Optics
Normal eye
A normal eye accommodates to focus anything from its near point (25 cm) out to infinity.
Normal eye — interactive Optics simulation. A normal eye accommodates to focus anything from its near point (25 cm) out to infinity. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Normal vision
A normal eye accommodates to focus anything from its near point (25 cm) out to infinity.
An emmetropic eye focuses parallel rays from infinity onto the retina without accommodation. The ciliary muscle can then adjust lens shape for closer objects down to the near point (~25 cm for adults). The preset models relaxed focusing for distant scenes.
- Near point ≈ 25 cm
- Far point ∞
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
A normal eye can focus on objects anywhere from its near point out to infinity by changing its internal lens's focal length (accommodation). Does the eye need a shorter or longer focal length to focus on a closer object?
Predictions to weigh
- Shorter — a closer object needs the eye's lens to bend light more strongly to still focus on the same retina distance.
- Longer focal length for closer objects.
- Focal length doesn't need to change for different object distances.
Variable roles
What you set:
- Object distance d_o (cm)
What you measure:
- Required focal length f (cm)
How the investigation runs
- Open the normal-eye preset and press Reset. The retina sits a fixed 2.2 cm behind the eye's lens.
- Enable the required-focal-length readout.
- Set the object distance for each trial and record the focal length the eye must adopt to keep the image on the retina.
Governing equation
Eye Accommodation — P = 1/f
To keep the retinal image sharp, the eye continuously adjusts its lens's focal length: f = d_o·R/(d_o + R), where R is the fixed retina distance. Closer objects demand shorter focal lengths (stronger focusing power).
What the printable worksheet asks students to work out
- For one trial, compute f = d_o·R/(d_o + R) using R = 2.2 cm (the retina distance). Compare to the table.
- Explain why f always stays just under R = 2.2 cm, approaching it as d_o grows very large (relaxed eye, focusing at infinity) and dropping further below it for closer objects (more accommodation effort).
Where this shows up beyond the lab
- A normal eye's near point (about 25 cm) sets the shortest focal length the eye's muscles can achieve; its far point (infinity) sets the longest (fully relaxed). Explain why aging typically raises the near point (presbyopia) even in an otherwise healthy eye — the lens loses flexibility, not focusing power at infinity.
- A camera autofocus system does mechanically what your eye does biologically: adjusts its lens's effective focal length (usually by moving the lens, rather than changing its shape) to keep the image sharp on the sensor at different object distances. Why might a camera lens move physically while your eye's lens instead changes shape?
- AP Physics 2 — Unit 13: Geometric Optics
- General High School Physics — Light & optics
- NGSS High School Physics — Wave properties
- Welcome to Normal Eye
- Select the eye
- Press Play
- Relaxed far vision
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.