# LIGHT. · From the Fenway Park bleachers, why do you see Jacoby Elsbury hit the baseball before you hear it?

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· From the Fenway Park bleachers, why do you see Jacoby Elsbury hit the baseball before you hear it?

LIGHT

What is Light? Early theories of light varied. One was that your eyes emitted beams that would illuminate things for you. That was why you could only see when your eyes were pointed at something(~500 BC). Eventually people came to believe that light was either a wave or a bunch of moving particles. Einstein coined the term “photon” which represents the tiny massless particle that light may exist as.

What is an electromagnetic wave?
Remember waves transfer ______ not matter. The waves we have studied so far have all traveled through matter. An electromagnetic wave does not need matter to travel through. It can travel in a vacuum. An electromagnetic wave is a wave of oscillating electric and magnetic fields that moves at the speed of light.

What is light? Since, em waves have both magnetic and electrical qualities;the two qualities exchange energy back and forth through induction. This causes the em waves to propagate and spread.

Speed of light in a vacuum is constant in universe: 300,000,000 m/s
Measuring the speed of light Just how fast is light? Speed of light in a vacuum is constant in universe: 300,000,000 m/s 7.5 round trips around the earth in one second 8 minutes from the sun to the earth 4 years from the nearest star, Alpha Centauri 100,000 years to cross our galaxy some galaxies are 10 billion light years away

Measuring the speed of light
A “light year” is a measurement of distance based on how far light can travel in a single Earth year. How far is a light year? Distance = velocity x time Light year = 300,000,000 m/s x 1 year = 300,000,000 m/s x 1 yr x 365 day/yr x 24 hr/day x 3600 s/hr Light year = 9.5 x 1015 m Or 9,500,000,000,000,000 m

So, the speed of light is:
What is the frequency of a beam of light that has a wavelength of 1014 m and it is traveling in a vacuum?

7 types of Electromagnetic waves
(7 types of light) radio waves, microwaves, infrared,Visible light, ultraviolet, x-rays, & gamma rays lowest frequency WE CAN SEE is red highest frequency WE CAN SEE is violet (more energetic)

Red Orange Yellow Green Blue Indigo Violet
ROY-G-BIV Red Orange Yellow Green Blue Indigo Violet

When light energy hits an object, the energy in the light causes the electrons in the material to vibrate. When this happens, a number of different things can occur. One of these things is:

The object can be: Transparent to Light
Transparent: lets light pass through in a straight line Glass and water are transparent to visible light Spring and ball model of light transmission light causes springs and balls to vibrate vibration transfers within the material energy is released on the other side

Transparent to Light Glass Example:
Glass, like what makes up our windows, is transparent to visible light (what lets us see) In other words, glass transmits visible light through it. This is a good thing! Mr. Stief wouldn’t be able to see if his glasses didn’t do this! BUT! Glass doesn’t transmit UV rays (in other words you are safe from sunburn due to UV radiation if you sit by a closed window). Glass likes to hold onto UV radiation. But what happens to that energy then? Results in heat

Opaque Materials An opaque material absorbs certain frequencies of visible light but reflects other frequencies (it does not transmit light). We perceive the color of something based on the colors that get reflected off an opaque material.

What is the speed of light?
Can we ever say that the speed of light is less than this number? How? What is the difference between a transparent material and an opaque material?

Shadows A shadow is formed when a light ray cannot reach a surface
total shadow: umbra partial shadow: penumbra light from another source fills in large source only partially blocked

Polarization Light is a transverse wave
Light from most sources vibrates in all planes Each light ray can be considered to have horizontal and vertical components Separating vertical and horizontal components is called polarization

Polarization Polarizing filters are like sewer gratings that look like slits. Light waves vibrating in the plane of the slit can make it through Light waves that vibrate perpendicular (at a 90 degree angle) to the grates cannot make it through

Polarization A single polarizing filter will let about one half of the light through Two polarizing filters aligned in the same direction will still let about one half of the light through Two polarizing filters aligned perpendicular to one another will let almost no light through

Applications of Polarizing Filters
Sun Glasses reduce glare block out half of the light 3-D movies

Why is the sky blue? Where do rainbows come from?

Diffraction When waves bend around an object
The smaller the object compared to the wavelength, the more the bending. Wavelength and velocity stay the same.

So, the sky is blue due to the fact that as white light (a combination of all the colors in the visible spectrum) hits our atmosphere, the light diffracts and spreads across the sky. However, it just so happens that due to the size of the gas particles in the sky, blue and violet diffract (bend around a barrier and spread out) the most, so we see the sky as blue. The sky is blue because: light diffracts in out atmosphere Blue and violet diffract the most so we see blue

Diffraction of Light What type of wave gets diffracted more?
Blue and violet bend the least Red bends more Lower frequencies/bigger wavelengths diffract the most Higher frequencies/smaller wavelengths diffract the least

· Why is it hard to pick a penny off of the bottom of a swimming pool?

Why is it difficult to catch a fish with a spear
Why is it difficult to catch a fish with a spear? (think of why it your hand looks all squishified when you look at it in a sink full of water?)

Refraction: when a wave passes from one medium to another This occurs in a change in wave speed If the incident wave is not straight on, the wave direction will change Some waves wavelength also change

Since the wave bends as it goes from one medium to another, the fish looks like it is in a different place

What does it mean for a wave to get diffracted? Example?
What does it mean for a wave to get refracted? Example? So where do rainbows come from?

Light can be bent as it passes in different media
Light can be bent as it passes in different media. This is how a mirage occurs. It is the bending of light as it passes through different temperatures of air. And it is why some roads look wet on a hot day

So, when a wave hits an object, a number of things can happen
So, when a wave hits an object, a number of things can happen. They are: Transmit (go through) Refract Get absorbed In some cases, get diffracted And they can get REFLECTED!

Reflection - how a mirror works
Smooth surface Definitions Incident ray is the ray moving toward the surface Reflected ray is the ray moving from the surface Normal is an imaginary line perpendicular to the surface

More Definitions Angle of incidence is the angle between the incident ray and the normal Angle of reflection is the angle between the reflected ray and the normal

Plane mirror

Bill looks at Emily’s reflection in the mirror
Bill looks at Emily’s reflection in the mirror. Draw the position of Emily’s image. Draw the path of the light rays that travel from Emily to Bill. Bill Emily

Concave Mirrors: (like a cave!)
Satellite dishes are like concave mirrors shaped like a parabola Image from a concave mirror depends on the placement of the object in relation to the mirror

Convex Mirrors A convex mirror is shaped like the outside bottom of a spoon. Convex mirrors are used as rearview mirrors in cars and in stores to observe shoppers. Both of these types of mirrors still obey the law of reflection

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