## Prism

Prism

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

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

â€¢ The net change in direction of the ray , the angle D is called angle of deviation.
â€¢ Further the away from optical center, itâ€™s Increase prismatic effects. Prism in air ,angle of deviation is determined by 3 factors.
âž¢Material of prism and refractive index.
âž¢Angle of incidence of ray.
âž¢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 itâ€™s apex.
â€¢ Base down prism â€“ upward
â€¢ Base up prism â€“ downward .
Unit:
â€¢ A prism of 1prism produce apparent lateral displacement of 1 CM of an object situated at 1 m.
â€¢ Denoted by symbol .

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

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

âž¢Therapeutic :
â€¢ Therapy of any binocular vision problems.
â€¢ Therapy of squints ( converges deficiency)
â€¢ Measurements of nystamgus.

Types of prism : There are 2 type of prism
1. Frensel prism.
2. Risley prism .
Frensel prism :
â€¢ 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.
â€¢ The overall prismatic effects is same as that of a single prism. Fresnel prism is only 1 mm thick.
â€¢ The sheet are lighter than glass prism and can be stuck on to the patients glasses.
â€¢ Thin and light weight.
â€¢ Flexible and can be applied to an existing spectacle.
â€¢ It can be cut into any shape with scissors.
â€¢ Reduce magnification difference considerably.
â€¢ Harder to clean than conventional lenses.
â€¢ Decrease visual acuity caused by reflection for prism than 10 .
âž¢Uses â€“ therapy purposes.
Risley prism :
â€¢ Pair of prism, one prism is mounted another.
â€¢ Combination of 2 prism.
â€¢ These prism are placed one on top of the other.
â€¢ Initially their base directions are exactly identical, but as the prism are rotated, their bases move by equal extent in opposite directions.
âž¢Uses â€“ Lensometer , All Phoropter.

For example â€“ 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 ðŸ˜Š.