Sunlight 1

SunlightSunlight

Chapter 14Chapter 14

1020C1020C

Sunlight 2

Introductory QuestionIntroductory Question

When you look up at the sky during When you look up at the sky during the day, is the light from distant the day, is the light from distant stars reaching your eyes?stars reaching your eyes?

A.A. YesYes

B.B. NoNo

Sunlight 3

Observations about Observations about SunlightSunlight

Appears whiter than most lightAppears whiter than most light Makes the sky appear blueMakes the sky appear blue Becomes redder at sunrise and Becomes redder at sunrise and

sunsetsunset Reflects from many surfaces, even Reflects from many surfaces, even

nonmetals (Reflection)nonmetals (Reflection) Bends and separates into colors in Bends and separates into colors in

materials (Diffraction)materials (Diffraction)

Sunlight 4

5 Questions about 5 Questions about SunlightSunlight

Why does sunlight appear white?Why does sunlight appear white? Why does the sky appear blue?Why does the sky appear blue? How does a rainbow break sunlight How does a rainbow break sunlight

into colors?into colors? Why are soap bubbles and oil films Why are soap bubbles and oil films

so colorful?so colorful? Why do polarizing sunglasses reduce Why do polarizing sunglasses reduce

glare?glare?

Sunlight 5

Question 1Question 1

Why does sunlight appear white?Why does sunlight appear white?

Sunlight 6

LightLight

Medium-wavelength electromagnetic Medium-wavelength electromagnetic waveswaves

The range of wavelengths we can The range of wavelengths we can see:see:

Sunlight 7

Spectrum of SunlightSpectrum of Sunlight

Thermal agitation make charges Thermal agitation make charges accelerateaccelerate

Highly agitated charges emit lightHighly agitated charges emit light The sun is a black-body at 5800 °CThe sun is a black-body at 5800 °C We perceive thermal light at 5800 We perceive thermal light at 5800

°C as “white”°C as “white”

Sunlight 8

Question 2Question 2

Why does the sky appear blue?Why does the sky appear blue? Why is the sun reddish at sunrise and Why is the sun reddish at sunrise and

sunset?sunset?

Sunlight 9

Rayleigh ScatteringRayleigh Scattering Rayleigh scattering occurs when light polarizes matterRayleigh scattering occurs when light polarizes matter

Passing sunlight polarizes tiny particles in the airPassing sunlight polarizes tiny particles in the air and that fluctuating polarization leads to light emission,and that fluctuating polarization leads to light emission, so air particles scatter light; they absorb and then reemit it.so air particles scatter light; they absorb and then reemit it.

Air particles are too small to be good antennas for Air particles are too small to be good antennas for light,light, so long-wavelengths (reds) scatter poorlyso long-wavelengths (reds) scatter poorly while shorter-wavelengths (violets) scatter better.while shorter-wavelengths (violets) scatter better.

Rayleigh scattered sunlight is bluish in appearanceRayleigh scattered sunlight is bluish in appearance and the missing blue light reddens the solar disk itself.and the missing blue light reddens the solar disk itself.

Sunlight 10

Introductory Question Introductory Question (revisited)(revisited)

When you look up at the sky during When you look up at the sky during the day, is the light from distant the day, is the light from distant stars reaching your eyes?stars reaching your eyes?

A.A. YesYes

B.B. NoNo

Sunlight 11

Question 3Question 3

How does a rainbow break sunlight How does a rainbow break sunlight into colors?into colors?

Sunlight 12

Light and RefractionLight and Refraction

Polarization of matter delays light’s passage,Polarization of matter delays light’s passage, so light slows as it passes through matter.so light slows as it passes through matter.

When any wave changes speed, it may When any wave changes speed, it may refractrefract It bends toward or away from the interfaceIt bends toward or away from the interface On slowing, it bends more perpendicular to On slowing, it bends more perpendicular to

interfaceinterface On speeding up, it bends less perpendicularOn speeding up, it bends less perpendicular

Each material has an “index of refraction,”Each material has an “index of refraction,” the factor by which light’s speed is reduced.the factor by which light’s speed is reduced.

Sunlight 13

Light and ReflectionLight and Reflection

Light polarizes different materials Light polarizes different materials differently,differently, which affects both how fast light travels in which affects both how fast light travels in

them andthem and the relationship between electric & magnetic the relationship between electric & magnetic

fields.fields. Changes in how light travels causes Changes in how light travels causes

reflectionsreflections As sunlight slows, some of it reflectsAs sunlight slows, some of it reflects As sunlight speeds up, some of it reflectsAs sunlight speeds up, some of it reflects

Sunlight 14

Light and DispersionLight and Dispersion

Light’s speed in a material depends Light’s speed in a material depends on coloron color Violet light usually moves slower than Violet light usually moves slower than

redred Refraction (bending) depends speed Refraction (bending) depends speed

changechange Violet light usually bends more than redViolet light usually bends more than red

Sunlight 15

RainbowsRainbows

Occur when sunlight encounters Occur when sunlight encounters water dropletswater droplets and undergoes refraction, reflection, and undergoes refraction, reflection,

and dispersion.and dispersion.

Sunlight 16

Question 4Question 4

Why are soap bubbles and oil films Why are soap bubbles and oil films so colorful?so colorful?

Sunlight 17

Light and InterferenceLight and Interference

Overlapping waves superpose and may Overlapping waves superpose and may interfereinterfere

Light following different paths can interfereLight following different paths can interfere constructively if fields are in same directionconstructively if fields are in same direction or destructively if fields are in opposite direction.or destructively if fields are in opposite direction.

The two reflections fromThe two reflections froma soap or oil film interferea soap or oil film interfere

Different colors mayDifferent colors mayinterfere differentlyinterfere differently

Sunlight 18

Question 5Question 5

Why do polarizing sunglasses reduce Why do polarizing sunglasses reduce glare?glare?

Sunlight 19

Reflection of Polarized Reflection of Polarized LightLight

Angled reflections depend on polarizationAngled reflections depend on polarization When light’s electric field is parallel to surfaceWhen light’s electric field is parallel to surface

there is a large fluctuating surface polarizationthere is a large fluctuating surface polarization and thus a strong reflection.and thus a strong reflection.

When electric field is perpendicular to surfaceWhen electric field is perpendicular to surface there is a small fluctuating surface polarizationthere is a small fluctuating surface polarization and thus a weak reflection.and thus a weak reflection.

Glare is mostly horizontally polarized lightGlare is mostly horizontally polarized light

Sunlight 20

Polarization and Polarization and SunlightSunlight

Polarizing sunglassesPolarizing sunglasses block horizontally polarized lightblock horizontally polarized light and thus block glare from horizontal and thus block glare from horizontal

surfaces.surfaces. Rayleigh scattering has polarizing Rayleigh scattering has polarizing

effects,effects, so much of the blue sky is polarized so much of the blue sky is polarized

light, too.light, too.

Sunlight 21

Summary about SunlightSummary about Sunlight

Sunlight is thermal light at about Sunlight is thermal light at about 5800 °C 5800 °C

It undergoes Rayleigh scattering in It undergoes Rayleigh scattering in the airthe air

It bends and reflects from raindropsIt bends and reflects from raindrops It interferes colorfully in soap and oil It interferes colorfully in soap and oil

filmsfilms It reflects in a polarizing fashion from It reflects in a polarizing fashion from

surfacessurfaces