Nature of Light Chapter 13

What is light? Electromagnetic radiation (needs no medium) Visible light is the electromagnetic radiation our eyes can detect 700nm to 400 nm wavelengths ROYGBIV from 700nm-400nm

Wave particle duality Light acts like a wave (interferes) Light also acts like a particle – distinct packet of energy - a photon This is a part of physics still being researched

Ray model of light Ray model – light travels in straight paths Very helpful in examining reflection and refraction

Speed of Light Until 17th century – all thought light arrived instantaneously From 1668-1674 Ole Roemer measured Io’s time behind Jupiter – found it took 22 minutes for light to cross the diameter of earth’s orbit This value was a little low

Speed of Light II Albert Michelson used earth-based techniques, the distance between 2 mountains 35 km apart In 1926 He got 2.997996 X 108 – very close to the value we use today - he won the Nobel prize for this work

Speed of light today The speed of light (in a vacuum) is an important universal constant Its symbol is c and its value is 299,792,458 m/s For our calculations use c = 3.00 X 108 m/s

Calculations Still a wave: v = λf Since light has a constant velocity (in vacuum) c = λf For light – higher frequency = higher energy EM Spectrum

Materials and light Transparent – lets light through without distortion (windows, clear glass) Translucent – lets light through but can’t clearly see objects (stained glass, frosted lightbulbs) Opaque – does not let light through (wood, brick, steel)

Reflection Law of reflection – a ray of light will reflect at an angle equal to the angle of incidence with respect to the normal Θinitial = Θreflected

Mirror Diagrams Use Ray diagrams to examine Mirror problems Flat mirrors Telescopes

Regular vs. Diffuse reflections

Refraction Light at transmission boundary Light is bent, depending on how optically dense the new material is Angle of refraction is angle with normal (perpendicular) in new material

Refraction II From less optically dense to more dense -> angle of refraction is smaller than angle of incidence From more dense to less dense -> angle of refraction is larger than angle of incidence Car on mud and road

Snell’s Law Angles are related – the ratio of sin Θi to sin Θr is related to the density of the 2 media (sin Θi)/(sin Θr)= nr/ni or ni sin Θi = nr sin Θr Where n is the index of refraction for the medium, n = (sin Θi)/(sin Θr) where light is passing from a vacuum into the medium

Snell’s Law example Light enters water (n = 1.33) from air (n = 1.00) at an angle of 35°. At what angle is the light refracted? ni sin Θi = nr sin Θr (1) sin(35) = (1.33) sin(x) Sin-1[(1)sin(35)/(1.33)] = x = 25.5°

Snell’s Law example 2 Light enters crown glass (n = 1.52) from water (n = 1.33) at an angle of 55°. At what angle is the light refracted? ni sin Θi = nr sin Θr (1.33) sin(55) = (1.52) sin(x) Sin-1[(1.33)sin(55)/(1.52)] = x = 45.8°

Uses of Refraction Lenses – focus or spread light Convergent Divergent Eyeglasses – near or far sighted Hologram

Eyeglasses and how they work

How to use a lens focus effectively 1/di + 1/do = 1/F

Speed of light in substances Revisit pavement and mud Vehicle turns because it moves slower in mud, faster on pavement Remember from Ch 12 that speed changes at boundaries and depends on the material Light is slower in materials than in vacuum

Speed of light in substances II How much slower is determined same way as how much the light bends, they are related Stickier mud would turn you sharper, slow you down more In light we use the index of refraction (n) vsubstance = c/nsubstance

Light speed problem Light enters water (n = 1.33). What is the speed of the light in the water? vsubstance = c/nsubstance vsubstance = [(3.00*108 m/s)/1.33] vsubstance = 2.26*108 m/s

Color from white light Prisms/rainbows – color from white light White light is made up of colored light – Light Box Light mixing website Red + Green + Blue = White These are the primary colors of addition

Light by addition Secondary colors – from pairs of primary colors Yellow, Cyan, Magenta are the secondary colors of addition Each 2nd color is a complementary color to the primary color that was removed to make it Ex: Blue + Red = Magenta, so Magenta is complement to Green

Colors by subtraction We see objects as colors because that is the wavelength (or combination of wavelengths) that it reflects, absorbing the rest Black objects absorb all colors, white reflects all colors Dyes are molecules that absorb certain wavelengths and reflect the rest Pigments are whole materials that do this

Colors by subtraction II A primary pigment is one that absorbs 1 primary color (thus it is the mix of the other 2) They are the same as the secondary (complementary) colors – Yellow, Cyan, Magenta Secondary pigments absorb 2 primary colors, therefore they are the primary colors (one is left)

Color summary Addition vs. Subtraction - Flag Primary colors = secondary pigments = Red, Blue, Green Secondary colors = complementary colors = primary pigments = Yellow, Cyan, Magenta

Polarized sunglasses Polarized sunglasses do what? How do they reduce glare? They use the fact that light can be polarized 3-D movies

Eyes Blind spot in the eye Rods – Dim light receptors Cones – Color receptors 3 types, if one is not working - colorblindness

Eyes

Why… Is the sky blue? Are clouds white? Molecules in the air scatter light The tinier the particle, the higher the frequency of light that will scatter Our eyes are not sensitive to violet Are clouds white? Droplets that compose a cloud come in a variety of sizes This causes a variety of colors to be scattered

Why… Are sunsets Red? Low frequencies of light are scattered the least by nitrogen and oxygen molecules Red, Orange, and Yellow light are transmitted through atmosphere more readily than violet and blue Red light passes through the most atmosphere without interacting with matter