1. The Eye

2. Eye Dissected  The eye is an optical lens system. Two single lensesTwo single lenses  The goal of the eye is to focus an image on the retina.

3. Corneal Lens  The cornea is a curved membrane with fluid behind it. Two indices of refractionTwo indices of refraction n corn = 1.376n corn = 1.376 n aq = 1.336n aq = 1.336  This is roughly the refraction from air to water. Eye works poorly underwaterEye works poorly underwater

4. Crystalline Lens  The lens is a denser material for fine focusing. n lens = 1.406 at maximum  Relaxed muscle provides little additional focus. For distant objects  Contracted muscles provide more focus. Relaxes tension and creates a bulge For near objects

5. Accommodation  The moon subtends an angle of about 0.009 rad as seen from the earth.  The size of the moon on the retina comes from the image triangle.  The focusing by the lens is called accommodation.  The relaxed lens forms a combination with the cornea. Object focus 15.6 mm in front of corneaObject focus 15.6 mm in front of cornea Image focus 24.3 behindImage focus 24.3 behind Optical center 17.1 mm in front of the retinaOptical center 17.1 mm in front of the retina  Typical shifts in accommodation are a few mm.

6. Retina  The retina is the screen for the eye. Light sensitive cells Neurons for signal transmission  Ideally it should be at the focal plane. Otherwise image is blurred

7. Pinhole Camera  A pinhole opening can act as a single lens. Forces ray convergence at the opening pointForces ray convergence at the opening point Iris in eye can act as pinhole lensIris in eye can act as pinhole lens  A pinhole camera is a box with a pinhole opening to focus light.

8. Single Lens Reflex  A single lens reflex (SLR) camera acts much like an eye. Shutter like irisShutter like iris Lens fixedLens fixed Adjust distance to screenAdjust distance to screen next