Waves

1. Waves By Neil Bronks

2. Some definitions… Crest Trough 1) Amplitude – this is height of the wave. 2)Wavelength ()– this is the distance between two corresponding points on the wave and is measured in metres: 3) Frequency – this is how many waves pass by every second and is measured in Hertz (Hz)

3. Some definitions… Transverse waves are when the displacement is at right angles to the direction of the wave… e.g.Light Longitudinal waves are when the displacement is parallel to the direction of the wave… e.g.Sound

4. The Wave Equation V  f The wave equation relates the speed of the wave to its frequency and wavelength: Wave speed (v) = frequency (f) x wavelength () in m/s in Hz in m

5. Diffraction Diffraction is when waves spread out from the edge of a gap. More diffraction if the size of the gap is similar to the wavelength More diffraction if wavelength is increased (or frequency decreased) Sound bends better around corners

6. Uses of Total Internal Reflection Optical fibres: An optical fibre is a long, thin, transparent rod made of glass or plastic. Light is internally reflected from one end to the other, making it possible to send large chunks of information Optical fibres can be used for communications by sending e-m signals through the cable. The main advantage of this is a reduced signal loss. Also no magnetic interference. It is important to coat the strand in a material of low n. The light can not leak into the next strand.

7. Detection • Waves invisible to the eye have to be detected using special apparatus • IR (Infra-Red) is a heat wave so a blackened thermometer bulb

8. UV Light • Ever walked into a nightclub • White cloth washed in optical brighteners glows in UV light

9. Coherent Waves • Same Frequency • In Phase Or Constant phase difference Phase difference in measured in degrees of a circle

10. Interference is where 2 coherent waves meet. The resultant is the algebraic sum of the 2 waves at any point. = + Constructive Interference

11. To Remember this we simplify it a little

12. Equation • sin  = /d  • d sin  =  • When more than one wavelength difference • d sin  = n d  

13. Metre stick Laser x θ D Diffraction grating MEASUREMENT OF THE WAVELENGTH OF MONOCHROMATIC LIGHT n = 2 n = 1 n = 0 n = 1 Tan θ = x/D n = 2

14. Polarization of Light Normally all e-m waves (Transverse) oscillate in all perpendicular planes at once. Polarization leaves only one plane of oscillation Sound is a longitudinal wave and so can not be Polarised

15. Standing Waves A n  When two coherent waves of the same amplitude traveling in opposite directions meet the waves combine to form a stationary wave We draw this as the two extremes

16. Real Standing Waves /2 /2 /4 Strings Closed Tubes Open Tubes

17. MEASUREMENT OF THE SPEED OF SOUND IN AIR Tuning fork λ=4(l1+0.3d) d Graduated cylinder l1 Tube Water

18. Harmonics Whole number multiples of the fundamental frequency that happen at the same time as the fundamental.

19. Formula for stretched string L=length T=tension =mass/unit length

20. INVESTIGATION OF THE VARIATION OF FUNDAMENTAL FREQUENCY OF A STRETCHED STRING WITH LENGTH Tuning Fork Paper rider l Bridge Sonometer

21. Musical Notes Music waves have a regular shape where noise is irregular Three Qualities – called the characteristics Pitch - This is frequency of the wave. Loudness - this is the amplitude of the wave. Timbre or Quality - The wave shape that is mainly due its overtones.

22. Resonance String • Transfer of energy between two objects with the same, or very similar, natural frequency. Barton’s Pendulum

23. Doppler Effect The apparent change in frequency due to the motion of the observer or the source • Hence the change in pitch as a car passes • Used by the Gardai in to detect speeding cars

24. Summary - Sound as a Wave Interference proves sound is a wave. If we twist a tuning fork near our ear it goes loud and soft. The two prongs of the fork are interfering with each other.

25. Threshold of Hearing • The absolute threshold of hearing (ATH) is the minimum sound level of a pure tone that an average ear with normal hearing can hear in a noiseless environment at 1kHz.

26. Limits of Audibility • The top and bottom values of the range are known as the limits of audibility. • For the human ear, the lower limit is approximately 20 Hz and the upper limit is 20,000 Hz. In other words, our ears are supposed to be able to hear sound with frequencies that are greater than 20 Hz and less than 20,000 Hz.

27. X-Rays High Tension Voltage • Electrons jump from the surface of a hot metal – • Thermionic Emission Accelerated by high voltage they smash into tungsten The electrons excite orbiting electrons to high energy orbits-see next few slides for details These fall back emitting high frequency waves. Most of the electron energy is lost as heat.-about90% X-rays very penetrating, fog film, not effected by fields.

28. The Problem e e e e e • If you shine light on the surface of metals electrons jump off Polished Sodium Metal • Electrons emitted • This is The PHOTOELECTRIC EFFECT

29. A charged Zinc plate is attached to an Electroscope When a U.V. lamp is shone on the plate the leaf collapses as all the electrons leave the surface of the zinc

30. Einstein’s Law So we define the Photoelectric effect as:- Electrons being ejected from the surface of a metal by incident light of a suitable frequency. Uncle Albert used Plank’s theory that as energy came in packets all energy 1 photon to 1 electron A small packet would not give the electron enough energy to leave Low frequency light had too small a parcel of energy to get the electron free. Energy of each photon = h.f

31. Photo-Electric Effect Electron Energy Frequency of light f0=Threshold Frequency Energy of incident photon = h.f = h. f0+ KE of electron Energy left over turned into velocity Work Function, Energy to release Electron