Question

A female patient complains of blurring of vision and was prescribed \(-0.5D\) spherical lenses. Retinoscopy done at \(1\ m\) with a plane mirror will cause the image to move in which manner?

• A. Image moves in the direction of the mirror
• B. Image moves in the opposite direction
• C. No movement of the image
• D. Scissors reflex is seen

Hint:

Compare the patient's far point distance to the examiner's 1 m working distance to find the movement direction.

Answer 1

The Correct Option is A

Step 1: Recall how retinoscopy movement works.
In retinoscopy, the examiner shines light into the patient's eye and rocks the mirror, watching how the red reflex in the pupil moves. The direction of the reflex tells us where the patient's far point sits compared to the examiner.

Step 2: Convert the working distance into a dioptric value.
The examiner is working at \(1\ m\).
\[ \text{Working distance power} = \frac{1}{1\ m} = 1.00\ D \]
This means an eye with exactly \(-1.00D\) of myopia would have its far point sitting right at the examiner's eye, giving no movement (a neutral point).

Step 3: Locate the patient's far point.
The patient needs \(-0.5D\) spherical lenses, so her own refractive error is \(-0.5D\) of myopia. The far point of a \(-0.5D\) myope lies at
\[ \frac{1}{0.5\ D} = 2\ m \]
in front of her eye.

Step 4: Compare the far point to the working distance.
The examiner sits at \(1\ m\), and the patient's far point is at \(2\ m\). Since \(2\ m\) is farther away than the examiner's position, the examiner is sitting inside the patient's far point.

Step 5: Apply the retinoscopy rule.
When the examiner is within the patient's far point (that is, the patient's myopia is less than the dioptric value of the working distance), the reflex moves "with" the mirror, meaning the image moves in the same direction as the mirror sweep. This is called a with movement, and it means more minus lens (or less plus) is needed to reach neutrality, matching a mild \(-0.5D\) myope tested at \(1\ m\).

Step 6: Rule out the other options.
A no movement (neutral) reflex would only be seen if her error were exactly \(-1.00D\). An against movement (opposite direction) happens only when the patient's myopia is greater than the working distance value, for example more than \(-1.00D\). A scissors reflex is seen in irregular astigmatism or keratoconus, not in a simple case like this.

Step 7: Final answer.
Since the patient's far point lies beyond the 1 m working distance, the reflex shows a with movement, so the image moves in the direction of the mirror.
\[ \boxed{\text{Image moves in the direction of the mirror}} \]