Physics – Light - Quick Guides

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Study GuidePhysics–Light1. DiffractionThomas Young’s double-slit experiment showed thatlight behaves like a wave. When light wavesoverlap, they caninterferewith each other—either reinforcing or canceling out.One important wave behavior isdiffraction. Diffraction happens when a wavespreads outas itpasses through an opening or bends around an obstacle.Why diffraction is easy to notice with sound but not lightDiffraction depends strongly onwavelength.•Sound waveshave long wavelengths, so they diffract easily.This is why you can hear sound through a doorway or even around corners.•Light waveshave extremely short wavelengths.Because of this, light diffraction is much harder to observe unless theopening is very small.1.Single-Slit DiffractionWhen light passes through asingle narrow slit, it does not form just a simple bright spot. Instead, itcreates a pattern ofbright and dark fringeson a screen.This pattern occurs because ofboth diffraction and interference.Imagine the slit is wide enough to allowmany waveletsto pass through. Each point across the slitacts as a source of waves, and these waves interfere with one another.

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Study Guide1.Analysis of Dark Fringes (Minima)In Figure 1, light rays from different points across the slit travel to a pointPon a distant screen.•Consider rays from pointsAandB.•The path length fromA to Pishalf a wavelength longerthan fromB to P.•Because of this half-wavelength difference, the two wavesdestructively interfereand canceleach other out.This same cancellation happens for all wavelets across the slit:•Every wave from one part of the slit has a partner wave from another part that cancels it.•Since the waves cancel in pairs,point P becomes a dark fringe (minimum).Because pointPis far from the slit, triangleACDis nearly a right triangle. Using geometry, we obtainthe condition for the first dark fringe:where:•(λ) is the wavelength of light•(w) is thewidth of the slit

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Study GuideWhenever thepath difference equals a whole number of wavelengths, complete cancellationoccurs and adark fringeappears.2.Partial Reinforcement and Bright FringesIn Figure 2, the slit is divided intothree equal regions.•Waves fromtwo regions cancel each otherthrough destructive interference.•Waves from theremaining region reinforce each other.Because only part of the slit contributes constructively, the result ispartial reinforcement, notmaximum brightness.This produces asecondary bright fringe.For this case, the angle of the fringe is given by:These secondary bright fringes aremuch weakerthan the central bright fringe.

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Study Guide3.IntensityPattern of Single-Slit DiffractionFigure 3 shows howlight intensity changes with angle.Key observations:•Thecentral maximumis thebrightest and widestregion.•This happens becauseall wavelets interfere constructivelyat the center.•On either side of the center, the intensity decreases.•Alternatingdark and weak bright fringesappear symmetrically.2.Diffraction in the Double-Slit ExperimentWhen usingtwo slits, it is impossible to observe only the double-slit interferencepattern.Why?•A double slit is actuallytwo single slits.•Each slit produces itsown diffraction pattern.

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Study GuideAs a result:•The observed pattern is acombination of double-slit interference and single-slitdiffraction.•The diffraction pattern acts like anenvelopethat controls the brightness of the interferencefringes.2. Characteristics of LightScientists have studied light for centuries, and our understanding of it has changed over time.1.Early Ideas About Light•Isaac Newtonbelieved that light was made of tiny particles.Hisparticle theoryhelped explain how light reflects from mirrors and refracts when it passesfrom air into water.•At the same time,Christiaan Huygensproposed that light behaves like awave.His wave theory also successfully explained reflection and refraction.Later, in the late 1800s:•James Clerk Maxwellpredicted the existence ofelectromagnetic waves.•Gustav Ludwig Hertzexperimentally confirmed these waves and showed that they travel atthespeed of light.Because of this work, light is now understood as:•awave•aparticle•a form ofelectromagnetic radiation

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Study Guide2.The Dual Nature of LightToday, scientists accept that light has adual nature.•In some experiments, light behaves like awave.•In other experiments, it behaves like aparticle.Because of this, asking whether light isonlya wave oronlya particle is not useful.Instead, it is better to think of waves and particles asmodelsthat help explain different behaviors oflight.3.Electromagnetic SpectrumMaxwell’s equations unified electricity, magnetism, and optics into one theory.Light is just asmall partof a much larger range of electromagnetic waves called theelectromagneticspectrum.Important points:•Visible light is the only part of the spectrum our eyes can detect.•Wavelengths are commonly measured in:

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Study Guide○nanometers (nm), where (1\text{ nm} = 10^{-9}\text{ m})○angstroms (Å), where (1\text{ Å} = 10^{-10}\text{ m})•The visible spectrum ranges:○fromviolet(shortest wavelength)○tored(longest wavelength)4.Speed of LightLight travels extremely fast—about:[3.15×108m/s]Because of this enormous speed, it was very difficult to measureaccurately in the past.5.Early Measurement by RoemerIn the late 1600s,Claus Roemerstudied the motion of Jupiter’s moons.•He noticed that the observed periods of the moons changed depending on Earth’s position.•He correctly concluded that light has afinite speed.•His calculated value was inaccurate, but his reasoning was correct.6.Fizeau’s ExperimentIn 1849,Armand Fizeaubecame the first person to measure the speed of light on Earth.

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Study GuideHow the experiment worked:•Light passed through a gap in arotating toothed wheel.•The light traveled to a mirror at distanced, reflected back, and passed through the wheelagain.•The wheel’s speed was adjusted so the returning light passed through a neighboring gap.If:•the wheel rotates with angular speed (\omega )•the angular separation between gaps is (\theta )Then the time taken for light to travel a distance (2d) is:The speed of light is then calculated as:Modern laser techniques now measure the speed of light with accuracy toat least nine decimalplaces.

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Study Guide7.Polarization of LightLight can bepolarizedbecause it is atransverse wave.•In transverse waves, the oscillations occurperpendicularto the direction of motion.•Longitudinal waves, such as sound, cannot be polarized.What Is Polarized Light?•Unpolarized lightvibrates in many different directions.•Polarized lightvibrates inonly one plane, perpendicular to the direction of travel.Unpolarized light from a bulb spreads out with vibrations in all directions.8.Mechanical Analogy of PolarizationPolarization can be explained using a mechanical example.

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Study Guide•A wave travels through two slotted frames (polarizers).•If the slots are aligned, the wave passes through.•If the second frame is turnedperpendicular, the wave is blocked and absorbed.9.Polarization by Selective AbsorptionSome materials polarize light byabsorbing certain vibration directions.•Tourmalineis a natural crystal that allows light to pass in only one polarization direction.•Such materials are calleddichroic substances.Polaroid Filters•Polaroidis an artificial dichroic material.•It is made of long-chain hydrocarbons aligned in one direction.•These molecules absorb light whose electric field isparallelto the chains.•Light with an electric fieldperpendicularto the chains is transmitted.

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