Prisms Done by: Othman Al- Abbadi, MD
Jan 21, 2015
PrismsDone by: Othman Al-Abbadi, MD
Reflection
• Occurs, at some degree, at all interfaces even when most of the light is transmitted or absorbed.
• Laws:– The Incident ray, the Reflected ray & the Normal..
All lie in the same plane.– Angle of Incidence = Angle of Reflection
Reflection at spherical mirrors
C FP
F CP
Clinical application
• The anterior surface of the cornea acts as a convex mirror.
Light Refraction
• It’s the change in direction of light when it passes from one transparent medium into another of different optical density.
• The higher the density, the slower light and the higher the refractive index.
• Absolute Refractive index: a comparison of light velocity in a vacuum to another medium.
• Refractive indices to some media:– Air= 1– Water= 1.33– Cornea= 1.37– Crystalline lens= 1.386-1.406– Crown glass= 1.52– Flint glass= 1.6
Snell’s law
• To measure the refracting angle, we use the Snell’s law of refraction
• On entering an optically dense medium from a less dense medium, light is deviated towards the normal.
• Medium 1nMedium 2 = n2 / n1
• When passing obliquely from air through a glass plate then to air, light is deviated laterally.. Thus, the direction of the light is unchanged but is laterally displaced.
Refraction on curved interfaces
• If n2 > n1 convergence
• If n1 > n2 divergence
• Refracting power= (n2 – n1 )/rr radius of curvature in meters
Critical angle
• As the rays meet the interface more obliquely, a stage is reached where the refracted ray runs parallel with the interface.
• More obliquely total internal reflection
• Critical angle for the cornea?? 48.5
• How to overcome this reflection?? applying higher refractive index
material
Light dispersion
• The dispersive power is not related to refractive index
• Shorter wavelengths more deviation• Rainbow total internal reflection with
dispersion– Primary– Secondary
Prisms • A portion of a refracting medium bordered by
two plane surfaces which are inclined at a finite angle.
• The ray is deviated towards the base of the prism obeying the Snell’s law… forming erect, virtual & apically displaced image.
• The net change in direction is the angle of deviation.
• Factors determining the angle of deviation in air:1. The refractive index of the prism material,2. The refracting angle of the prism,3. The angle of incidence of the ray.
Primary positions
1. Angle of minimum deviation,
2. Prentice position.
Angle of minimum deviation
• When the angle of incidence equals the angle of emergence.
• Symmetrical refraction.• Under these strict condition,
D= (n-1) * A• For glass prisms
D= A/2
Prentice position
• When the incidence angle is zero, so that all the deviation takes place at the other surface of the prism.
• The “prentice position” power of the prism is greater than the “angle
of minimum deviation” power.
In ophthalmic use
• The prentice position is specified for glass ophthalmic prisms… & the angle of minimum deviation power is specified for the plastic prism.
• Stacking prisms one on top of another doesn’t bring cumulative power of both prisms, so it’s not advised… except if used on different planes (one H & one V) because of perpendicular planes which result in independence refraction.
Notations of prisms
1. Prism dioptre ( Δ )Linear of apparent displacement of 1 cm of an object that’s situated at 1 m.
2. Angle of apparent deviation ( θ )1 prism dioptre = ½ angle of apparent deviation
3. The centad ( Ϫ )The image is measured along an arc of 1 m from the prism.Slightly greater than the prism dioptre.
4. Refracting angle of the prism
Vector addition
• When the patient needs prismatic correction both H & V, we can give one stronger prism mounted at an oblique angle.
• Calculation:1. Graphically; by drawing a rectangle,2. Mathematically; by the Pythagoras’ theorem.
Risley prism
Orthoptic reports1. The synoptophore
Measuring the angle between the visual axes of the eyes in degrees… using (+) for XT & (-) for ETe.g: Synopt without gls +20
2. Prism cover testMeasuring the angle of squint by the alternating cover test while placing prisms of increasing power before one eye until movement is eliminated… measured in dipotrese.g: PCT= distance eso +40Δ
Prescribing prisms
• The correction is split between the two eyes upon prescribing.
• E.g:– Convergence both base-out– Divergence both base-in
(the apex of the prism is directed toward the deviation)
– Hypper-/hypotropia one base-up & the other base-down
1. The following are true about prism:
a. its orientation is defined by its apexb. light is deviated towards the apexc. light with shorter wavelength is deviated more
than light with longerwavelengthd. the angle of the prism apex is called the
refracting anglee. all the ophthalmic prisms are calibrated according
to the Prentice's position
1. The following are true about prism:
a. its orientation is defined by its apexb. light is deviated towards the apexc. light with shorter wavelength is deviated
more than light with longerwavelengthd. the angle of the prism apex is called the
refracting anglee. all the ophthalmic prisms are calibrated
according to the Prentice's position
2. The image formed by a prism is:a. erectb. magnifiedc. laterally invertedd. virtuale. deviated towards the apex
2. The image formed by a prism is:a. erectb. magnifiedc. laterally invertedd. virtuale. deviated towards the apex
3. The angle of deviation of a prism is determined by:a. the refracting angleb. the angle of incidence of the rayc. the refractive index of the prism materiald. the width of the basee. the thickness of the prism
3. The angle of deviation of a prism is determined by:a. the refracting angleb. the angle of incidence of the rayc. the refractive index of the prism materiald. the width of the basee. the thickness of the prism
4. The following are true about prisms:a. they can control torsional diplopiab. they can control diplopia caused by an eye which is deviated out and upc. the prism power can be calculated form the refracting angle aloned. a prism with 2 doptres will produce a linear displacement of 2 cm of an object situated at 1 me. a prism with 1 dioptre produce a stronger deviation than one with an angle ofapparent deviation of 1
4. The following are true about prisms:a. they can control torsional diplopiab. they can control diplopia caused by an eye which is deviated out and upc. the prism power can be calculated form the refracting angle aloned. a prism with 2 doptres will produce a linear displacement of 2 cm of an object situated at 1 me. a prism with 1 dioptre produce a stronger deviation than one with an angle ofapparent deviation of 1