Light Effects

blue skies and rainbows!

Let’s re-group and re-view

We pulled a Friday…so let’s review.

• Before we can – we need to step back and take a look at our eye:

Cornea – transparent covering, helps focus the lightIris – colored part that controls the amount of light that entersPupil – opening that appears blackLens – focuses light onto retinaRetina – tissue on the back of the eye

Rods - brightness receptors, see in the darkCones - color receptors

Optic nerve – carries info to your brain, cause the blind spot

More on eye issues to come…

• We will review the various “near sighted” and “far sighted” issues and how physics can help us understand our eye appointments when we draw lens diagrams next week.

Incandescent vs Fluorescent

Regular light

Enhancing red + green = yellow

Enhancing blue = blue

Incandescent bulbs enhance redFluorescent bulbs enhance blue

To save energy costs, find the bulbs with the light output you need, then choose the lowest watts.

Brightness is based on LUMENS not WATTS.

Why don’t people buy energy efficient bulbs?

• When the medium changes, light can1. absorb (stop)2. reflect (return back through the same medium)3. transmit (continue on to the new medium)

a. refract (bend because of changes in speed)b. disperse (refract differently for each color)c. diffract (bend around a hole)d. polarize (transverse wave effect)

4. or a combination of 1−3.

(b) Dispersion

Dispersion

• is the separation of light into different colors

• occurs because different colors have slight changes in the speed of light (and thus different indices of refraction).

• "Red Refracts Rarely"• "Blue Bends Best"

Why is the sky blue?

lots ofblue

red sunsets

Rainbows

Inside the raindrop…1.Refraction2.Reflection3.Dispersion

"Red Refracts Rarely" (outside of rainbow)"Blue Bends Best" (inside of rainbow)

Double Rainbows!

Iridescence

• is a rainbow-like effect caused by reflection off of a thin layer (film)

(c) Diffraction

Diffraction• is the bending of a wave around the corner of

a hole.• The smaller the hole, the more the wave

bends.

Light: Particle or Wave?Particle Wave Light

Who?Newton Huygens

Einstein

Reflection yes yes yes

Refraction yes yes yes

Diffraction no yes yes

Momentum yes no yes

Diffraction Grating• is a piece of plastic with

parallel holes cut into it• Light is bent into a

"diffraction pattern"• (lab 227 Tuesday)

(d) Polarization

Polarization• Light is a transverse wave.• Luminous sources give off

light in all transverse directions (unpolarized).

• A "polarizing filter" (or polarizer or polaroid) makes light go in one transverse direction.

• A second polarizer in the same direction passes light; at a right angle blocks light.

Glare and Sunglasses

• Glare is the light reflecting off a horizontal surface. It is horizontally polarized.

• Polarized sunglasses are vertical polarizers; they block the horizontal glare, and pass some of the unpolarized light you want to see.

Laser

• Light Amplification by Stimulated Emission of Radiation

• One color (monochromatic), crests occur at same time (coherent), narrow beam (collimated)

Laser Safety

1. Always know where your beam is going.2. Never point towards faces or eyes.3. If only one person can see the beam, it is

strong enough to blind.4. Turn off when you are done.

Doppler Effect

moving towards moving away

higher frequency lower frequency

shorter wavelength longer wavelength

blue shift red shift

ParallaxThe apparent shift of an object against a background due to a change in observer position.

3D Movies and TVHow does 3d polarization work? http://www.youtube.com/watch?v=cxzosL4oyJM&feature=related 4min

Polarized light_3dmovieshttp://www.youtube.com/watch?v=PCE1JZ3bKMg 3min

Reflection and Refraction

Changes in the Medium• When light travels through the same medium,

it travels in a straight line.• When the medium changes, light can

1. absorb (stop)2. reflect (return back through the same medium)3. transmit (continue on to the new medium)

a. refract (bend because of changes in speed)b. disperse (refract differently for each color)c. diffract (bend around a hole)d. polarize (transverse wave effect)

4. or a combination of 1−3.

