Dr. Mohammad Shehadeh. Physiological optics 6 th lecture. 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.
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Dr. Mohammad Shehadeh
Physiological optics 6th lecture
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
Water (incl. Aqueous)= 1.33
Crystalline lens= 1.386–1.406
Crown glass= 1.52
Flint glass= 1.6
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 :
converge to a focus if n2 is greater than n1,
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:
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
surgical intraocular light source and
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.