P3 Physics Medical applications Section a) The structure of the eye. The structure of the eye is limited to: ■ retina ■ lens ■ cornea ■ pupil /iris ■ ciliary muscle ■ suspensory ligaments.

P3 Physics Medical applications Section b) Correction of vision using a convex lens to produce an image on the retina: ■ long sight, caused by the eyeball being too short, or the eye lens being unable to focus

P3 Physics Medical applications Section b) Correction of vision using a convex lens to produce an image on the retina: ■ long sight, caused by the eyeball being too short, or the eye lens being unable to focus

P3 Physics Medical applications Section b) Correction of vision using a convex lens to produce an image on the retina: The correcting lens is a converging one: it aids the eye by converging the rays BEFORE they reach the eye. The amount of converging depends on the focal length. Lens power, P, is equal to the inverse of the focal length in metres or P = 1/f e.g. a converging lens of focal length 20cm: f = 0.2m So P = 1/0.2 = +5D (Dioptres)

P3 Physics Medical applications Section b) Correction of vision using a concave lens to produce an image on the retina: ■ short sight, caused by the eyeball being too long, or the eye lens being too strong to focus on the retina

P3 Physics Medical applications Section b) Correction of vision using a concave lens to produce an image on the retina: ■ short sight, caused by the eyeball being too long, or the eye lens being too strong. The correcting lens must DIVERGE the light BEFORE it enters the eye. The lens is a DIVERGING LENS so it has a NEGATIVE POWER P = 1/f e.g. f = 25cm which is 0.25m So P = 1/0.25 = -4D

P3 Physics Medical applications Section c) Range of vision. The eye can focus on objects between the near point and the far point. Candidates should know that the near point is approximately 25 cm and the far point is infinity for a normal eye. For a short sighted eye, the near point is closer than 25cm but the far point is closer than distant objects so they appear out of focus. For a long sighted eye, the near point is further than 25cm so it is hard to see close by objects clearly e.g. writing, so a long sighted person often needs glasses for reading. Distant objects are still seen clearly.

P3 Physics Medical applications Section d) Comparison between the structure of the eye and the camera. Both have a converging lens. Both control the amount of light entering Both have a light sensitive layer 1.The eye lens can change shape to adjust for objects at different distances 2.The camera has to move the lens away from the film

P3 Physics Medical applications Section d) Comparison between the structure of the eye and the camera. Both control the amount of light entering 1.The eye has an entrance hole called the pupil. Its size is controlled by the iris and controls the light entering the eye. 2.The camera has an APERTURE which controls the amount of light reaching the film or CCD

P3 Physics Medical applications Section d) Comparison between the structure of the eye and the camera. Both have a light sensitive layer 1.The retina is sensitive to light and is a 3D structure 2.Older cameras used film but digital cameras use a CCD which is a light sensitive structure of many picture elements or PIXELS

P3 Physics Medical applications Section d) The eye and the camera. Film, retina, CCD: