Optics And Refractive States Of The Eye

Question 1

Define the following terms : refractive index, focal point, focal length, and diopter.

Answer 1

Refractive index: the ratio of the speed of light in a vacuum to its speed in a specific substance.
Focal point : the point somewhere along the principal axis of a lens at which the paraxial rays converge or diverge.
Focal Length : the distance between the focal point and the lens.
Diopter : the unit of measure of the power of a lens

Question 2

Name the principal refractive properties of a convex lens and a concave lens.

Answer 2

A convex lens converges light rays; a concave lens diverges light rays.

Question 3

Distinguish between emmetropia and ametropia.

Answer 3

Emmetropia : the normal refractive state of an eye, in which light rays for distant objects are focused clearly on the retina by the relaxed lens without any accommodative effort.
Ametropia : the abnormal refractive state of the eye, in which light rays from distant objects cannot be focused clearly on the retina due to a refractive error.

Question 4

Define and describe the physiologic characteristics of myopia, hyperopia and astigmatism.

Answer 4

Myopia : nearsightedness; the cornea and lens have too much plus power for the length of the non accommodating eye, so that the light rays from distant objects are focused in front of the retina.

Hyperopia : farsightedness; the cornea and lens have too little plus power for the length of the non accommodating eye, so that the light rays from distant objects are focused theoretically behind retina.

Astigmatism : blurred vision of both distant and near objects due to a toric cornea or whose surface is irregular, so light rays are brought to a single focal point.

Question 5

Describe presbyopia

Answer 5

Progressive loss of accommodative ability of the crystalline lens due to a natural process of aging.

Question 6

Name at least 3 methods of correcting refractive errors of the eye.

Answer 6

Eyeglasses, contact lenses, intraocular lens implants, refractive surgery

Question 7

Define the state and purpose of an add power in multi focal lenses

Answer 7

Lower segment(s) of a multifocal lens; used to provide near vision to patients with presbyopia.

Question 8

Define a spherocylinder

Answer 8

Lens that has different curvatures in each of 2 perpendicular meridians (like a football), each of which possesses refractive power.

Question 9

State the principal refractive characteristic of a prism

Answer 9

Refracts light rays toward its base

Question 10

State the purpose of incorporating a prism into a corrective lens

Answer 10

To correct diplopia caused by visual misalignments.

Question 11

The refractive power of a prism is measured in units called _____.

Answer 11

Prism Diopters

Question 12

A 0.5 /\ prism deviates parallel rays of light a1 cm at a distance of ______ m from the prism.

Answer 12

2

Question 13

Name the kind of ophthalmic lens used to correct the following refractive errors : myopia, hyperopia and astigmatism

Answer 13

Myopia : concave, or minus, sphere.

Hyperopia: convex, or plus, sphere.

Astigmatism : spherocylinder, or cylinder.

Question 14

Define refraction as the term is used in eye care

Answer 14

Process of measuring a persons refractive error and determining the optical correction required to provide clear vision.

Question 15

Name the 2 main components of refraction

Answer 15

Refractometry and clinical judgement

Question 16

Name the 3 separate steps of refractometry.

Answer 16

Retinoscopy (objective refractometry), refinement (subjective refractometry) , binocular balancing

Question 17

Distinguish between cycloplegic refraction and manifest refraction

Answer 17

Cycloplegic refraction : uses cycloplegic drops to temporarily paralyze the ciliary muscle and block accommodation.

Manifest Refraction : is obtained without cycloplegia

Question 18

Define and distinguish between objective refractometry and subjective refractometry.

Answer 18

Objective Refractometry is retinoscopy, which does not require require responses from the patient.

Subjective Refractometry is refinement, which requires the patient to participate by choosing the lens (usually presented by means of a phoropter or trial frame and lenses) that provides the better clarity.

Question 19

Name 4 instruments or devices that may be used during refraction

Answer 19

Retinoscope, refractor (or phoropter), trial lens set and trial frame, cross cylinder

Question 20

Define ‘with motion’ and ‘against motion’.

Answer 20

With Motion : the retinoscopic reflex of the eye that moves in the same direction as the retinoscope streak of light.

Against Motion : the retinoscopic reflex of the eye that moves in the opposite direction from the retinoscope streak of light

Question 21

Name the 3 components of the lens prescription shown

+2.75 -1.50 X135
A. B. C.

Answer 21

A . sign and power of sphere

B. Sign and power of cylinder

C. Axis of Cylinder

Question 22

Transpose the following lens prescription :

+4.25 +1.50 X 135

Answer 22

+5.75 -1.50 X 45

Question 23

Define lensometry.

Answer 23

Measurement of the prescription of eyeglasses lenses or the power of contact lenses

Question 24

List the 4 properties of lenses that can be measured by a lensometer

Answer 24

Power in diopters, axes of cylinder component, presence and direction of prism, optical centers

Question 25

Name the type of lens identified by lensometry when the single and triple perpendicular line come into focus simultaneously.

Answer 25

Sphere

Question 26

The lensmeter power-drum reading on the distance portion of a multifocal lens measures -1.50 sph. The power-drum reading for the bifocal is +0.25. What is the bifocal add?

Answer 26

+1.75

Question 27

Define Keratometry

Answer 27

Measurement of a patients corneal curvature; provides an objective, quantitative measurement of corneal astigmatism

Question 28

List the 4 conditions that must be met simultaneously to obtain a proper measurement with a keratometer.

Answer 28

1. The reticule is in the center of the bottom-right circle.
2. The instrument is focused with fusion of the bottom-right circles.
3. The pulses between the circles are in the same plane and are fused.
4. The minuses between the circles are in the same plane and are fused.