Absorbption, Reflection, Transmission

Absorbtion

Absorbtion• Opaque objects absorb all light; no

light passes through.• Translucent objects absorb some

light, and scatter the other light so it does not pass through clearly.

• Transparent objects pass light through.

• Colored objects absorb some colors of light; the other colors are reflected or transmitted.

Reflection

Specular – reflection off smooth surfaces

Diffuse – reflection in many directions from a rough surface

Sound Reflection

Echo: a reflection of soundReverberation: persistence of sound due to

multiple reflections

Measuring Reflection• incident ray: the incoming ray of light

(draw solid with arrow heads)• reflected ray: the outgoing ray of light

(draw solid with arrow heads)• reflection point: where the incident and reflected rays

hit the mirror• normal line: passes through the reflection point, and

makes a right angle to the mirror (draw dotted)• angle of incidence, θI: angle between the incident ray

and the normal line.• angle of reflection, θR: angle between the reflected ray

and the normal line.

Angles are always measuredto the normal line.

never to the mirror!

Reflected rayIncident ray

Normal

line

Angle of incidence

Angle of reflection

Reflectionpoint

WRONG ANGLE!

WRONG ANGLE!

Angles are always measured to

the normal line.

Law of Reflection

• The angle of incidence equals the angle of reflection:

θI = θR

Specular – reflection off smooth surfaces

Diffuse – reflection in many directions from a rough surface

Periscope

Car headlight

Other uses of reflectors…

Refraction

Refraction

• is the change in direction of a wave as it moves between two mediums in which the wave travels at different speeds.

• toy car video• fish tank demo

Refraction

Measuring Refraction• incident ray: (same)• refracted ray: the outgoing ray of light• refraction point: where the incident and

refracted rays hit the lens• normal line: passes through the refraction

point, and makes a right angle to the lens• angle of incidence, θI: (same).

• angle of refraction, θR: angle between the refracted ray and the normal line.

Angles are always measured to

the normal line.

How much light refracts depends…

1. the angle the light enters2. type of materials

air less dense medium light is faster light bends away from the normal

glass more dense medium light is slower light bends toward the normal

Index of Refraction, n: the ratio of the velocity of light in a vacuum to that in a medium [no units]

n = index of refraction c = speed of light in vacuum = 3.0 x 108 [m/s] v = speed of light in given medium

↓speed of light = ↑ n valueslower light travels = the higher the index of refraction

v

cn

Example: If light travels at 1.90x108 [m/s] in crystal, what is the crystal’s index of refraction?

smx

smxn

/1090.1

/1000.38

8

58.1n

v

cn

Snell’s Law

θi = angle of incidence θr = angle of refraction

ni = incident index of refractionnr = refracted index of refraction

rrii nn sinsin

ni sinӨi = nr sinӨr

In the picture below, light is traveling from water into air. The angle of incidence is 30°. What angle of refraction?

Substance (n)

Air 1.00

Water 1.33

1.33 sin 30° = 1.00 sinӨr

Өr = 41°

1.33 sin 30° =sinӨr

1.00

Өr = sin-1 0.66

Is the fish higher or lower than it

appears?

lower

total internal reflection – occurs when the incident angle is greater than the critical angle

critical angle – incident angle that causes the refracted ray to lie along the boundary

Fiber Optics

demo

Internal Reflection is what makes a diamond sparkle

Sound Refraction – sound travels faster in warm air

Mirage – image that appears in the distance due to the refraction of light

Prezi “now you see me”

• http://prezi.com/jtbjhlaqv3nd/untitled-prezi/

• Colors http://www.youtube.com/watch?v=Hbxy1W9O_Wk

• Bill Nye (6min)• http://

www.youtube.com/watch?v=gtgBHsSzCPE

refraction

• http://www.youtube.com/watch?v=1Ppjl0iKFnE

• Aim at the tail of the fish

• http://www.youtube.com/watch?v=evQsOFQju08&list=UU6nSFpj9HTCZ5t-N3Rm3-HA

• Is your red my red? (9min)