Optics
The eye and optical instruments
Study image formation by a converging lens and an eye model: vary object distance and focal length to determine image character. Verify the thin-lens formula and magnification.
⚠ Never point a telescope or binoculars at the Sun — it can cause permanent blindness.
Equipment
- 3D model of rays through a lens/eye (two panels)
- Incidence-angle, refractive-index, and focal-length sliders
- Image-position readout and virtual retina
Procedure
Rays through a curved surface
In panel 1, keep index 1.5 and vary «Incidence angle»: rays through the curved surface bend toward the axis. Shorten/lengthen «Focal length» to see stronger/weaker convergence — short f means stronger curvature and a «stronger» lens.
Verify the thin-lens formula
Place the object at several distances d (inside and outside f) and read the image position d'. Check for d > f (real, inverted image) and observe the upright enlarged virtual image for d < f — the magnifier regime.
Eye model and accommodation
In panel 2, vary «Focal length» to mimic the crystalline lens changing curvature: distant objects need large f (flattened lens), near objects need short f (bulging). Notice the retinal image is always real and inverted — the brain «re-erects» it.
Predict the instruments
Predict the setup for a magnifier (d < f), a microscope (short-f objective forms a real magnified image, the eyepiece acts as a loupe), and a telescope (distant object → image at the focal plane). Set the sliders accordingly and check with the ray paths.