but close objects are blurred
• This is because thier eye lens is not powerful enough or their eybal is too short
• This causes the light rays to come into focus behind the retina
• Their vision can be fixed by wearing glasses with a converging lens
• Rays refract together before passing through eye lens bringing rays to focus on retina

The focal length of a lens is determined by: refractive index of the material from which the lens is made and curvature of the two surfaces of the lens

• Given focal length greater refractive index, thinnner the lens can be manufactured