Optics

·         Minimal angle of resolution of an eye with a 6/6 acuity: One degree of arc

·         An eye with 6/6 visual acuity read the 6/24 lines from 24 m away.

·         Snell's law of refraction: the incident and refracted rays and the normal to the surface at the point of incidence lie in the same plane and the ratio of the sine of the angle of incidence i to the sine of the angle of refraction r is a constant for any two media. This constant is called the relative index of refraction ie. sine i / sin r = refractive index.

·         Apex angle in degrees of a 10 dioptres prism made of glass: 10 degrees

·         The nodal point of a thin lens is at the intersection between the principle axis and the principle plane.

·         The refracting power of a cylindrical lens is at 90 degrees to the axis.

·         The image produced by a negative lens is virtual, erect & diminished.

·         A focimeter measures the BACK VERTEX POWER of a lens.

·         A decentration of 10 mm from the optical centre of a +5 dioptre lens produces a prismatic effect of  5 prism dioptres.

·         A X8 loupe has an equivalent power of 32  dioptres.

·         A lens of +10 dipotres fully correct an hyperopia and now the lens is moved forward 10mm, what is the new lens power needed to correct the hyperopia: +9D

Dn = Do/1-dDo

 where Dn = the new power

Do = the old lens power

d = difference in the location (in meters), -ve if it is moved forward and +ve if it is moved backward.

·         In trifocals the intermediate lens usually has half power over the distance correction.

·         The LTF stands for LUMINANCE TRANSMISSION when considering tints.

·         Relative spectacle magnification = Actual size corrected with spectacle / size seen by the emmetropic eye

·         Visible light contains wavelengths between 400 and 780nm. Ultraviolet A has wavelength of 315-400nm whereas ultraviolet C 200- 280nm. Thus in order of increasing wavelengths: ultraviolet C, ultraviolet B, ultraviolet A; visible light; infrared A, infrared B and infrared C. The lens is very efficient at absorbing ultraviolet than infrared light. The thermal burn in eclipse burn is caused by infrared light.

·         Red-green defect is seen in acquired optic nerve disease, cone dystrophy and Stargardt's disease.

Blue-yellow defects are seen in most retinal dystrophy, glaucoma and autosomal dominant optic neuropathy.

·         Farnsworth-Munsell hue 100 test contains 84 colour discs to be arranged in order of closest colour match with the reference colours at each end. The colours only differ in hue with same brightness and saturation. Ishihara test plates are mainly for congenital red-green colour defect. Wavy lines are used in Ishihara plates for illiterates or children. Lanthony New Colour Test can be used in children.

·         Illuminance and irradiance are terms used to refer to the amount of light arriving at a given point. Radiance refers to the amount of light leaving a certain point.

·         The refractive index of a human lens increases with age due to the development of cataract. In addition, the human lens does not have uniform refractive index being higher in the nucleus (1.400) than the cortex (1.380) in a non-cataractous lens.

·         The image formed by a prism is erect, virtual and deviated towards the apex.

·         Fresnel prisms reduce the weight of conventional prism and used widely in treating patients with strabismus. They are made up of a series of small prisms. The most common type are made up of polyvinyl chloride. They reduce the visual acuity mainly through chromatic aberrations. They are usually applied to the the back of patients' glasses.

·         Prentice rule: Prism dioptre = distance from the optic centre (cm) X dioptre power of the lens.

The induced prismatic effect is 1D.

·         The vergence power of a lens is affected by the vergence power of each surface, thickness of the lens and the medium on either side of the lens. The later explain why cornea has a stronger refractive power than lens in human eye although its refractive index is less. The wavelength of the light also affects the vergence power of the lens.

·         The first focal length has the same length as the second focal length only if the media on either side of the lens are the same. The first focal length of a convex lens is to the left of the lens whereas that of the concave lens is to the right of the lens.

By convention, the second focal length has a positive sign for convex lens and a negative sign for concave lens. As lenses are designated by their second focal lengths, a convex lens is also called a plus lens and a concave lens a negative lens.

·         The magnifying power is calculated to be M/4 where M is the power of the lens in dioptres.

·         The increasing prismatic effect of the more peripheral parts of a spherical lens is responsible for:

· spherical aberration

· ring scotoma

· jack-in-the-box effect

· image distortion so that a thick plus lens gives a pin-cushion effect and a thick minus lens gives a barrel effect.

·         A cylindrical lens has one plane surface and the other with curve surface. It has no power along its axis. Its power is 90 degrees to the axis and the lens forms a focal line parallel to its axis. It is used in Maddox rod for the measurement of phoria.

Maddox rod is used to measure distant phoria and Maddox wing for near phoria. Double Maddox rod can be used to measure cyclotorsion.

·         The circle of least confusion of the conoid of Sturm is located at the focal point of the spherical equivalent lens.

·         Duochrome test is a subjective test using the principle of chromatic aberration. It uses letters or numbers of the same colour usually black against different backgrounds: red and green. It is sensitive changes in refraction of 0.25D or less. To an emmetropic patient, letters of both the colours look equally sharp; while to a slightly myopic patient the red letters appear sharper and to a slightly hypermetropic patient the green letters look sharper.

