CHAPTER 12- WAVES

WHAT IS A WAVE? Mechanical waves vs non-mechanical waves?

LONGITUDINAL VS TRANSVERSE

KEY TERMS DISPLACEMENT AMPLITUDE WAVELENGTH PERIOD FREQUENCY PHASE DIFFERENCE

PHASE AND PHASE DIFFERENCE This is a measure of how “in step” the different waves (or particles) are.

CALCULATING PHASE DIFFERENCE

PHASE DIFFERENCE If the particles are moving together we say they are “in phase” If the particles are “out of phase” by a half cycle they are π radians out of phase. A phase difference of π/2 radians shows they are out of phase by ¼ of a cycle

WAVE PROPERTIES- PART 1 Reflection Refraction Diffraction

REFLECTION The law of reflection from a plane mirror (any flat surface)? What is TIR? When does it occur?

DIFFRACTION When waves pass through apertures or around obstacles they tend to spread out. This spreading out is called DIFFRACTION.

MAXIMUM DIFFRACTION Maximum diffraction will occur when the gap or obstacle size is close to the wavelength of the wave. php

REFRACTION Definition ?? What would happen if… Waves in a ripple tank move from shallow to deeper water? Light travels from air into glass

HOMEWORK! Discuss the implications of reflection and diffraction on satellite dish design. Explain one use of TIR in technology

WAVE EQUATION Derivation of the wave equation

SNELL’S LAW

SNELL’S LAW - DEFINITION It is the ratio of the velocity of the wave in the two media Or the ratio of the sine of the angle of incidence to the sine of refraction REMEMBER – because it is a ratio, this means the refractive index is a DIMENSIONLESS QUANTITY

REFRACTION- KEY POINTS Frequency, wavelength and speed? Which quantities change at the boundary?

TIR – TOTAL INTERNAL REFLECTION Can only take place if… The incidence substance has a LARGER REFRACTIVE INDEX than the other substance The angle of incidence is greater than the CRITICAL ANGLE

TIR – TOTAL INTERNAL REFLECTION

TIR - USES Multipath dispersion – Spectral dispersion -

SUPERPOSITION AND INTERFERENCE Wiki Sim – Huygens’ Principle of SuperpositionHuygens’ Principle of Superposition

HUYGENS PRINCIPLE OF SUPERPOSITION IMPORTANT – The displacement of any point due to the superposition of wave systems is equal to the sum of the displacements of the individual waves at that point. REMEMBER- Crest meets crest – SUPERCREST Trough meets trough – SUPERTROUGH Crest meets trough – cancel each other out – DISPLACEMENT IS ZERO

STATIONARY WAVES Formation of stationary waves Stationary waves are formed when two progressive waves meet each other travelling in opposite directions. Wiki – Waves on a stringWaves on a string Definitions Progressive wave -

STATIONARY WAVES The result of this is a wave where the peaks simply move up and down but DO NOT PROGRESS.

STATIONARY VS PROGRESSIVE WAVES StandingProgressive AmplitudeAll points on the wave have different amplitudes. The maximum being 2A (at the antinodes) and 0 (at the nodes) All points on the wave, have the same amplitude FrequencyOscillate with the same frequency (except at nodes) Oscillate with the same frequency WavelengthTwice the distance from one node to the next node Shortest distance between two points that are in phase Phase differenceAll points between one node and the next node are in phase (=mπ) m is the no. of nodes between the two particles All points along a wavelength have different phases (=2πd/λ) EnergyEnergy is not transmitted by the wave but it does have an energy associated with it. Energy is transmitted by the wave.

FORMATION OF STANDING WAVES

FORMATION OF WAVES IN CLOSED PIPES

FORMATION OF WAVES IN OPEN PIPES

YOUNG’S DOUBLE SLIT EXPERIMENT Key words – For his experiment Young used a COHERENT and MONOCHROMATIC light source Monochromatic Coherent

DOUBLE SLIT INTERFERENCE Young’s Double Slit Experiment

DOUBLE SLIT EQUATION w = fringe spacing in metres (m) λ = wavelength of the light in metres (m) D = distance between the double slits and the screen in metres (m) s = slit separation in metres (m)

DOUBLE SLIT EXPERIMENT What would happen if…. I used a non-coherent light source? I swapped a red laser for a blue laser? I made the distance from the slits to the screen shorter? I increased the slit separation? A white light source?

DIFFRACTION FROM A SINGLE SLIT More in depth look at diffraction Huygens’ Principle php

DIFFRACTION FROM A SINGLE SLIT What is the diffraction pattern shown on a screen when a wave is diffracted around a single slit?

SINGLE SLIT KEY POINTS The central fringe is twice as large as the outer fringes The intensity of each fringe decreases with the distance from centre. Each of the outer fringes are the same width The outer fringes are much less intense than the central fringe.

EXPLAINING THE DIFFRACTION PATTERN

SINGLE SLIT EQUATION What is the equation if… Increasing the wavelength gives wider fringes Making the slit narrower gives wider fringes Increasing the slit to screen distance gives wider fringes λ = the wavelength of the light D = the slit to screen distance a = the width of the slit

COMPARING THE SINGLE AND DOUBLE SLIT EXPERIMENTS

DIFFRACTION GRATING

DIFFRACTION GRATING EQUATION Where… d= θ= n= λ=

MAXIMUM NUMBER OF ORDERS What is the maximum number of n? Occurs when sin θ = sin 90 What does the equation become? KEY POINT!! -