Waves, Sound, Light & the EM spectrum

• A wave is a cyclical repetitive motion that transfers energy and information but not matter from one place to another
• Particles in a matter wave vibrate or oscillate but do not move anywhere; they stay in position
• In a transverse wave, vibrations are perpendicular to the direction of travel; a water wave is a good example
• In a longitudinal wave, particles vibrate parallel to the direction of travel; sound is a good example
• In a longitudinal wave, areas where particles get closer together are compressions with slightly higher density
• In a longitudinal wave, areas where particles get further apart are rarefactions with slightly lower density
• Electromagnetic waves are transverse waves and can travel through a vacuum
• The speed of a wave is the distance traveled per unit time
• The frequency of a wave is the number of complete waves passing a point per unit time
• The wavelength of a wave is the distance between two adjacent crests or troughs
• The amplitude of a wave is the maximum displacement from the equilibrium position
• Equation linking wavelength and frequency: v = fλ or wave speed = frequency × wavelength
• Wavefront is a line drawn along the points of a wave at a given point in the cycle, always perpendicular to the direction of wave travel
• When a wave reaches a boundary between two materials, it could be refracted, reflected, absorbed, or transmitted
• Water waves refract on entering shallow regions due to a change in speed
• Water waves reflect on hitting a plane barrier
• Sound waves are produced when things vibrate and are passed on through the medium as longitudinal waves
• Sound waves cause changes in pressure. At any one instant there will be series of compressions (where air molecules are compressed together) and rarefactions (where the air molecules are further away). Theses then travel away from the source
• Sound cannot be passed on in the absence of a medium
• Speed of sound in different media: Air - 340 m/s, Water - 1500 m/s, Solids - 5000 m/s
• Increasing the amplitude of a sound wave makes the sound louder
• Increasing the frequency of a sound wave increases the sound's pitch
• When a sound wave enters the human ear canal, the changing pressure causes the eardrum to vibrate. These vibrations are passed on to tiny bones which amplify the vibrations. The vibrations are passed on to the liquid inside the cochlea which contains a long, narrow membrane which also vibrates. Different parts of the membrane detect different frequencies. The membrane is covered in tiny hairs and the hair cells convert vibrations into electrical signals. The brain interprets these signals from different parts of the membrane as different pitches of sound.
• Humans can hear frequencies from 20Hz to 20kHz
• Ultrasound has a frequency greater than 20kHz, while infrasound has a frequency less than 20Hz
• Sonar uses reflection of ultrasound pulses to find the depth of the sea
• Earthquake or seismic waves are detected with a seismometer
• Primary earthquake waves are longitudinal (P-waves)
• Secondary earthquake waves are transverse (S-waves)
• The fact that s-waves from an earthquake are not detected on the opposite side of the Earth, shows that the outer core of the earth is liquid
• Law of reflection: angle of incidence = angle of reflection
• Smooth surfaces like mirrors reflect light evenly, called specular reflection. The light will form an image
• A rough surface reflects light in many directions, known as diffuse reflection
• A surface looks red if it reflects red light and absorbs all other colors
• A green filter transmits green light
• A wave crossing a boundary at an angle will change direction if it changes speed
• If a wave slows down, it is refracted towards the normal
• On exiting a medium into air, light will bend away from the normal
• The critical angle is the angle of incidence where light just exits the medium and travels parallel to the edge of the medium
• Up until the critical angle, some light reflects and some refracts