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Physiological optics 5 th lecture Dr. Mohammad Shehadeh.

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1 Physiological optics 5 th lecture Dr. Mohammad Shehadeh

2 Reflection of Light When light meets an interface between two media, its behaviour depends on the nature of the two media involved. Light may be absorbed by the new medium, or transmitted onward through it it may bounce back into the first medium. This 'bouncing' of light at an interface is called reflection. It occurs, to some degree, at all interfaces even when most of the light is transmitted or absorbed. It is by the small amount of reflected light that we see a glass door and thus avoid walking into it.

3 Laws of Reflection (1) The incident ray, the reflected ray and the normal to the reflecting surface all lie in the same plane. (The 'normal' is a line perpendicular to the surface at the point of reflection.) (2) Angle of incidence, i, equals the angle of reflection, r.

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5 Reflection at an Irregular Surface When parallel light encounters an irregular surface, it is scattered in many directions,This is called diffuse reflection. It is by diffuse reflection that most objects are seen, e.g. furniture, etc. A perfect reflecting surface (free from irregularities causing diffuse reflection) would itself be invisible. Only the image formed by light reflected in it would be seen.

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7 Reflection at a Plane Surface: Plane Mirrors In Fig., light from object O is reflected at the surface according to the laws of reflection. If the reflected rays are produced behind the surface, they all intersect at point I, the image of object O.

8 The brain always assumes that an object is situated in the direction from which light enters the eye. Light from object O appears to come from point I, the image of O. However, if the observer actually goes to point I, there is no real image

9 Virtual Images: it could not be captured on a screen. real images: Images which can be captured on a screen.

10 Characteristics of plane mirror image upright (erect), virtual, laterally inverted. It lies along a line perpendicular to the reflecting surface as far behind the surface as the object is in front of it.

11 Rotation of a Plane Mirror If a plane mirror is rotated while light is incident upon its centre of rotation, the reflected ray is deviated through an angle equal to twice the angle of rotation of the mirror.

12 Reflection at Spherical Reflecting Surfaces a spherical mirror : A reflecting surface having the form of a portion of a sphere. concave mirror: If the reflecting surface lies on the inside of the curve. convex mirror: If the reflecting surface lies on the outside of the curve.

13 the centre of curvature, C, is the centre of the sphere of which the mirror is a part, the pole of the mirror, P, is the centre of the reflecting surface, CP = the radius of curvature, r.

14 An axis is: any line passing through the centre of curvature and striking the mirror. the principal axis : That passing through the pole of the mirror; subsidiary axis : any other axis. Rays parallel to the principal axis are reflected towards (concave) or away from (convex) the principal focus, F of the mirror The distance FP is the focal length, f, of the mirror and is equal to half the radius of curvature.

15 the image is constructed using two rays: (1) A ray parallel to the principal axis and reflected to or away from the principal focus. (2) A ray from the top of the object, passing through the centre of curvature and reflected back along its own path.

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17 Image formation by the concave mirror. (a)Object outside the centre of curvature, C: 1. Image real, 2.inverted, 3.reduced in size 4.lying between C and principal focus F

18 (b) Object between centre of curvature C, and principal focus F. 1.Image real, 2.inverted, 3.enlarged, 4.lying outside the centre of curvature C.

19 (c) Object inside principal focus F. 1.Image erect, 2.Virtual 3.Enlarged

20 Image formation by the convex mirror. The object may be located at any distance from the mirror. The image is 1.virtual, 2.Erect 3. diminished.

21 For any position of the object, the position of the image formed by a spherical mirror can be calculated using the formula:

22 Magnification is : the ratio of image size to object size The magnification can be calculated using the formula:

23 When using these formulae, the sign convention must be adhered to

24 All distances are measured from the pole of the mirror. Distances measured in the same direction as the incident light are positive, those against the direction of the incident light are negative. The incident light is usually shown coming from the left in optical diagrams. Image size is positive for erect images and negative for inverted images

25 Clinical Application The anterior surface of the cornea acts asa convex mirror and is used as such by the standard instruments employed to measure corneal curvature Images formed by the reflecting surfaces of the eye are called catoptric images

26 catoptric images Purkinje–Sanson images Each refracting interface in the eye also acts as a spherical mirror, reflecting a small portion of the light incident upon it Four images are therefore formed by reflection at the four interfaces, the anterior and posterior corneal surfaces and the anterior and posterior lens surfaces

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28 Images I, II and III (from anterior corneal, posterior corneal and anterior lens surfaces respectively) are erect, virtual images because they are formed by convex reflecting surfaces Image IV (from the posterior lens surface) is a real, inverted image because it is formed by a concave reflecting surface

29 Positions of the images image I lies just behind the anterior lens capsule, image II is close behind it. Image III is located in the vitreous, image IV is inverted and in the anterior lens substance.

30 Uses of the first image (image I) 1.to study the anterior corneal curvature is routine in clinical practice. 2.The regularity of the curvature is examined using Placido's disc, 3. the radius of curvature is measured using the keratometer. 4. The first image is also used in the diagnosis and measurement of squint.

31 Uses of the second image (Image 2) 1.Corneal thickness when measuring the distance between image 1 and 2 2.Anterior chamber depth when measuring the distance between image 2 and 3

32 Uses of images III and IV. Much information regarding the changes in lens form during accommodation has also been obtained from the study of images III and IV.


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