Medical Physics Lecture Notes PDF

 Physics of the Optical System Eye as an optical system: Eye is like a camera. Light enters the eye through a small hole called the pupil and is a focusing lens, which focuses images from different distances on the retina, which is like a camera film. At the center of the iris...

 Physics of the Optical System Eye as an optical system: Eye is like a camera. Light enters the eye through a small hole called the pupil and is a focusing lens, which focuses images from different distances on the retina, which is like a camera film. At the center of the iris colored membrane, controls the amount of light entering the eye by opening and closing the pupil, a muscle-controlled, variable-diameter hole, or aperture, which Controls the amount of light that enters the eye. It appears black because essentially all of the light that enters is absorbed inside the eye A tough white sheet called sclera covers the outside of the eye except the cornea. The front of the sclera is transparent to allow the light to enter the eye. Ciliary muscles control the focusing of the lens automatically. Cornea a clear transparent membrane covers the front part of the eyeball, and is made of living cells without blood supply The cornea focuses by bending (refracting) the light rays. The amount of bending depends on the curvatures of its surfaces and the speed of light in the lens compared with that in the surrounding material. The index of refraction is nearly constant for all corneas, but the curvature varies considerably from one person to another and is responsible for most our defective vision Retina is facing the cornea with a mesh of nerve fibers lining the back half of the eyeball. it converts light images into electrical impulses, sent to the brain by the optic nerve. Near the center of the retina is a small depression which is called the fovea central is. This small part of the retina is responsible for our highest visual acuity. It consists entirely of cones packed closely together. When the eye scans a scene, it projects  The cavity of the eye is filled with two types of fluid: Optics of the Eye The cornea and crystalline lens together act like a single, convergent Lens. Light entering the eye from an object passes through this lens system and The eye focuses on objects at varying distances by accommodation, or the use forms an inverted, real of muscles to change the curvature, and thus the focal length, of the crystalline lens. In its most relaxed state, the crystalline lens has a long focal length, and image on the retina but the eye can focus the image of a distant object on the retina. The farthest the brain automatically distance at which the eye can accommodate is called the far point for distinct corrects for this. vision. For a normal eye, the far Point is infinity. Focusing The cornea and eye lens form a compound lens system, producing a real inverted image on the retina. From air to cornea: large bending, the main focusing. From cornea to eye lens, less focusing power. The eye has a limited depth of field. We cannot see things close and far at the same time. FOCUSING ELEMENTS OF THE EYE The eye has two major focusing components: 1-The cornea is a fixed focus element. 2-The lens is variable in shape and has the ability to focus objects at various distances. Accommodation The eye focusing is not done by change the distance between the lens and retina. Rather, it is done by changing the focal length of the eye lens. Ciliary muscles help to change the shape of the lens accommodation. Muscles relax, long focal length, see objects far way; Muscles tense, short focal length see objects close. Normal eyes can see 25cm to infinity Eye is said to be normal, when in a state of full relaxation, it can focus on the retina objects at an infinite (∞) distance. Looking to a near object, the eye accommodates itself by changing the power of its lens in order to form the image on the retina. Far point: is the distance between the eye and the furthest object that can be brought into focus. The far point is effectively infinity (∞)for normal vision. the near point as the closest distance at which it can see clearly Astigmatism is caused by a distortion of the cornea shape. Normally the cornea is almost spherical but in astigmatism, its curvature is greater in one region than another. Vision is blurred at all distances. Astigmatism usually occurs with either short or long sight. Presbyopia: As we age, the ability of our eyes to focus both near and far begins to diminish. The lens inside the eye becomes less flexible with time In presbyopia, the image of close objects is focused behind the retina Common eye defects  ‫تم بحمد هللا‬ ‫كل ما هو خير كان من عند هللا‬ ‫وكل ماهو تقصير نتلمس العذر فيه‬ ‫وسعينا الى ان نكون على قدر ايصال المعلومة الكافية‬ ‫بارك هللا فيكم وجهودكم مباركة‬