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Mirrors and Optical Principles in Ophthalmology
Optics & Refraction
Teaching images
111. Fundamentals of Reflection - The Mirror Analogy
Exam pearl
- Clinical Analogy: The cornea, functioning as the most powerful refracting surface of the eye, also acts as a mirror - this is the basis of Placido disc keratoscopy, corneal topography, and the Purkinje-Sanson images. Understanding mirrors is not abstract physics; it is the bedrock of every instrument you will use in clinical practice.
Laws of Reflection
Teaching image
1- The incident ray, reflected ray, and normal at the point of incidence are all coplanar.
- The angle of incidence (i) equals the angle of reflection (r) - measured from the normal.
Exam pearl
- These laws apply to both plane and curved mirrors. For a spherical mirror, the normal at any point is the radius joining that point to the centre of curvature.
Plane Mirrors - Image Characteristics
A plane mirror forms a virtual, erect, laterally inverted image at the same distance behind the mirror as the object is in front.
| Property | Plane Mirror | Concave Mirror | Convex Mirror |
|---|---|---|---|
| Image type | Virtual, erect | Real/Virtual (depends on object distance) | Virtual, erect |
| Magnification | m = 1 always | • m > 1 (virtual) • m > 1, = 1, or < 1 (real) | m < 1 always |
| Focal length (f) | ∞ | +R/2 (positive, converging) | –R/2 (negative, diverging) |
| Power | 0 | Converging (used in ophthalmoscopy) | Diverging (rear-view) |
| Clinical use | • Visual acuity testing | • Indirect ophthalmoscopy | • Keratometry (convex surface model) |
| Property | Plane Mirror | Concave Mirror | Convex Mirror |
|---|---|---|---|
| Image type | Virtual, erect | Real/Virtual (depends on object distance) | Virtual, erect |
| Magnification | m = 1 always | • m > 1 (virtual) • m > 1, = 1, or < 1 (real) | m < 1 always |
| Focal length (f) | ∞ | +R/2 (positive, converging) | –R/2 (negative, diverging) |
| Power | 0 | Converging (used in ophthalmoscopy) | Diverging (rear-view) |
| Clinical use | • Visual acuity testing | • Indirect ophthalmoscopy | • Keratometry (convex surface model) |
2. Curved Mirrors - Concave and Convex
Exam pearl
- Clinical Analogy: The concave mirror in the indirect ophthalmoscope condenses light toward the patient's fundus. The convex surface of the cornea acts like a convex mirror - Purkinje image I is reflected from this surface. Understanding these is essential for keratometry, retinoscopy, and ophthalmoscopy.
Sign Convention (New Cartesian)
Teaching image
1- All distances measured from the pole of the mirror.
- Distances in the direction of incident light → positive.
- Distances opposite to incident light → negative.
- Heights above principal axis → positive; below → negative.
Mirror Formula & Magnification
- Mirror formula: 1/f = 1/v + 1/u
- Magnification: m = –v/u (also = h_i / h_o)
- f = R/2 for paraxial rays from spherical mirrors (concave: –ve; convex: +ve)
Image Formation by Concave Mirror - Position Summary
Teaching image
1| Object position (u) | Image position (v) | Nature | Magnification | Ophthalmology relevance |
|---|---|---|---|---|
| • u > 2f (beyond C) | • f < v < 2f | • Real, inverted | • Diminished | - |
| • u = 2f (at C) | • v = 2f | • Real, inverted | • Same size (m=–1) | • Calibration reference |
| • f < u < 2f | • v > 2f | • Real, inverted | • Magnified | - |
| • u = f (at F) | • v = ∞ | • At infinity | - | • Collimation in slit lamp beam |
| • u < f (inside F) | • Virtual, behind mirror | • Virtual, erect | • Magnified | • Concave mirror as magnifier |
| Object position (u) | Image position (v) | Nature | Magnification | Ophthalmology relevance |
|---|---|---|---|---|
| • u > 2f (beyond C) | • f < v < 2f | • Real, inverted | • Diminished | - |
| • u = 2f (at C) | • v = 2f | • Real, inverted | • Same size (m=–1) | • Calibration reference |
| • f < u < 2f | • v > 2f | • Real, inverted | • Magnified | - |
| • u = f (at F) | • v = ∞ | • At infinity | - | • Collimation in slit lamp beam |
| • u < f (inside F) | • Virtual, behind mirror | • Virtual, erect | • Magnified | • Concave mirror as magnifier |
Convex Mirror - Key Facts
- Always forms a virtual, erect, diminished image regardless of object position.
- Image is always between the pole and focus (behind the mirror) for a finite object; for an object at infinity, the image forms at the focus.
- Field of view is wider than plane or concave mirror - hence used in road safety mirrors.
Oral-exam tip
- In keratometry, the cornea behaves as a convex mirror.
- The mire image formed is virtual, erect, and diminished - located behind the corneal surface.
- Keratometer measures the size of this image to calculate corneal curvature.
Purkinje-Sanson Images - Mirror Basis
Teaching image
1| Purkinje image I (PI) | Purkinje image II (PII) | Purkinje image III (PIII) | Purkinje image IV (PIV) |
|---|---|---|---|
| • Anterior corneal surface | • Posterior corneal surface | • Anterior lens surface | • Posterior lens surface |
| • Virtual, erect, bright | • Virtual, erect, faint | • Virtual, erect, large | • Real, inverted, small |
| • Convex mirror | • Convex mirror (weaker) | • Convex mirror (shallow) | • Concave mirror (steep) |
| • Keratometry basis | • Phakometry - posterior corneal curvature | • Phakometry - lens power | • Phakometry - posterior lens |
| Purkinje image I (PI) | Purkinje image II (PII) | Purkinje image III (PIII) | Purkinje image IV (PIV) |
|---|---|---|---|
| • Anterior corneal surface | • Posterior corneal surface | • Anterior lens surface | • Posterior lens surface |
| • Virtual, erect, bright | • Virtual, erect, faint | • Virtual, erect, large | • Real, inverted, small |
| • Convex mirror | • Convex mirror (weaker) | • Convex mirror (shallow) | • Concave mirror (steep) |
| • Keratometry basis | • Phakometry - posterior corneal curvature | • Phakometry - lens power | • Phakometry - posterior lens |
Exam pearl
- Purkinje image IV (PIV) is the only real, inverted Purkinje image because the posterior lens surface is a steep concave mirror. Object (light source) lies beyond its centre of curvature.
- Examiner favourite: 'Which Purkinje image is real and inverted?' → Answer: Purkinje image IV (PIV) only. Purkinje image II (PII) is fainter than Purkinje image I (PI); Purkinje image IV (PIV) is also fainter than Purkinje image I (PI).
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Keep reading Mirrors and Optical Principles in Ophthalmology
11 more sections are included with the Clinical plan.
- Mirrors in Ophthalmic Instruments
- Keratometry - Convex Mirror Principle
- Retinoscopy - Plane Mirror in Clinical Use
- Slit Lamp - Mirrors, Prisms & Illumination
- Ophthalmoscopy - Mirrors & Lens Systems
- Optical Aberrations Relevant to Mirrors