Physics 3A GCSE AQA

X- Rays:

• These are high frequency, short wavelength electronmagnetic waves - ionising
• They pass through health tissue, but are absorbed by denser materials eg. bones and met
• Are dangerous to health and to developing babies - ionising but image quality is great

Uses:

• X-ray photographs are used to diagnose medical conditions like bone fractures or dental pro
• CT Scans - produce a 3D image        - Treat cancer

Ultrasound:

• Is over 20,000 Hz - so very high frequency, higher than a humans, non- ionising - safer
• Ultrasound waves get parcially reflected at a boundary between media. You can use an oscilloscope traces to find the boundaries
• Image quality isnt that great but is a lot safer to use than x-rays

Uses:

• Pre-natal scanning      - Breaking down kidney stones     - Scanning the sea bed

Every transparent material has a Refractive Index - the ration of speed of light in a vacuum to speed of light travelling through a medium

Refraction is caused by the waves changing speed as they enter a different medium. This is caused by the density change from one medium to another - changing the speed

• When waves slow down, they change direction towards the imaginary normal line
• If a wave hits a boundary at 90 degrees (eg along the normal line) it will not change direction
• When light hits a different medium (eg plastic or glass), some of the light will pass through but some will be reflected, depends on the angle of incidence (the angle it hits the medium)

The angle of incidence (i), angle of refraction (r) and refractive index (n) are all linked

Refractive Index = Sin (i)/Sin (r) 

• Convex - bulges outwards
• Produces inverted images
• Causes the rays to move together and meet at the focus point parallel to the axis - in front of the lens

Rules of Refraction:

1) A ray hitting parallel refracts through the focal point

2) A ray stricking through a focal point refracts parallel

3) A ray refracts through the centre of the lens continues in the same direction (not bent) - any angle

• Concave -  caves inwards (like a capital I)
• Image is the same side as the object - virual (not real)
• Rays are more spread out - meet up behind the lens

Rules of Refraction and Ray Diagrams:

1) A ray passing through the centre of the lens will carry on in the same direction

2) A ray hitting parallel will appear to have come from the focal point on the near side of the lens (behind it)

The image is:

- Virtual (not real)

- Diminished (smaller)

- Right way up

Magnifying Glasses

• Use converging lenses
• Create a magnified virual image

1) The object being magnified must be closer to the lens than the focal length

2) Since the image produced is a virtual image, the light rays don't actually come from the place where the image appears to be

Magnification = Image Height/Object Height

Powerful lenses have a short focal length. The more powerful the lens, the more strongly it converges rays of light, so shorter the focal length.

Power (D) = 1/Focal Length (m)      P =1/f

The focal length is determined by two factors:

• The refractive index of the lens material
• The curvature of the two surfaces of the lens

Structure of the Eye:

Cornea: -  refracts light - bends it as it enters the eye

Iris: - Pigmented - decides the colour of your eyes. Its muscles contract and relax to alter the size of its central hole or pupil. Controls how much light enters the pupil

Retina: - Contains the light receptors

Optic nerve: - Bundle of sensory neurones at back of eye. Carries impulses from the eye to the brain

The eye can focus on objects between the near and far points

A camera works in roughly the same way as the eye.

• The image on the film is a real image because the rays of light actually meet there
• The image is smaller than the object, because the object is alot further away than the focal length.         - The image is inverted

Lens: - Behind the iris attached by the suspensory ligaments to the ciliary muscles. Focuses light onto the retina

Short sight is corrected by DIVERGING LENSES

• Short-sighted people can't focus on distant objects - their far point is closer than infinity
• This is caused by the eyeball being too long, or by the cornea and lens's being too powerful - this means the eye lens cant produce a focused image on the retina
• The images of far away objects are brought into focus in FRONT of the lens
• To correct short sight, a diverging lens (negative power) in worn in front of the eye. This diverges light before it enters the eye, so the image can be focused on the retina

Long sight is corrected by CONVERGING LENSES

• Long-sighted people cant focus on near objects - their near point is further away than normal (25cm or more)
• This happens when the cornea and lens are too weak or the eyeball is too short - so near objects are brought into focus BEHIND the retina
• To correct long sight, a converging lens (positive power) is worn in front of the eye. The light is refracted and starts to converge before it enters the eye, so the image can be focused on the retina

These are used to surgically correct eye problems (laser eye surgery)

• A laser is a narrow, intense beam of light
• They can be used in surgery to cut through body tissue
• They cauterise (burn and seal shut) small blood vessels as they cut the tissue. This reduces blood loss and reduces the chance of infection

Surgical Uses:

• Skin correction -  eg acne scars - burn the stop layer of tissue, leaving health tissue underneath
• Eye Surgery - A laser vaporises some of the cornea to change its shape - changing its focusing ability. This can increase or decrease the power of the cornea so the eye can focus images properly on the retina

Total Internal Reflection can only happen when a wave travels through a dense substance like glass or water towards a less dense substance, like air

The value of the critical angle depends on the Refractive Index   Refractive Index = 1/sin (c)

Uses of Total Internal Reflection:

• Optical Fibres - small thin plastic tubes which carry visible light over great distances
• Endoscopes - use lots of optical fibres to let surgens look inside the body