Lecture 3: Refractive Errors and Correction

Question 1

Define/Explain Emmetropia

Answer 1

The eye has adequate refractive POWER to focus parallel light rays from a distant object precisely on the retina.
Hence, there is no refractive error.

Question 2

Define/Explain Myopia

Answer 2

When the eyeball is too powerful for its axial length, either lens/cornea being too powerful or an axial length too long for its power.
This leads to light rays being focused in front of the retina.

Question 3

Define/Explain Hyperopia

Answer 3

When the eyeball is not powerful enough for its axial length, hence light rays focus behind the retina.

Question 4

Define/Explain Far point (denoted by ‘Mr’)

Answer 4

The far point of the eye is a point in space that is conjugate with the retina of the UNACCOMMODATED eye.

Question 5

Explain far point in emmetropic/myopic/hyperopic eye

Answer 5

Far point in emmetropic eye is at infinity, though anything at 6m or further is counted as distant object, and its light rays will reach the eye with zero vergence ie. parallel.

Far point in myopic eye is closer than infinity, hence the light rays from it will be divergent.

Far point in hyperopic eye is behind the retina, hence only at that position will light rays from distant object come as convergent rays, but must be further converged by cornea and lens to focus on retina precisely.

Question 6

Explain when object is nearer than far point

Answer 6

General rule: nearer than far point means (more) divergent rays, hence more refractive power is needed to focus rays onto retina. This is done through accommodation.

Unaccommodated, light rays will focus behind the retina (relaxed eye not enough power to focus divergent rays onto retina). Accommodated, refractive power increases and is able to focus rays onto retina precisely.

Question 7

Define ocular refraction (denoted by ‘K’)

Answer 7

It is the dioptric distance to the eye’s far point/ refractive error of the eye. Calculated by reciprocal of far point.