Waves

Waves

1. UNITS NEEDED TO KNOW

   • Degree (°)
   • Hertz (hz)
   • Metre (m)
   • Metre/second (m/s)
   • Second (s)

2. Transverse/longitudinal waves

   • Transverse
     • Vibrations go up and down line of travel
       • Eg. is light and electromagnetic waves
   • Longitudinal
     • Vibrations in same direction as travel
       • Compressed where vibrations are close
         • Eg. is sound waves
           • Sound can be refracted, reflected and diffracted like light (eg. echo)
             • For humans the frequency for hearing is 20 Hz - 20,000 Hz
               • More vibrations = higher frequency = higher pitch
                 • Bigger vibration = higher amplitude = louder
             • Microphone detects sound waves
               • Feeds info to oscilloscope to display wave
                 • Experiment
                   • Have an noise made into a microphone attached to an oscilloscope, for example have someone try to sing a note. See how many oscillations there are per second, this will be your frequency. Try changing the pitch of the note and see it the number of oscillations per second changes. An oscillation is the completion of one wave i.e from one peak and the next.
           • Speed of sound ex.
             • Measure the distance between two places, have a sound made in one place, as soon as you see the sound has been made start a stop watch, as soon as you hear the sound made stop the stopwatch.

3. Wave terms

   • (Time) period: time taken for source to produce 1 wave
   • Amplitude: distance between furthest point up and line of direction
   • Frequency: number of waves per second (in Hz)
   • Wavelength: tough to trough, or crest to crest

4. Waves transfer energy/info without matter

   • A wave is a transfers energy through a space or object, but it does not move the particles in it. If you stand on one side of a door and say 'Hi' a person on the other side will be able to hear you saying 'Hi'. This is because the vibrations that you made have travelled through the door to the other side, the energy moving from one particle of the door to another: but the door its self has not move, none of its particles have changed position.

5. EQUATIONS NEEDED TO KNOW

   • Wave speed = frequency x wavelength (v = f × λ)
   • Frequency = 1/time period (F=1/T)
   • Refractive index = sin(i)/sin(r) |(n=sin(i)/sin(r))
   • Sin(crit.angle) = 1/refractive index (sin(c) = 1/n)

6. Diffraction: when light meets a barrier it will carry on through the gap and spread to area beyond

   • Refraction: light waves change speed when through different densities
     • Causes to change direction
       • Original density = original direction
     • Refraction experiment
       • Place a block of glass on a piece of paper, drawing an outline. At one point, draw the normal line. Draw a line at 30 degrees to the normal line, shine a ray of light down this line. Draw a line where the light comes out the other side. Connect the two lines, drawing the refracted ray. Measure the angle of the emergent ray. Repeat for different shaped glass.
     • Refractive index experiment
       • Shine a ray of light through a glass block, measure the angle of incidence and the angle of refraction. Do sin(i) divided by sin(r) and you will have the refractive index of glass.
     • Total internal reflection
       • Reflecting light past critical angle causes it to be reflected back to the medium
         • Hence it can travel through medium to send info (optic fibres)
       • Critical angle (c): where past a certain angle the light will be refracted back to medium
   • Reflection: light hits reflective surface; bounces back at same angle as it was hit
     • The mirror- a straight line with hatchings to show the side with the reflective coating on it. Incident ray- line with arrows pointing towards the mirror. Reflection ray- line with arrows pointing away from the mirror. The image- (where the reflection appears to be behind the mirror) dashed line. The angle of incidence should equal the angle of reflection. A perpendicular line from the object to the mirror, if repeated the other side of the mirror, shows where the image appears to be in the mirror. Draw a line from the image to the eye, where this passes the mirror is where the angle of incidence should also meet the mirror.
     • Angle of incidence (i) = angle of reflection (r)

7. Uses of electromagnetic radiations

   • Radio: broadcasting
   • Microwaves: cooking
     • Harmful: heating of body tissue, damages cells
   • Infra-red: heaters + night vision
     • Harmful: skin burns
   • Visible light: optical fibres
   • Ultraviolet: fluorescent lamps
     • Harmful: blindness and damages surface cells
   • X-ray: x rays
   • Gamma: sterilsing food + medical equipment
     • Harmful: cancer, mutation

8. Analogue/digital signals

   • Analogue: amplitude and/or frequency vary
   • Digital: consists of pulses with states on/off

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