Reflection of Light Waves Refleted wave momentum Incident wave momentum i r What direction will the reflected wave be? No sideways force acts on the light beam Parallel momentum will be conserved Total energy is conserved Perpendicular momentum must be equal and opposite
Double Reflections C First beam comes in 10 degrees away from vertical Mirrors are at right angles B A Which direction is the twice-reflected beam? 10o Therefore 10o from vertical First reflection beam is 100 from vertical 10o 90o Reflected at 80o from horizontal Right triangle angles add up to 180o 90o 80o Beam comes in at 800 from horizontal 10o Therefore parallel to incoming
Refraction and Reflection If light moves from one region to the another with a different index of refraction, it will be bent – refracted Some of the light will normally also be reflected 1 2 1 We can “see” glass mostly because it partly reflects light If you match the index of refraction, all the light is transmitted
Light inside a medium Light interacts with particles inside materials This slows down the light The speed of light inside a medium is less than c n is called the index of refraction It is always bigger than 1 Material n Air 1.000 Water 1.333 Glass 1.6 Diamond 2.419
Snell’s Law Snell’s Law What happens to an EM wave that comes in at an angle? d 2 1 Consider where the wave crests are impacting the glass
Snell’s Law 2 1 index n2 index n1 When light goes from low index medium to high index medium, the angle gets smaller When light goes from high index medium to low index medium, the angle gets bigger
Quiz A) B) C)
Quiz
Quiz 1 2 5 3 4 Which rays are reflected and which rays refracted? Rays 2 and 3 are reflected, 4 and 5 refracted Rays 2 and 4 are reflected, 3 and 5 refracted Rays 2 and 5 are reflected, 3 and 4 refracted Rays 3 and 4 are reflected, 2 and 5 refracted
Multiple Refractions Material n Air 1.000 Water 1.333 F Glass 1.6 Diamond 2.419 F g g d d w w 370 A light beam starting in air enters a series of parallel sheets, passing through water, diamond, and then glass. If the initial angle is 37o, what is the final angle F ? 37o B) Less than 37o C) More than 37o