The Human Eye

The Human Eye Its Function and Limitations

Light Refracts (Changes Direction) as it Moves from One Medium to Another - so lenses can focus light

The Basic Steps Light enters the pupil enters the pupil is refracted and focussed by the lens is refracted and focussed by the lens to form an image on the retina. to form an image on the retina. The energy of the light has a chemical effect on the pigments (enzymes) in the receptor cells that make up the retina, and has a chemical effect on the pigments (enzymes) in the receptor cells that make up the retina, and an electrical signal is sent along the optic nerve to the brain. an electrical signal is sent along the optic nerve to the brain. The brain does the rest!

Some Important Concepts 1. Dynamic Range (day vs night) How it’s achieved (in part): the pupil expands! in dim lightvs in bright light Much more important: the replenishment of the pigments – hence, we get dark-adapted.

2. Accommodation [focussing our eyes] We lose this ability as we age – hence, reading glasses.

3. Persistence of Vision Allows us to watch movies, TV, etc.

4. Colour Vision 4. Colour Vision To discriminate colours, we need at least two different kinds of receptors with different enzymes, having different sensitivities. Our retinas contain rods and cones.

The Difference

One Implication, and One Puzzle 1. The implication: When it’s dark, there is too little light to stimulate the Cones, so everything looks shades of black and white at night. With rare (bright) exceptions, the stars look colourless! (But have you ever noticed the subtle ‘redness’ of Betelgeuse, Antares, and Mars?) 2. The puzzle: If our Cones are sensitive to red, green and blue, why do some things look yellow, orange or purple?

The Puzzle Answered Together, red light and green light give us the sensation of yellow! Similarly, green and blue sensation of yellow! Similarly, green and blue yield teal; red and blue produce magenta. (All three mixed together produce white.) The light from a yellow object stimulates both the red and green receptors to some extent – and we see yellow. That is how old colour televisions work: electrons stimulate various tiny spots on the screen to glow red, green or blue – in combination, this generates all colours.

5. Resolution How much detail can you discern?

Finer Grain yields Better Resolution How many pixels, or How many pixels, or “picture elements,” are there? My camera has 12 megapixels. In your eye: 125 million rods and cones! (But they are clumped in groups, so not really quite that many in effect.)

The Size of the Image Also Matters If two photons (‘lumps’of light) arrive at well-separated spots on the retina, landing on different ‘pixels,’ they can be seen to be coming from two separate objects. But two photons that arrive closely side by side can’t be distinguished (the details are “unresolved”).

Example: To the Unaided Eye, Saturn is Just a Point of Light Its image formed on the retina is tiny – just a dot! To see details, either: Move closer to the target Move closer to the target so that it looks bigger (that is, the image is spread over a larger area on the retina) Or use a telescope and various optical elements to magnify the image. Or use a telescope and various optical elements to magnify the image.

Galileo’s Telescope Not invented by Galileo, but he saw its potential for astronomy.

Guilty of Heresy, Sentenced to House Arrest ”Wine is light held together by moisture.” – G Galileo