• Primary refractive surface

• Contribute about 2/3 of eye's total refractive power

• Highly transparent structure

2. Crystalline Lens 🔦

• A transparent, biconvex structure

• Fine-tunes the focus of light into retina

3. Aqueous Humour 🔦

• A clear, watery fluid between cornea and lens

• Maintains intraocular pressure and provide nutrients

4. Pupil 🔦

• Opening in center of iris

• Control amount of light that enters eye

5. Retina 🔦

• Contain photoreceptors (rods and cones)

• Act as a screen where image falls on

6. Vitreous Humour 🔦

• A clear gel between lens and retina

• Maintain eye shape and allow light pass through to retina

• Reduced to a single lens and surface in a simplified model, making it easier to study the basic principles of image formation and refraction

  
  

2. Easy to calculate 👁️

• Allow for quicker, more intuitive calculations

3. Helps students easily understand fundamental concepts such as focal length, image formation, and refractive errors 👁️

4. Useful in Instrument Design and Prototyping 👁️

• These models are fast and efficient for

  simulation and testing

1. Easy to understand 🔎

• Combines multiple refractive surfaces into one and makes calculations much easier

2. Useful for calculating image formation, focal length and refractive errors 🔎

   
   

3. Simplifies ray-tracing and vergence calculations without needing a full multi-surface model 🔎

   
   

DISADVANTAGES

1. Inaccuracy in depicting the eye's true anatomical structure 🔎

2. Not suitable for precise modeling of vision problems like astigmatism, presbyopia or complex refractive surgeries 🔎

3. Ignore Optical Abberations 🔎

• Real eyes have spherical or chromatic aberrations and imperfections that the reduced eye model does not accounted for

• Pinhole discs are used to differentiate between refractive errors and pathological causes of reduced vision.

• If vision improves with a pinhole, it suggests a refractive error.

2. Predicting Corrective Potential:

• The pinhole test helps estimate how much visual acuity can improve with corrective lenses.

📝Measure cornea’s curvature by treating it like a biological mirror to analyzing how light bounces off its surface (like a spoon’s reflection)

• Use a doubling prism that splits the reflection into 2 overlapping images.

• Aligns these images - Remove error from eye movement.

b) A thick lens where the BVP is F’v =+5.00D

The back surface power is F2= -6.00D, the axial thickness is t= 9mm, refractive index is ng = 1.5

➤Bennett, A. G., & Rabbetts, R. B. (2007). Clinical visual optics (4th ed.). Butterworth-Heinemann. https://doi.org/10.1016/B978-0-7506-8874-1.X5000-3 (ISBN 9780750688741)

➤Chen, A.-H., Norazman, F. N., & Buari, N. (2012). Comparison of visual acuity estimates using three different letter charts under two ambient room illuminations. Indian Journal of Ophthalmology, 60(2), 101. https://doi.org/10.4103/0301-4738.90489 

➤Hodges, I. (2023). Basic principles of ophthalmic lens and dispensing optics. Alexis Press. ISBN 978-1-64532-015-9

➤Holladay, J. T., & Prager, T. C. (1998). Clinical evaluation of keratometry and computerized videokeratography: Intraobserver and interobserver variability on normal and astigmatic corneas. British Journal of Ophthalmology, 82(6), 704–711. https://doi.org/10.1136/bjo.82.6.704

➤Kaiser, P. K. (2009). Prospective evaluation of visual acuity assessment: A comparison of Snellen versus ETDRS charts in clinical practice. American Journal of Ophthalmology, 147(1), 187–193. https://doi.org/10.1016/j.ajo.2008.07.027

➤Loshin, D. S. (2025). The geometrical optics workbook (1st ed.). Elsevier Health. ISBN 9780750690522

Tunnacliffe, A. H. (2024). Introduction to visual optics (4th ed.). ABDO College. ISBN 9780900099281