Sunlight Sunlight Sunlight makes the sky appear blue Sunlight - PDF document

Sunlight 1 Sunlight 2 Observations about Sunlight Observations about Sunlight � Sunlight appears whiter than most light Sunlight appears whiter than most light Sunlight Sunlight � Sunlight makes the sky appear blue Sunlight makes the sky appear blue � Sunlight becomes redder at sunrise and sunset Sunlight becomes redder at sunrise and sunset � It reflects from many surfaces, even nonmetals It reflects from many surfaces, even nonmetals � It bends and separates into colors in materials It bends and separates into colors in materials � Sunlight a key source of renewable energy Sunlight a key source of renewable energy Sunlight 3 Sunlight 4 5 Questions about Sunlight 5 Questions about Sunlight Question 1 Question 1 � Why does sunlight appear white? Why does sunlight appear white? � 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 Sunlight 6 Light Light Spectrum of Sunlight Spectrum of Sunlight � Light is Light is a class of electromagnetic waves a class of electromagnetic waves � Sunlight is thermal radiation Sunlight is thermal radiation— —heat from the sun heat from the sun � � Long Long- -wavelength EM waves: Radio & Microwave wavelength EM waves: Radio & Microwave � Charges in the sun’s hot photosphere jitter thermally Charges in the sun’s hot photosphere jitter thermally � Medium Medium- -wavelength: IR, Visible, UV light wavelength: IR, Visible, UV light � Accelerating charges emits electromagnetic waves Accelerating charges emits electromagnetic waves � Short � Sh rt Short-wavelength: X Sh rt wavelength: X-rays & Gamma l n th X r l n th X rays & Gamma- r & G mm r & G mm -rays rays � Th � The sun emits a black Th The sun emits a black- n mit n mit bl bl k k b d -body spectrum at 5800 K b d body spectrum at 5800 K p p tr m t 5800 K tr m t 5800 K � We perceive thermal light at 5800 K as “white” We perceive thermal light at 5800 K as “white” 1

Sunlight 7 Sunlight 8 Question 2 Question 2 Rayleigh Scattering Rayleigh Scattering � Rayleigh scattering occurs when Rayleigh scattering occurs when � Why does the sky appear blue? Why does the sky appear blue? � passing sunlight electrically polarizes tiny particles in the air. passing sunlight electrically polarizes tiny particles in the air. � Why is the sun reddish at sunrise and sunset? Why is the sun reddish at sunrise and sunset? � That alternating polarization That alternating polarization acts as a source of light waves acts as a source of light waves, , � so air particles scatter light so air particles scatter light— —they absorb and then reemit it. 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 so long- -wavelengths (reds) scatter poorly wavelengths (reds) scatter poorly � while shorter while shorter- -wavelengths (violets) scatter better. wavelengths (violets) scatter better. � Rayleigh scattered sunlight is bluish in appearance Rayleigh scattered sunlight is bluish in appearance � The missing blue light reddens the solar disk itself The missing blue light reddens the solar disk itself � particularly at sunrise and sunset particularly at sunrise and sunset Sunlight 9 Sunlight 10 Question 3 Question 3 Light and Refraction Light and Refraction � How does a rainbow break sunlight into colors? How does a rainbow break sunlight into colors? � Sunlight slows while it passes through matter Sunlight slows while it passes through matter � Light waves electrically polarize matter Light waves electrically polarize matter � That polarization delays the light wave’s passage That polarization delays the light wave’s passage � Each material has an “index of refraction” � E E Each material has an “index of refraction”— h m t ri l h h m t ri l h n “ind n “ind f r fr f r fr ti n” ti n” —the factor th f the factor th f t r t r by which it reduces light’s speed. by which it reduces light’s speed. � When light changes speed at an interface, When light changes speed at an interface, � it bends toward the perpendicular if it slows down it bends toward the perpendicular if it slows down � it bends away from the perpendicular if it speeds up it bends away from the perpendicular if it speeds up Sunlight 11 Sunlight 12 Light and Reflection Light and Reflection Light and Dispersion Light and Dispersion � Light polarizes different materials differently, Light polarizes different materials differently, � The different colors of light in sunlight The different colors of light in sunlight � which affects both how fast light travels in them and which affects both how fast light travels in them and � have different frequencies have different frequencies � relationship between its electric & magnetic fields. relationship between its electric & magnetic fields. � and polarize a material slightly differently, and polarize a material slightly differently, � Changes in how light travels causes reflections Changes in how light travels causes reflections Ch Ch i h i h li h li h l l fl fl i i � so they travel at slightly different speeds. so they travel at slightly different speeds. th th tr tr l t li htl diff r nt p l t li htl diff r nt p d d � � Violet light usually travels slower than red Violet light usually travels slower than red � Sunlight partially reflects from most surfaces Sunlight partially reflects from most surfaces � Refraction (bending) depends speed change Refraction (bending) depends speed change � Sunlight reflects almost completely from metals Sunlight reflects almost completely from metals � so violet light usually bends more than red so violet light usually bends more than red 2

Sunlight 13 Sunlight 14 Rainbows Rainbows Question 4 Question 4 � Occur when sunlight encounters water droplets Occur when sunlight encounters water droplets � Why are soap bubbles and oil films so colorful? Why are soap bubbles and oil films so colorful? � � and undergoes refraction, reflection, and dispersion. and undergoes refraction, reflection, and dispersion. Sunlight 15 Sunlight 16 Light and Interference Light and Interference Question 5 Question 5 � Overlapping waves superpose and may interfere Overlapping waves superpose and may interfere � Why do polarizing sunglasses reduce glare? Why do polarizing sunglasses reduce glare? � Light following different paths can interfere Light following different paths can interfere � constructively if fields point in the same direction constructively if fields point in the same direction � or destructively if fields point in opposite directions. or destructively if fields point in opposite directions. d d i i l if fi ld l if fi ld i i i i i i di di i i � The two reflections from The two reflections from a soap or oil film interfere a soap or oil film interfere � Different colors often Different colors often interfere differently interfere differently Sunlight 17 Sunlight 18 Reflection of Polarized Light Reflection of Polarized Light Polarization and Sunlight Polarization and Sunlight � Angled reflections depend on polarization Angled reflections depend on polarization � Polarizing sunglasses Polarizing sunglasses � � When light’s electric field is parallel to a surface When light’s electric field is parallel to a surface � block horizontally polarized light block horizontally polarized light � � and thus block glare from horizontal surfaces. and thus block glare from horizontal surfaces. � there is a large fluctuating surface polarization there is a large fluctuating surface polarization � Rayleigh scattering has polarizing effects, R R Rayleigh scattering has polarizing effects, l i h l i h i i h h l i i l i i ff ff � and thus a strong reflection. and thus a strong reflection. d h d h fl fl i i � When electric field is perpendicular to a surface When electric field is perpendicular to a surface � so much of the blue sky is polarized light, too. so much of the blue sky is polarized light, too. � 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 polarized parallel to the surface Glare is mostly polarized parallel to the surface 3

Sunlight 19 Summary about Sunlight Summary about Sunlight � Sunlight is thermal light at about Sunlight is thermal light at about 5800 K 5800 K � � 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 4