·         High-order aberrations are ones that cannot be corrected by simple spherocylindrical systems, such as spectacles or contact lenses. They are caused by minute misalignments of the eye's optical components and include, in order of visual significance, spherical aberration, coma, higher-order astigmatism, and others. They can be detected with wavefront analysis performed with an instrument called aberometer. Theoretically, an ablation that removes aberrations increases visual contrast and the spatial detail of images seen by the eye.

·         Purkinje-Sanson images: The images are formed at 4 surfaces: the anterior (image 1) and posterior (image 2) corneal surfaces and the anterior (image 3) and posterior (image 4) lenticular surfaces. The first three images are erect and virtual whereas the last one is inverted and real. The first image is used for keratometry and images 3 and 4 are used for accommodation.

Hirschberg's test made use of the first captoptric image which is located on the anterior corneal surface for measuring ocular deviation. This image is also used for keratometry.

·         An image known as Sturm's conoid is produced by an astigmatic eye; such image can only be corrected with a spherocylindrical lens.

·         Unilateral nucleosclerosis and not posterior subcapsular cataract can lead to index myopia. Central serious retinopathy can cause hypermetropic shift.

·         SRK formula is used for IOL calculation. However, it is not accurate for eye less than 22mm or longer than 24.5mm. SRK II and SRK-T are more accurate than SRK. The SRK II uses adjusted A constant depending on the length of the eye. For eye with an axial length shorter than 21mm, Hoffer Q is preferable.

·         The Franklin design is a split bifocals in which a distance lens is mounted on a near lens. The problem of prismatic jump is related to the power of the lenses as well as the distance between the distance of the interface from its optical centre. Image jump can be reduced by moving the optical centres towards the junction of the two portions as in the executive glasses. Alternatively, a base up prism can be incorporated into the reading section.

Varifocal glasses has three sections: distance, intermediate (for VDU or working at arm length) and near (for reading). Varifocal or progressive lenses have no visible interface between the distance and near portions unlike bifocal or trifocal lenses. There is a power progression corridor (intermediate portion) which reduces the image jump seen in bifocal glasses; however on either side of the corridor aberration or astigmatism induced can become intolerable.

·         Soft contact lens with toric surface can be used to correct higher astigmatism.

Piggyback contact lens involves the use of two lenses, the soft contact lens provides the fitting surface for the rigid gas permeable lens.

·         Because of the prismatic effect of glasses with reading (base in in myopes and base out in hypermetropes), the myopes need less convergence and accommodation than when using the contact lenses, the converse is true for hypermetropes. Because of the additional accommodation and convergence required, contact lenses can cause eyestrain in presbyopic at an earlier age.

·         Corneal warpage refers to change in the corneal curvature associated with contact lens wear. Corneal oedema is absent. It is commoner with rigid gas permeable contact lens than soft contact lens. The result is reversible.

·         Giant papillary conjunctivitis is more common with soft than RGP lens. It is thought to be caused by deposits of denatured proteins. Ptosis is more common with RGP lens. It is thought to be related to the way the RGP lens is removed ie. temporal stretching of the lids combined with forceful lid closure.

·         Diabetic maculopathy is better detected with indirect ophthalmoscope than direct one.

·         The slit-lamp can be used to detect the presence of macular hole using the Watzke's sign.

·         The Hruby lens is a powerful plano-concave lens which gives a virtual, erect and diminished image. It is difficult to master and the image is too small for reliable photocoagulation. Coupling solution is not required as the lens does not come into contact with the eye. It is held with the concave side towards the patient. The image is formed within the eye.

·         The International Safety Classification of Lasers divides the lasers into 4 groups. Group 3 is subdivided into 3a and 3b. Class 3b and above is damaging to the eye and their powers are 5MW and above. All lasers used in  ophthalmology are classed as 3b and above. Safety goggles should always be worn by people in the vicinity.

·         Karl Himly (1806) was the first to employ the technique of oblique illumination examination.

·         Gullstrand invented the slit-lamp.

·         Babbage in 1848 invented the direct ophthalmoscope.

·         von Helmholtz in 1850 reinvented and popularised the ophthalmoscope.

·         Nagel in 1864 invented the indirect ophthalmoscopy.

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·         Two types of keratometers used in practice are Javal-Schiotz model and Bausch & Lomb model.

·         LASIK is laser-assisted in-situ keratomileusis. It is performed using ALK (automated lamellar keratectomy) machine and the excimer laser. This procedure is good for myopia of more than – 8D.

·         types of retinoscopes:

1.       Mirror retinoscopes, which may consist of a simple plane mirror or a combination of a plane mirror (on one end) and a concave mirror (on the other end). e.g., Pristley-Smith’s mirror.

2.       Self-illuminated streak retinoscope.

·         uses of red and green glasses or filters:

1.       Diplopia charting

2.       Worth’s four-dot test

3.       Malingering test

While testing, the red glass is kept in front of the right eye and the green glass is kept in front of the left eye.

·         Crown glass with refractive index 1.5223 is most commonly used for making spectacles.

·         +20 D is the power of the convex lens most commonly used in indirect ophthalmoscopy