Elementary Optics and Physiology of Vision MCQs

Oxygen-permeable or rigid gas-permeable lenses are made of a firm, durable plastic that transmits oxygen, e.g., a copolymer of PMMA and silicone and cellulose acetate butyrate. As they do not contain water, they resist deposits and are less likely than soft contact lenses to harbour bacteria. Oxygen-permeable lenses are easy to clean and disinfect, do not dehydrate and last longer than soft lenses. The rigidity of oxygen permeable lenses also means that they are easier to handle than soft lenses. As they retain their shape better, they provide sharper vision. Reference: Parson ch 8, P. 87

A sympathomimetic drop would dilate the pupil, not constrict the pupil. It would not produce a myopic shift, and the pupil would remain reactive to light and near stimuli. A prostaglandin inhibitor would have no effect on the pupil, and an alpha adrenergic agent would have the same effect as a sympathomimetic. Only a parasympathomimetic drop would constrict the pupil and induce the myopic shift. Reference: A.K.Khurana, 5th Edition, Pg: 222/424.

1/ 0.1 - 1 / infinity = 10 D Range of accommodation Distance between far (infinity) and near point (30 cm) Power of accommodation Refractive power of eye in fully accommodative state - power at rest = power to focus at near - power to focus at far. The amplitude of accommodation is a measure of the closest point at which the eyes can focus: it is the range from the far point to the near point in dioptres. Because it is measured from the far point, the measurement needs to be taken with the distance correction in place. The amplitude of accommodation (Am) is mathematically expressed as the dioptric difference between the far point (fp) and the near point (np) of the eye and it is related to the maximum power variation that can perform an eye. Reference: Khurana Optics 2/e

Take half the value of the cylinder and Add it to the sphere power. In other words, as a formula the spherical equivalent Spherical Equivalent = Sphere + (Cylinder)/2 Reference: Khurana Optics 2/e

The angle of incidence for which the angle of refraction is 90 degrees is called as the critical angle. The critical angle is the angle of incidence above which the total internal reflection occurs. Reference: Khurana Optics 2/e

When streak retinoscopy is performed, the width of the reflex widens progressively as the neutralization is approached, and at the end point, streak disappears and the pupil appears completely illuminated or completely dark Reference: Khurana Optics 2/e

If the letters of the green side are clearer +0.25 D sphere is added If the letters on the red side are clearer -0.25 D sphere is added Reference: Khurana Optics 2/e

Types of contact lenses Hard contact lenses Semi soft Soft Modern soft contact lenses are made of hydrogel and silicon hydrogel polymers. Hard contacts are made of polymethyl methacrylate (PMMA) or Plexiglas. Soft contacts are mass-produced, but hard contact lenses are made to fit the wearer. The main hydrogel material used for soft contacts is poly(2-hydroxyethyl methacrylate), usually called poly-HEMA Soft contact lenses Problems of deposits Daily disposables, Weekly, Monthly Continuous wear lenses – over 30 nights Increased water content and permeability for oxygen Reference: Khurana Optics 2/e