Sunlight 1 Sunlight Chapter C

Sunlight 2 Introductory Question When you look up at the sky during the day, is the light from distant stars reaching your eyes? When you look up at the sky during the day, is the light from distant stars reaching your eyes? A. Yes B. No

Sunlight 3 Observations about Sunlight Appears whiter than most light Appears whiter than most light Makes the sky appear blue Makes the sky appear blue Becomes redder at sunrise and sunset Becomes redder at sunrise and sunset Reflects from many surfaces, even nonmetals (Reflection) Reflects from many surfaces, even nonmetals (Reflection) Bends and separates into colors in materials (Diffraction) Bends and separates into colors in materials (Diffraction)

Sunlight 4 5 Questions about Sunlight 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 into colors? How does a rainbow break sunlight into colors? Why are soap bubbles and oil films so colorful? Why are soap bubbles and oil films so colorful? Why do polarizing sunglasses reduce glare? Why do polarizing sunglasses reduce glare?

Sunlight 5 Question 1 Why does sunlight appear white? Why does sunlight appear white?

Sunlight 6 Light Medium-wavelength electromagnetic waves Medium-wavelength electromagnetic waves The range of wavelengths we can see: The range of wavelengths we can see:

Sunlight 7 Spectrum of Sunlight Thermal agitation make charges accelerate Thermal agitation make charges accelerate Highly agitated charges emit light Highly agitated charges emit light The sun is a black-body at 5800 °C The sun is a black-body at 5800 °C We perceive thermal light at 5800 °C as “white” We perceive thermal light at 5800 °C as “white”

Sunlight 8 Question 2 Why does the sky appear blue? Why does the sky appear blue? Why is the sun reddish at sunrise and sunset? Why is the sun reddish at sunrise and sunset?

Sunlight 9 Rayleigh Scattering Rayleigh scattering occurs when light polarizes matter Rayleigh scattering occurs when light polarizes matter Passing sunlight polarizes tiny particles in the air Passing 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 light, Air particles are too small to be good antennas for light, so long-wavelengths (reds) scatter poorly so long-wavelengths (reds) scatter poorly while shorter-wavelengths (violets) scatter better. while shorter-wavelengths (violets) scatter better. Rayleigh scattered sunlight is bluish in appearance Rayleigh 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 (revisited) When you look up at the sky during the day, is the light from distant stars reaching your eyes? When you look up at the sky during the day, is the light from distant stars reaching your eyes? A. Yes B. No

Sunlight 11 Question 3 How does a rainbow break sunlight into colors? How does a rainbow break sunlight into colors?

Sunlight 12 Light 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 refract When any wave changes speed, it may refract It bends toward or away from the interface It bends toward or away from the interface On slowing, it bends more perpendicular to interface On slowing, it bends more perpendicular to interface On speeding up, it bends less perpendicular On 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 Reflection Light polarizes different materials differently, Light polarizes different materials differently, which affects both how fast light travels in them and which affects both how fast light travels in them and the relationship between electric & magnetic fields. the relationship between electric & magnetic fields. Changes in how light travels causes reflections Changes in how light travels causes reflections As sunlight slows, some of it reflects As sunlight slows, some of it reflects As sunlight speeds up, some of it reflects As sunlight speeds up, some of it reflects

Sunlight 14 Light and Dispersion Light’s speed in a material depends on color Light’s speed in a material depends on color Violet light usually moves slower than red Violet light usually moves slower than red Refraction (bending) depends speed change Refraction (bending) depends speed change Violet light usually bends more than red Violet light usually bends more than red

Sunlight 15 Rainbows Occur when sunlight encounters water droplets Occur when sunlight encounters water droplets and undergoes refraction, reflection, and dispersion. and undergoes refraction, reflection, and dispersion.

Sunlight 16 Question 4 Why are soap bubbles and oil films so colorful? Why are soap bubbles and oil films so colorful?

Sunlight 17 Light and Interference Overlapping waves superpose and may interfere Overlapping waves superpose and may interfere Light following different paths can interfere Light following different paths can interfere constructively if fields are in same direction constructively 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 from a soap or oil film interfere The two reflections from a soap or oil film interfere Different colors may interfere differently Different colors may interfere differently

Sunlight 18 Question 5 Why do polarizing sunglasses reduce glare? Why do polarizing sunglasses reduce glare?

Sunlight 19 Reflection of Polarized Light Angled reflections depend on polarization Angled reflections depend on polarization When light’s electric field is parallel to surface When light’s electric field is parallel to surface there is a large fluctuating surface polarization there is a large fluctuating surface polarization and thus a strong reflection. and thus a strong reflection. When electric field is perpendicular to surface When electric field is perpendicular to surface there is a small fluctuating surface polarization there is a small fluctuating surface polarization and thus a weak reflection. and thus a weak reflection. Glare is mostly horizontally polarized light Glare is mostly horizontally polarized light

Sunlight 20 Polarization and Sunlight Polarizing sunglasses Polarizing sunglasses block horizontally polarized light block horizontally polarized light and thus block glare from horizontal surfaces. and thus block glare from horizontal surfaces. Rayleigh scattering has polarizing effects, Rayleigh scattering has polarizing effects, so much of the blue sky is polarized light, too. so much of the blue sky is polarized light, too.

Sunlight 21 Summary about Sunlight Sunlight is thermal light at about 5800 °C Sunlight is thermal light at about 5800 °C It undergoes Rayleigh scattering in the air It undergoes Rayleigh scattering in the air It bends and reflects from raindrops It bends and reflects from raindrops It interferes colorfully in soap and oil films It interferes colorfully in soap and oil films It reflects in a polarizing fashion from surfaces It reflects in a polarizing fashion from surfaces