Light & Eye

K.7 SOL

The student will investigate and understand that shadows occur when light is blocked by an object. Key concepts include *shadows occur in nature when sunlight is blocked by an object; and * shadows can be produced by blocking artificial light sources.

5.3 SOL

The student will investigate and understand basic characteristics of white light. Key concepts include * the visible spectrum, light waves, reflection, refraction, diffraction, opaque, transparent, translucent; *optical tools (eyeglasses), lenses, flashlight, camera, kaleidoscope, binoculars, microscope, light boxes, telescope, prism, spectroscope, mirrors); and * historical contributions in understanding light.

What is Light?

The nature of light Light is a form of energy Light interacts differentially

with matter Vision occurs as light

energy is processed by the human eye

Light is Energy

Light is a form of radiant energy. It travels in waves.

Visible light is part of the electromagnetic spectrum

Radiant energy travels in straight lines

Wave characteristics

Wave length: Distance between Maximas

Frequency: The number of waves per second

Energy Relationships

Wavelength =

Frequency = v (hz) (cps)

1, 2, 3 have passed me

Light is fast

Frequency and Velocity

c = velocity of light in a vacuum c is constant c = 300 million m/sec c = /sec x m/

The Electromagnetic Spectrum

A continuous spectrum of electromagnetic waves (Maxwell, 1880), (Hertz,1888) with varying wavelengths, frequencies and energies.

The Electromagnetic Spectrum

Energy

E = mc2

Energy = mass x velocity squared

As velocity increases, energy increases

Relationships

As wavelength, increases

frequency, , decreases

energy decreases.

As wavelength, decreases

frequency, , increases

energy increases

Visible light

A portion of the EM spectrum 700 nm - 400 nm ROYGBIV Infrared - longer than red Ultraviolet - shorter than violet

Which has the most energy?

Color

A wonderful resource - http://www.thetech.org/

exhibits_events/online/color/overview/

White light & Prisms & Newton

Newton conducted prism experiments White light (sun light) contains all the

visible wave lengths of light

Rainbow

When white light is broken (refracted) by rain drops into the visible spectrum

White

Rattlesnake- Infrared Sensor

Fluorescence

Light is absorbed at one wavelength (energy) (color) and emitted at another

Light interacts with matter

reflection

refraction

diffraction

absorption/transmission

Interactions depend on the nature of matter and light

The denser the matterdenser the matter the more interactions

The greater the energygreater the energy of light the more interactions.

Why is the sky blue and the sun yellowyellow?

Reflection

light returns returns after striking an object

the angle of reflection equals the angle of incidence

Angles of Reflection

Normal, at 90o from surface

Angles of reflection

Angle of incidence = Angle of reflection

NormalNormal is an imaginary line perpendicular to the reflecting surface

Activity: Reflection

Bouncing light off mirrors using a flashlight and mirror in a dark room.

Place a mirror on the floor and shine the light on the mirror at an angle. Hold a book in the path of the reflected light. Using a protractor can you measure the angle? (Ticotsky, p 30)

The Case of the Vanishing Reflection Carefully cut 8 - 10 inches of foil and look at

your reflection. Crinkle the foil and flatten it again. Can you still see your reflection? (Churchill, p 95)

Refraction

Light is bent bent when it traveled from one medium to another of a different density.different density.

Snell(1621) The law of refraction When the light passes through a denser

medium, the light is bent toward the normal, because the light slows down (velocity decreases).

When light passes through a less dense medium the light is bent away from the normal.

AIR

WATER

GLASS

AIR

Refraction occurs at the interface between two mediums

Water more dense than air-->

Air less dense than glass -->

Activity: Refraction

You can see this effect when you place a coin under a clear glass. Look straight down, what do you see? Now look from an angle. What happens?

Turn this around. Place a coin in a bowl and move so that you can not see it. Have your partner slowly add water to the bowl, what happens

Diffraction

light that passes very close to an edge bendsedge bends

Theoretical

Diffracted

light

Obstacle

Transmission - Absorption Light that passes through

an object is transmittedtransmitted Light neither transmitted

nor reflected is absorbedabsorbed Transmitted light strikes

the retina of the eye to stimulate vision

Transparent

All light is transmitted

Opaque

No transmission. Only reflects and absorbs.

Translucent

Reflects, transmits and absorbs

Absorbed light causes the heating effect

Prove that dark colors absorb light and light colors reflect light. Put two cups containing water and a

thermometer behind a white sheet of paper and a black sheet of paper. Place a lamp very close to the cups, allow the lamp to shine only on the paper not the thermometer or water. What happens?

What colors do we wear in the summer? Why?

Light travels in a straight line

What Path Does Light Follow?

Set up 3 or 4 index cards standing up. Punch a hole in each card. With a flashlight on one side, move the cards until the light passes through each. Look through the last back at the light. What will you see? Move the cards, what happens?

