Dr. Mohammad Shehadeh

 Refraction: is the change in direction of light when it passes from one transparent medium into another of different optical density.  The incident ray, the refracted ray and the normal all lie in the same plane.  The velocity of light varies according to the density of the medium through which it travels. 

 The more dense the medium the slower the light passes through it.  When a beam of light strikes the interface separating a less dense medium from a denser one obliquely, the edge of the beam which arrives first, A, is retarded on entering the denser medium.

 The opposite side of the beam, B, is meanwhile continuing at its original velocity.  The beam is thus deviated as indicated in Fig being bent towards the normal as it enters the denser medium.

 It is a comparison of the velocity of light in a vacuum and in another medium (optical density of that medium)

 As the optical density of air as a medium is negligible under normal conditions

 Air= 1  Water (incl. Aqueous)= 1.33  Cornea= 1.37  Crystalline lens= 1.386–1.406  Crown glass= 1.52  Flint glass= 1.6  Diamond= 2.5

 On entering an optically dense medium from a less dense medium, light is deviated towards the normal.  The incident ray makes an angle, i, the angle of incidence, with the normal.  The angle between the refracted ray and the normal is called the angle of refraction, r.

 Snell's law states that the incident ray, refracted ray and the normal all lie in the same plane  angles of incidence, i, and refraction, r, are related to the refractive index, n, of the media concerned by the equation

 where the first medium is a vacuum, n is the absolute refractive index,  and in air n is the refractive index.  on passing from medium1 into medium2, the index of refraction is given by

 Light passing obliquely through a plate of glass is deviated laterally and the emerging ray is parallel to the incident ray.  Thus the direction of the light is unchanged but it is laterally displaced

 some reflection also occurs at every interface  a lens or window with a refractive index of 1.5 in air reflects 4% of light from the anterior surface and transmits the remaining 96% to the posterior surface;  a further 4% of this is reflected so that the lens transmits only 92.16% of normally incident light

 a sheet of glass as an image-splitter, e.g. the teaching mirror of the indirect ophthalmoscope.  Most of the light is refracted across the glass to the examiner's eye.  However, a small proportion is reflected at the anterior surface of the glass and enables an observer to see the same view as the examiner.

 Light passing across a curved interface between two media of different refractive indices obeys Snell's law.  A convex spherical curved surface causes parallel light to : 1. converge to a focus if n2 is greater than n1, 2. diverge as from a point focus if n2 is less than n1

 The refracting power or vergence power of such a surface is given by the formula: r: is the radius of curvature of the surface in metres The surface power is measured in dioptres Surface power is positive for converging surfaces and negative in sign for diverging surfaces

 Objects situated in an optically dense medium appear displaced when viewed from a less dense medium.  This is due to refraction of the emerging rays which now appear to come from a point I, the virtual image of object O.  Objects in water seem less deep than they really are.

 Ray A strikes at 90° to the interface and is undeviated  Ray B emerges after refraction.  Ray C, runs parallel wi h the interface (the critical angle)

 Rays striking more obliquely than the critical angle still fail to emerge from the denser medium and are reflected back into it as from a mirror.  The critical angle is determined by the refractive indices of the media involved and can be calculated using Snell's law.  The critical angle for the tear film/air interface is 48.5°, and for a crown glass/air interface the critical angle is 41°.

 Total internal reflection is used in optical instruments: Fibre optic cables 1. surgical intraocular light source and 2. the transmission of laser light from the laser tube to the delivery system of the laser slit lamp.

 Total internal reflection also occurs at surfaces within the eye  the cornea:air interface, and prevents visualisation of parts of the eye, e.g. the angle of the anterior chamber and peripheral retina.  The problem is overcome by applying a contact lens made of material with a higher refractive index than the eye and filling the space between eye and lens with saline, thus destroying the cornea/air refracting surface and allowing visualisation of the anterior chamber angle (gonioscopy) and peripheral retina (three-mirror).

 In fact, the refractive index of any medium differs slightly for light of different wavelengths.  Light of shorter wavelength is deviated more than light of longer wavelength, e.g. blue light is deviated more than red.  The refractive index of a material is normally taken to mean that for the yellow sodium flame.  The angle formed between the red and blue light around the yellow indicates the dispersive power of the medium  This is not related to the refractive index of the material.

 Total Internal Reflection and Dispersion  When sunlight enters a raindrop it is dispersed into its constituent spectral colours  Under certain circumstances, the angle of incidence is such that total internal reflection then occurs within the drop.  The dispersed light finally emerges, each wavelength or colour making a different angle with the horizon.  To see the rainbow, the observer must look away from the sun.

 The observer receives only a narrow pencil of rays from each drop, i.e. only one colour.  The whole rainbow is the result of rays received from a bank of drops at increasing angle to the observer's eye  Violet, the colour making the smallest angle to the horizon, is received from the lower drops while red, making the greatest angle with the horizon, is received from the highest drops

 Thus the red is on the outside of the primary rainbow.  The secondary rainbow is formed by rays that have twice undergone total internal reflection within the raindrops, and the colours are seen in reverse order: violet is on the outside of the bow.