Physics Mrs. Tobler

What is Light?

• Debated by scientists–Dual “Personality”

•Acts like transverse waves •Acts like it is made of particles –Photons (packets of energy)

• Light is a form of energy

Light as a Wave• Light travels as a transverse wave

– Made up of Electric and Magnetic Fields moving perpendicular to one another

• Light is a range of frequencies of electromagnetic waves– A.K.A. Electromagnetic Radiation

Properties of Electromagnetic Waves

• Travel at 3.0 x 108 m/s in a vacuum (empty space)

• Light year = distance light travels in one year (about 6 trillion miles)– The light from a star 4 light years away

traveled for 4 years before reaching your eyes

– Light from our own sun takes 8 minutes to reach Earth

– When you look at the stars you are looking into the past

• Transmit energy via Radiation– Don’t need a medium

• Slow down when it enters a medium.– Diamond slows it down to 124 m/s

Visible Light

• Remember “ROY G BIV”• Which has a longer wavelength

Orange or Blue?– Orange

Complete the Spectrum

Uses for Electromagnetic Waves

• Radio Waves– Transmit TV, AM &

FM– Satellite radio– Cell phones– RADAR for

communication and police

• Microwaves– Cook food by

exciting water molecules to produce heat

• Infrared– Keep food warm (like at McDonald’s or

Luby’s)– Night vision

• Ultraviolet– Suntans, skin cancer– Sterilize goggles and other items– Fluorescence (Black Light)

• X-rays– For looking at bones such as teeth or broken

arm• Gamma

– To kill off cancer cells– Can also damage healthy cells

Uses Continued

Why can’t we see all electromagnetic waves?

• Human eyes contain rods and cones in the retina that are sensitive to only a small range of frequencies

• We see only reflected or emitted light– Reflected = waves bounce off of a

surface back to our eyes– Emitted = object produces light at

frequencies our eyes can detect

What color is white light?• Isaac Newton

– Discovered white light consists of various colors of light in 1666 using prisms

• Made of the 3 Primary Colors of Light– Red– Blue– Green

How we See Color• Our eyes detect the color

(frequency) of light that is being Reflected off of a surface.– If an object appears blue that

means blue light is bouncing off of the object and the other colors of light are absorbed

• What happens to the light if the object appears white?

• What if it appears black?

More Fun with Color• Primary colors of Pigments

• Magenta• Cyan• Yellow

– Color printers use these same colors

• Color Filters– If an object reflects red light when

exposed to white light, then when exposed to green light the object will appear almost black because there is NO red light to reflect.

Behaviors of Light• All electromagnetic waves

experience the following wave behaviors similar to mechanical waves– Reflection– Refraction– Diffraction– Interference

• Constructive• Destructive

– Polarization

Words to Know• Ray = beam of electromagnetic radiation

(light)• Normal = imaginary line that is

perpendicular to the surface of an object• Incident Ray = emitted or incoming light

ray• Reflected Ray = light ray that bounces

off a surface; outgoing wave• Angle of Incidence = angle at which the

incoming light ray hits a surface• Angle of Reflection = angle at which

reflected light ray leaves the surface

Reflection of Light• Occurs when a wave reaches a

boundary between 2 different mediums which causes– The wave to bounce back into the 1st

medium (totally reflected)– Or some of the wave bounces back and

some is absorbed into the 2nd boundary (partially reflected).

• Depends on how rigid the boundary is – Very rigid then most of the wave is reflected – Less rigid then only some of the wave is

reflected

Law of Reflection• When a wave

hits a boundary the angle of incidence equals the angle of reflection.– Also true when

playing wall ball, basketball and tennis

Complete the Reflection Diagram

Diffraction• Bending of a wave around a

barrier, such as an obstacle or the edges of an opening.– Amount depends on the size of

the wavelength compared with the size of the obstruction or opening.•Wide opening = less bending •Narrow openings = more bending

• Thomas Young’s Double Slit Experiment

Diffraction Gratings

• A series of closely spaced parallel slits or grooves

• Used to separate colors of light by interference.– Used instead of a prism– Found in objects used to reflect

various colors of light •Back of CDs & DVDs•Bird feathers

Total internal Reflection• At a certain angle (critical angle)—

no ray will emerge into the air above the surface.– For water it is 48 which does not

allow the ray to enter the air, it is only reflected inside the water.

– Other Examples•Glass (43)•Mirrors (90-95)•Diamonds (24.6)

Review Refraction• Bending of a wave as it enters

a new medium due to a change in velocity.– Angle of Incidence is NOT EQUAL

to the Angle of Refraction• Refraction of Sound

•Even though speed of sound is greater in warm air; sound waves bend away from the ground which doesn’t allow it to carry as well.

• In cold air sound bends toward the ground and sound carries well.

Refraction of Light• Less dense to More

dense medium – Speed of light slows

down – Refracted Ray bends

toward the normal• air to glass

• More dense to Less dense medium – Speed increases – Ray bends away from the

normal.• water to air

• As it leaves the new medium it goes back to the original angle.

Complete the Refraction Diagram

Observing objects in Water

Refraction of Light

Causes white light to separate into the

primary colors as it passes through

the water droplets

How are Rainbows Produced?

• Water droplets act like prisms• Each color of light is refracted

at a different angle due to its frequency– Violet has the highest

frequency and is refracted the most.

– Red has the lowest frequency and is refracted the least.

• Sun has to be behind the observer

• Circular rainbows can be seen from airplanes

Atmospheric Refraction

• Mirage – caused by change in air temperature resulting in light refraction– Road appears

to be wet•Actually reflection of the sky

Why is the Sky Blue?• Light from the

sun gets scattered as it collides/passes through molecules in the atmosphere– Blue light is

scattered the most because of its short wavelength

Index of Refraction• Unique to the substance

– Based on density

• Ratio between the speed of light in a vacuum to the speed in a particular material

v

cn

n = index of refraction

c = speed of light in vacuum

(3x108 m/s)

v = velocity of light in material

Example Index of Refractions

Material Refractive Index

Air 1.0003

Water 1.333

Glycerin 1.473

Immersion Oil 1.515

Crown Glass 1.520

Flint Glass 1.656

Zircon 1.920

Diamond 2.417

Lead Sulfide 3.910

Polarization of Light• The aligning of vibrations in an

electromagnetic wave, usually by filtering out waves of other directions.

• Polarized Filter– Only allows waves through that are

parallel to the slits

• Occurs only with transverse waves – Not all light is polarized

How Polarization Works

Fiber Optics• A.K.A. Optical fibers or light

pipes• Thin glass fibers

– Use total internal reflection • Developed for communication

and data transmission– Replacing electric circuits and

microwave links • Thousands of miles of it in our

phone lines

Polarized Sunglasses.

• Only allow light that is in the same direction as its polarized axis to pass.– If polarization axis of 2 filters are at right

angles to each other then the light can’t pass through.

Traditional 3-D Glasses

• The red and blue lenses filter the two projected images allowing only one image to enter each eye.

• Brain combines to images to look 3-D

Modern 3-D Glasses

• Uses polarized filters instead of color

• Allow only one of the images intoeach eye because each lens has a different polarization.