Refraction, Lenses, Eye and Eyesight etc

refraction is the change of direction of light as it passes from one (transparent) medium to another.

waves change speed when they cross a boundary - this causes a change in direction, unless the waves are travelling along a normal.

air > glass: light ray SLOWS down, bends TOWARDS normal.

the refractive index of a substance is a measure of how much the substance can refract a light ray

sin i/ sin r

sin -1 for working out i or r

Refraction is the bending of a light ray at the boundary of two medium as the light ray propagates from a medium to another with difference optical density.

when light ray refracts from glass to air, a partially reflected ray is also seen. 

if the angle of incidence in the glass is gradually increased, the angle of refraction increases until the refracted ray emerges along the boundary - this angle of incidence = THE CRTICIAL ANGLE

if the angle of insidence is increased beyond the critical angle the light ray undergoes total internal reflection - the angle of reflection is equal to angle of incidence anf there is no refraction. - all light is reflected now!!!

a device that's used to look inside a patient's body without cutting it open, or when performing KEY HOLE surgery. 

it contains bundles of optical fibres - very thin, flexible glass fibres.  visible light is sent along these fibres by T.I.R

laser light may be used as the energy source in an endoscope 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 absorption.

egs: eye surgery on the retina 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.

An optical fibre is a thin rod of high-quality glass. Very little light is absorbed by the glass. Light getting in at one end undergoes repeated total internal reflection, even when the fibre is bent, and emerges at the other end.

Information such as computer data and telephone calls can be converted into electrical signals. These can be carried through cables, or transmitted as microwaves or radio waves. However, the information can also be converted into either visible light signals or infrared signals, and transmitted by optical fibres.

Optical fibres are also used in endoscopes that allow surgeons to see inside their patients.

converging:

parallel rays of light that pass through are refRACTED so that they CONVERGE to a point.  this point = the principal focus - and the distance from the centre of the lens to focal point = the focal length

principal focus on each side of lens because light can pass through in either direction. 

if the object is further away from the lens than the principal focus, an inverted, real image is formed.  size of the image depends on the position of object - nearer to lens = larger image

if object is nearer to lens than principal focus, an upright, virtual image is formed behind the object - image is MAGNIFIED. 

parallel rays of light that pass through the lens are REFRACTED so that they diverge away from a point - principal focus.

the distance from centre of lens to principal focus = focal length

ALWAYS VIRTUAL IMAGE.

diverging = concave lens

CONVErging = CONVEx lens

RI, VU > real, inverted.  virtual, upright.

DRAW RAY DIAGRAMS

light enters the eye through the CORNEA

the cornea and the eye lens focus the light onto the RETINA

the iris adjusts the size of the pupil to control the amount of light entering the eye

the ciliary muscles alter the thickness of the lens to control the fine focusing of the eye - they are attatched to the lens by the suspensory ligaments.

near point: 25cm

far point: infinity

power of lens - DIOPTRES, D (1/ focal length of the lens, m)

can see close objects clearly, but blurred distanct objects because the image is formed IN FRONT OF THE RETINA.  far point is closer than infinity.

• eyeball to long, or
• lens too powerful - can't be made thin enough to focus on near

corrected by a DIVERGING lens which diverges the rays before they enter the eye and make them converge LATER - so image formed on retina now :)

can see distant objects clearly, but close objects blurry - iamge is formed behind the retina

• eyeball too short
• eye lens too weak - cant become thick enough for close up focusing

correct with converging lens - make rays refract more, so meet sooner (on retina)

focal length of a lens is determined by:

• refractive index of the material the lens is made from
• curvature of the two surfaces of the lens

for a lens of a given focal length, the greater the refractive index of the lens material, the flatter and thinner the lens can be manufactured