Light and VisionHow do we see?

The Electromagnetic Spectrum

Things to Think About Today

• Light waves travel in straight lines until they meet matter• Then, light is reflected, absorbed, refracted• Why do animals have eyes? How did eyes evolve?• What are the parts of the eye and how do they work to

bend light

Light Energy Meets Matter

Reflected

Refracted (Bent)

Absorbed

• Light waves travel in straight lines until they meet matter

• Some matter ( lenses) focus rays and bend them in defined ways

• Pin activity=Boxes with solutions/laser pointer, ray box

• Ray boxes with lenses, gelatin lenses

Convex Lenses

Magnifying Lenses are Convex lenses

Concave Lenses(divergent lenses)

Eyes: Detect and Focus Light

• Eyes are organs that detect light, and convert it to electro-chemical impulses in neurons

• In higher organisms the eye is a complex optical system which:

– collects light from the surrounding environment

– regulates its intensity through a diaphragm

– focuses it through an adjustable assembly of lenses to form an image

– converts the image into a set of electrical signals

– transmits these signals to the brain, through complex neural pathways that connect the eye, via the optic nerve, to the visual cortex and other areas of the brain.

3 types of eyes have evolved• Single centered lens

– Advanced mollusks (octopus), spiders,vertebrates

• Many tiny lenses– Arthropods (trilobites)– Ex. Ants (50 images) Horsefly

(7,000 images) Dragonfly (30,000 images)

• Hole without lens– snakes

Parts of the Eye that you can See!

Functions of Parts of the Eye• Eyelid- protection of eye

from injury• Pupil- hole in the center

of the iris that lets light in– Appears black because the

eye tissue absorbs most of the light

• Sclera-tough outer layer– collagen and elastin

• Iris-colored area surrounding pupil– Composed of smooth

muscle

Interior Parts of the Eye

Path of an Image

• Goes through the Cornea and Lens

• Hits the back of the retina

• Activates rod and cone cells

• Transmit impulse through optic nerve to the brain

Rod and Cone Cells• Rod Cells- numerous ( 130 million in each eye

– Responsible for Twilight vision, Gray shades– Very sensitive to light– Found on outside of

retina

• Cone Cells-– less numerous

• ( 7 million)

– Responsible for – Ability to see Color– Found in center of the retina

• (macula)

Now, let’s think about what might go wrong

Myopia- Near-sighted

• Genetic Basis

• Eye is longer than normal, Cornea steeper

• Correct with glasses, contacts, surgery

• Very common!

• Can make it better by squinting!- Changes the eyeball shape!

Myopia= Near Sightedness

Hyperopia- Far-sighted

• Image is focused behind the eyeball

• Cornea is flatter, eye is shorter, focusing power is weaker

• Young people can “accommodate” by changing shape of eye= As people get older, lens get harder and can’t be bent.

• Lasix surgery changes shape of cornea– http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0004853/

Hyperopia+ Farsightedness

Cataracts

• Clouding of the lens

• Caused by aging or damage to the proteins that make up the lens

• Chemicals, Diabetes, Injury, Smoking are causative

• Surgery to remove lens

Normal/Cataract

Retinitis Pigmentosa

• Rare genetic disease (1/4000 people in US)

• Rods die and leave dark deposits in retina

• Lose peripheral

and night vision

Normal/Retinitis

What about compound vision?

A Grid- Seen by a Human and an

Insect

A spider web- seen by a human and an

insect

Bee as seen by a mammal and an

insect

Insect images

Insects See Colors Differently

http://www.naturfotograf.com/UV_flowers_list.html

What about other animals?

Bird Vision

• How do nocturnal birds see?– Rod cells are very sensitive in low light

• Humans have around 200,000/mm2, some owls have over a million/mm2

– Birds also have larger pupils which let more light in

• What about day foragers– Have more cones than humans

– May have more than one fovea area of sharpness)

Ruminant Pupils

• Slit-like in bright light, circular in dim light

• Gives permanent wide-angle vision whether in dim or bright light -makes watching for predators more efficient.

Making the connection between physical science and biology

• Light travels in straight lines

• Light bends when it meets matter

• Lenses are matter that bend light

• How animals see• Eye diseases• Do you need light to see?

• Ray box

• Bending Experiment

• Play with Lenses

• Eye Dissection

• Annenberg video

Video

Wrap up Questions

• What is seeing?

 

• Can a human being see in the dark? Why?

• How would a scientist test if you could see in the dark?

Reading Resources

• A Natural History of the Senses-Diane Ackerman

• Video- Annenberg Foundation

• GEMS- http://lawrencehallofscience.org/gems/gemsguidestopic.html

Exploring Light and Lenses

• Concave Lenses and light • Pinhole Viewer (Camera) • Camera Obscura

Camera Box Camera Obscura (CA)