Shadows Shadows are a consequence of light moving

in a straight line. If an object passes in front of the light source,

the light is absorbedabsorbed(blocked) and a shadow forms.

Shadows are always darkest in the middle and lighter on the edges.

Why Do Shadows Change in Size?

The more light rays that are blocked by the object, the larger the shadow.

The closer the object to the

light source the BIGGER the shadow

Activity: Shadow

Place a projector across the room from a black wall, turn the projector on and the lights off. Slowly walk toward the projector. What happens to the size of the shadow formed?

Why are shadows darker in the middle, lighter at the edges?

DiffractionDiffraction

Optical tools

Lenses Eyeglasses Microscope Camera Kaleidoscope Binoculars

Flashlight Light boxes Telescope Prism Spectroscope Mirror

Lens Lenses alter the angle of incidence, and

thus the angle of refraction of light rays Lenses determine how much light rays

are bent Lenses determine in which direction light

rays are bent Lenses bring light rays together

(converge) Lenses spread light rays apart (diverge)

Magnifying lens

Tools that use lenses / mirrors Eyeglasses - to correct poor

vision Telescopes- to see things that

are far away Microscope - to see very small

things Binoculars - to enlarge an image Camera - focus image on film

Binoculars

Telescope

Telescope

Mirror

Kaleidoscope

Sir David Brewster, 1817

internal mirrors repeat the pattern

http://kaleidoscopeheaven.org/info.html

Lasers

Light Amplification by the Stimulation Emission of Radiation

History

Mesopotamia Ptolemy Classical Greeks Pythagoreans Newton Young & Helmholtz

Mesopotamia (1500 BC)

polished metal used as mirror piece of glass used as burning lens

Ptolemy (127-135 AD)

Alexandria rules of additive light mixtures spinning disks* light bends when passing

through glass (refraction) Wrote Optica on optical

phenomena

Classical Greeks

Sought underlying principles Observation without

experimentation Light travels in straight lines Reflected light: the angle of

incidence = the angle of reflection

Plato: believed that light was

emitted by the eye. This was believed as late

as 1644 when Descartes published a book elaborating a similar theory.

Aristotle (350 BC)

all colors are derived from mixture of black and white, White light is the purest color, others are contaminated

“These juxtapositions involving simple ratios may the most pleasing colors such as purple or crimson, like the concords in music. Irrational ratios may produce impure colors”

Sense and the Sensible

Isaac Newton (1600’s)

Beginning of modern scientific understanding /testing

Prisms white light consists of light of

may different colors with different “refrangibility” and power

Perception of color is due to physical properties of light

Make a prism

Make a prism by placing a small mirror in about an inch of water in a baking pan. Lean the mirror against one edge near bright sun light and direct the reflection to a white surface.

Young (1802) & Helmholtz

trichromatic (three color receptor) theory of color vision

demonstrated that the wide range of colors can be reproduced by superimposing various proportions of red, green and blue light

there are three color receptors in the eye

Today

the three cone pigments genes have been located on specific chromosomes

Human Eye

Human eye structures

Cornea - most anterior Aqueous humor - watery solution

behind cornea Lens - fairly rigid structure Vitreous Humor- jelly like colloid Retina -delicate light receiving

layer Sclera - tough protective white

outer layer

corneaVitreous Humor

Aqueous humor

Refractive surfacesMedium Refractive index

Air 1

Cornea 1.3

Aqueous humor 1.3

Lens 1.42

Vitreous 1.42

Where does most refraction occur? At the air/cornea interface. This is the site of astigmatism A normal cornea is like a

baseball - evenly curved in all directions

An astigmatic cornea is like a football, more curved in one direction than the other.

cornea

Vitreous Humor

Aqueous humor

Human Lens

The lens in the human eye is convex, but unlike a glass lens, it is elastic so that it can change shape to focus on objects at varying distances. The lens becomes short and fat when viewing close objects and elongated and thin when viewing distant objects.

Hyperopia (far sighted)

Sometimes eye muscles are unable to focus light on the retina, the screen at the back of the eyeball. If the image forms behind the retina for nearby objects, farsightedness (results. Convex lenses are prescribed for hyperopicshyperopics to assist the eye in making light converge on the retina for nearby objects

Hyperopia & Myopia

Presbyopia - old eyes

As the eye ages, the lens becomes less transparent and absorbs more blue light.

What is the effect on vision?

Cataracts

A loss of transparency - usually of the cornea

May be congenital or aquired (uv light)

What is the effect on vision?

Vision occurs as

Light energy collected by the eye

Is transduced in the retina to neural energy

Travels via nerves and is processed by the

brain

Color vision

The eye has three types of light receiving units, red, green and blue cones

Differential stimulation of cones If you stimulate

only red and green cones, not blue, you see YELLOWYELLOW

Try this with different colors of cellophane on an overhead projector or tied over flash lights.

White

Colorblindness

the British Navy

The result of cone (pigment) dysfunction