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The Prism
This document discusses prisms, their properties, and uses in optometry. It defines key prism terminology like apex, base, axis, and apical angle. It explains how prisms refract light and cause deviation according to Snell's law. Prisms are used clinically for diagnosing and managing eye alignment conditions like esotropia. They help measure deviation angles and correct angles of deviation. Prisms are also used in ophthalmic instruments like slit lamps and microscopes due to their light reflecting properties.
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The Prism
- 1. Prism By -Shwetali Bhagat . Bsc. Optometry 3rd yr.
- 2. Prism- It issimply a refracting medium with triangular cross section having apex and base. Neoclature of prism : i. Apex - The point where the two refracting surface are meet. ii. Base - refered to the bottom of prism which opp. to the apex. iii. Axis - refered to the line bisecting the prism. iv. Apical Angle - formed B/W two refracting surface of prism. It is also called as refracting angle.
- 3. Apex – Tipof the prism where the two refractive surfaces meet Apical angle - angle between two refractive surfaces ‘α’ Refractive surface – The two refractive surfaces of a refractive prism inclined at the apical angle Reflecting surface – In some prism, internal ray hitting the 2nd surface is subjected to total internal reflection (reflecting prisms) Axis – Line Bisecting The Refractive Angle Base – Bottom of the prism / side opposite to apex or optical angle. Helps in orientation of ophthalmic prisms
- 4. Prism –Refraction Refractive angle / Apical angle - angle between two refractive surfaces ‘α’ Angle of deviation – Angle between the incident and the emergent rays. Obeying Snell's Law of refraction light passing through the prism is deviated to the base. Where, Prism produce displacement of object seen through it towards the apex. Factors responsible for ‘D’ Angle of Deviation - refers to the net change in direction of light ray passing through prism. Angle of Minimum Deviation - produced when the angle of incidence is equal to angle of emergence.
- 5. Prism and refraction A δm N N Prism A is the angle of prism N is normal on the surface of prism δm is the deviation of emerging ray from initial direction
- 6. Dispersion of whitelight of sun by prism Red 6600 Å Orange 6100 Å Yellow 5800 Å Green 5500 Å Blue 4700 Å Indigo 4400 Å Violet 4100 Å A prism Spectrum wavelength λ of the center of color band in angstroms If a shaft of light entering a prism is sufficiently small such that the coloured edges meet, a spectrum results
- 7. δR δV Θ A δm ⇒ mean deviation or deviation for yellow Angle of dispersion θ θ = δV - δR Dispersive power ω θ δm ω = δm ω = δV - δR Deviation by prism δR ⇒ deviation for red δV ⇒ deviation for violet δm
- 8. Refraction AndReflection Of Prism
- 9. Characteristics ofPrism - Thickest edge the Base & Thinnest edge the Apex. Displace the incident ray towards the base. Displace the image towards the apex. Does not change vergence of the rays. Does not magnify & minify the image.
- 10. Prismatic Effect Spherical lens Spherical lens behave like prisms in all quadrants except the center The refractive angle between the lens and the surface incident at the edges. Cylinder lens Cylindrical lenses have no power along the axis meridian, hence the cylinder can exert no prismatic effect along its axis. Most commonly if correction is not equal in both eyes. High myopes Aphakia
- 11. Types OfPrisms In Clinical Practice 1. Loose Prisms 2. Prisms Bars 3. Trial Prisms 4. Fresnel Prisms 5. Rotating Prisms 6. Risley Double Prisms – 2 rotating Prisms of same strength on a rotating frame Prism Bars Loose Prisms Rotating Fresnel Prisms Prisms
- 12. Uses ofPrism Prisms are used in orthoptic settings, mainly for diagnosis and management Diagnosis Diagnosis microtropia Measurement of angle of deviation To correct angle of deviation Management Eso/ exo deviation of concomitant / incomitant Amblyopia, Nystagmus, Malingering, Visual Field defects
- 13. Prisms in ophthalmicinstruments Reflecting and dove prism are used in almost all of the of the ophthalmic instrument and operating micro-scopes Using the property of total internal reflection prisms have basically replaced mirrors in SLB, microscopes, ect.
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