Test two 1. The following about Fresnel's prism are true: a. its power is determined the thickness of the base  b. the visual acuity is affected by the power of the prism  c. it is normal fitted to the front of the spectacle  d. the maximum prismatic power than an adult can tolerate is usually      around 15 dioptres  e. dividing the strength of the prism between the two eyes improves the      comfort of the patient Answer1.    a.F    b.T    c.F    d.F    e.T Fresnel's prism is composed of contiguous small prisms which have the same apex angle  the power is determined by the apex angle the visual acuity is affected by increased prismatic power it can be used as an adhesive for the spectacle and is usually fitted on the posterior surface the maximum prismatic power than most adult can tolerate is 30 dioptres patient's comfort can be improved if the prism strength (especially if high power) is divided between the two eyes. 2. Regarding the fusional reserves: a. the horizontal fusional reserve is higher than the vertical fusional reserve  b. the positive fusional reserve is higher for distance than near  c. the positive fusional reserve is affected by the subject's      accommodative reserve  d. the horizontal fusional reserve decreases with age  e. the vertical fusional reserve is decreased in patient with reduced     accommodative reserve Answer2.     a.T    b.F    c.T    d.T    e.F Fusional reserve: refers to the amount of prism that needs to be placed in front of the eye to induce diplopia the horizontal fusional reserve can be positive (the amount of base-out prism required to induce diplopia) or negative (the amount of base-in prism required to induce diplopia) the positive fusional reserve is affected by the accommodative reserve and therefore decreases with age. It is higher for near than for distance the horizontal fusional reserve is higher than the vertical fusional reserve. The vertical fusional reserve is measured by placing a base-up prism before one eye and base-down before the other until fusion is broken. Accommodative reserve is not involved in vertical fusional reserve   3. In the normal eye, resolution: a. depends on cone separation  b. is 6/6 or better  c. depends on the size of the pupil  d. is about 10 minutes of arc  e. can be tested with the Sheridan-Gardner test Answer3.     a.T    b.T    c.T    d.F    e.T Resolution: is the least separation of two images so that they are seen as separate and is affected by the cone separation in normal subject is 6/6 or better is dependent on the size of the pupil. A small pupil causes increased diffraction and therefore reduced resolution, pupil of about 2.5 mm is best for resolution above which optical aberration can affect the resolving power is 1 minute of arc in normal subject can be tested with Sheridan-Gardner test which is a modified form of Snellen chart for testing children and the illiterate   4. The vernier acuity (hyperacuity): a. is the ability of  the eye to detects  two images placed close together  b. depends on the separation of cone  c. in normal eye is less than 8 seconds of arc  d. requires the presence of binocular single vision  e. is used when performing applanation tonometer Answer4.    a.F    b.F    c.T    d.F    e.T Vernier acuity: is the determination of the eye to detect a break in a line and this may be as little as 3 to 5 seconds of arc is not determined by cone separation because the normal acuity is only a fraction of the width of the cone, the mechanism involves in Vernier acuity is still unclear  does not requires binocular single vision  is required in using optical instruments such as keratometer or applanation tonometer   5. True statements about diffraction include: a. diffraction is increased when light passes through a smaller than a      bigger aperture  b. diffraction is increased with shorter than longer light wavelength  c. diffraction which occurs when the light passes through the pupil is      referred to as Fraunhofer diffraction  d. produces an Airy's disc which affects the resolving power of the eye  e. produces an Airy's disc which is bigger in a smaller than a bigger pupil Answer5.    a.T    b.T    c.T    d.T    e.T Diffraction: occurs when a light passes through a gap or round object produces a change in the direction of the light wave is increased with smaller than bigger pupil is increased with longer wavelength than shorter wavelengths, and therefore red light diffracts more than blue Fraunhofer diffraction refers to the production of an interference pattern by diffraction through a single slit (pupil) and this gives an Airy's disc the Airy's disc affects the resolving power of the eye. The minimum resolvable distance is about the radius of the Airy disc the Airy's disc is increased with a smaller pupil than a bigger pupil. The resolving power is therefore decreased with a smaller pupil 1 Click here to return to Contents