Refraction• Refraction is the bending oflight as it passes from onemedium to another medium.• It bends because the light slowsdown as it enters thenew medium.
Table of Observations# Activity1 Invisible coin2 Broken pencil3 Glass block4 From air to water5 From water to air6 Dispersion of light7 Light ray thru’glassObservations1.1. Coin disappears whenCoin disappears whenviewed from sideviewed from side2.2. Pencil appears to bendPencil appears to bend3.3. Coin disappears whenCoin disappears whenviewed from sideviewed from side4.4. Light bends as it entersLight bends as it enterscontainercontainer5.5. Light bounces off waterLight bounces off water6.6. Light spreads out intoLight spreads out intocolours ROYGBVcolours ROYGBV7.7. Light bends twice as itLight bends twice as itenters and exits the glassenters and exits the glass
Why is the Sky Red at Sunset?• Dust particles in the air scatter lightfrom the sun and the sky.• Blue light is scattered more than red.• When the sun is high in the sky, it looksyellow-white because the light travelsvertically through a fairly clear and thinatmosphere, so there is little refraction.• At dusk, the sun’s rays travel horizontaland through more dust filled air.• More of the blue light is scattered sothat only the reddish rays hit our eyes.
Dispersion• Dispersion is the spreading outof light into its componentcolours ROYGBV.• It does this because thedifferent colour light rays slowdown at different speeds asthey enter a new medium.
Recomposition• Colours of the spectrum may berecombined by means of a lens toform white light in a processcalled recomposition.• A series of mirrors can shine thedifferent coloured lights together.• Newton’s colour disk worksbecause of the persistence ofvision, in which the image ofcolours remain on the retina andcombine to form white.
Rainbows…• A rainbow is the sun’s spectrumproduced by water droplets inthe atmosphere.• Light enters the spherical raindrop where it is refracted,reflected internally and finallydispersed.
…Rainbows• The rainbow arc appears atspecific points in the skybecause only droplets of waterthat are located along that arcwill reflect the spectrum at thecorrect angle in to the eye ofthe observer.• The sunlight is behind theobserver, while person has toangle his/her vision up by 42 o.
9Refraction• White light can be separatedinto its constituent colours bydispersion.• A prism can be used to do this.• It can be recombined with aNewton Disc or a lens.
Refraction• Refraction is the bending of alight ray as it enters a newmedium and changes speed.• The angle is measured from thenormal – an imaginary line at90° to the point of intersection.• Light bends towards the normalif it enters an optically densersubstance and v.v. 10
Refractive Index - n• The refractive index, n, is a measureof how much light bends as it entersthe substance.• n = c/v, where c = 3x108m/s• v = velocity of light in new medium• Air has a refractive index of 1.• Diamond bends light the most (n=2.42).• Table of n values – page 79.11
12Snell’s Law• In air, n = sin Θi / sin Θr• Snell’s Law: n1sinΘ1 = n2sinΘ2• The left side refers to the medium inwhich the light is incident.• The right side refers to the mediumto where the light exits.
Activity• P. 81, Q. 1-3• P. 83, Q. 1-2• P. 86, Q. 3-5, 7
Total Internal Reflection• This occurs when a ray of lightpasses from an optically densematerial (big n) to an opticallyLESS dense material (low n).• If the angle of incidence isgreater than a certain angle –the critical angle - the light willNOT refract out, but will reflectinternally. 14
16Critical Angle• In TIR situations, there comes a point atwhich the angle of refraction increasesuntil it leaves the medium.• In this case the angle of refraction can beconsidered to be equal to 90o.• The angle of incidence at which an angle ofrefraction of 90° first occurs is the CriticalAngle.• Thus for Critical Angle questions, the angleof refraction is assumed to be 90°.
18Snell’s Law & TIR• n1sinΘ1 = n2sinΘ2• Thus the Θ2 is 90o, always.• The Θ1 is called Θc , the critical angle.• As n increases, the Θc decreasescausing more TIR, which is whydiamonds appear so brilliant.• Page 88, Q. 1, 2, 6
19Fibre Optics• This is especially useful in fibreoptics.• Light enters a optic fibre andreflects on the inside of thecable instead of escaping.• So signals can be transmitted atthe speed of light, much fasterthan the speed of electricity.
20Lab Activity• Change: Draw a 10 cm by 10 cm cross(with accurate 90oangles) on a blank sheetof paper.• Make a table as in the book up to 60o.• In pairs, do the lab.• Report due beginning of next class • Determine the Critical Angle of Lucite.
21Chapter Summary• Dispersion is the separation of whitelight into its component colours by aprism.• The colours of the spectrum, whenrecombined, form white light.• White objects tend to reflect light.Black objects tend to absorb light.• Subtractive Theory of Colour: thecolours of the spectrum contained inwhite light are subtracted by filters ordyes until the desired colour remains.
22• Additive Theory of Colour: anyother colour of the spectrum canbe produced by mixing additiveprimary colours (RBG).• IR and UV light are radiationsbeyond the red and violet areas ofthe spectrum, respectively.Neither is visible to the humaneye.• Rainbows are formed by thedispersion and internal reflectionof the white light from the sun bywater droplets in the atmosphere.
23• Light refraction terms: incidentray, angle of incidence, normal,reflected ray, angle of refraction.• When light enters a moreoptically dense medium, its speeddecreases.• n = c/v• Law of Refraction• Snell’s Law n = sin i / sin r• The incident ray and refracted rayare on opposite sides of the normal
24• When a ray of light passes intoan optically denser medium, itbends towards the normal.• When a ray of light passes intoan optically less dense material,it bends away from the normal.• When a ray of light passes intoa medium at an angle of 90۫, itdoes not refract.
25• Total Internal Reflection TIR• The ray of light passes from onemedium into another of loweroptical density.• The angle of incidence is greaterthan the critical angle.• When the angle of incidence isgreater than the critical angle, therefracted angle is 90۫.• n1sin Θ1 = n2 sin Θ2