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Prism is a refracting medium bordered by 2 plane surface inclined at a finite angle .
Properties :
• A prism displace incident ray towards the base.
• Prism is thickest edge at base and thinnest edge at Apex.
• Prism do not change the Vergence.
• The Image is erect virtual , and image displaced towards the apex of prism .
• Prism do not magnified or minifying the Image.
• Prism disperse the pencil ray into component color.

Refraction through prism:
• Light passing through the prism obeys the Snell law at each surface.
• The ray is deviated toward the base of prism.
• This cause object to displaced away from the base of prism towards the apex .

• The net change in direction of the ray , the angle D is called angle of deviation.
• Further the away from optical center, it’s Increase prismatic effects. Prism in air ,angle of deviation is determined by 3 factors.
➢Material of prism and refractive index.
➢Angle of incidence of ray.
➢Refracting angle of prism.

Light ray entering and leaving the prism are bent towards the base of prism. This cause object to displaced away from the base and towards it’s apex.
• Base down prism – upward
• Base up prism – downward .
• A prism of 1prism produce apparent lateral displacement of 1 CM of an object situated at 1 m.
• Denoted by symbol .

• Centrad : An arc 1 m from the prism.
➢Prism can be oriented in front of the eye notation.
• Base In -Left
• Base Out- Right

• Base Up – Downward
• Base Down- Upward
Uses : It can be use as diagnostic and therapeutic.
➢Diagnostic :
• Measure of squints.
• Measure of convergence ( movement in different
directions in same time).

➢Therapeutic :
• Therapy of any binocular vision problems.
• Therapy of squints ( converges deficiency)
• Measurements of nystamgus.

Types of prism : There are 2 type of prism
1. Frensel prism.
2. Risley prism .
Frensel prism :
• To understand Frensel prism work, imagine cutting off the top of large no and equally powered prism and gluing them, one over the other, onto a thin plastic of sheet.
• The overall prismatic effects is same as that of a single prism. Fresnel prism is only 1 mm thick.
• The sheet are lighter than glass prism and can be stuck on to the patients glasses.
• Thin and light weight.
• Flexible and can be applied to an existing spectacle.
• It can be cut into any shape with scissors.
• Reduce magnification difference considerably.
• Harder to clean than conventional lenses.
• Decrease visual acuity caused by reflection for prism than 10 .
➢Uses – therapy purposes.
Risley prism :
• Pair of prism, one prism is mounted another.
• Combination of 2 prism.
• These prism are placed one on top of the other.
• Initially their base directions are exactly identical, but as the prism are rotated, their bases move by equal extent in opposite directions.
➢Uses – Lensometer , All Phoropter.

For example – suppose 2 prism of 10 each are placed on top of one another base to base. The total
prismatic effects is 20 . But if they are placed base to Apex then their prismatic
effects is 0 Zero.

Varsha Gupta

Optometrist for us vision is the art of seeing what is invisible to other. A better vision is our responsibility and say good bye to poor sight 😊.

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