WAVES
Waves are rhythmic disturbances that carry energy without carrying matter. For example, water waves created by a boat make objects bob up and down, but the water molecules themselves don't travel outward — only the energy does.
Mechanical waves require matter (a medium) to transfer energy. They cannot travel through empty space. The medium can be a solid, liquid, or gas.
Transverse Waves
Matter in the medium moves perpendicular (at right angles) to the direction the wave travels. The high points are crests; the low points are troughs.
Example: rope wave, water surface
Compressional Waves
Matter in the medium moves in the same direction as the wave. Dense areas are compressions; spread-out areas are rarefactions.
Also called longitudinal waves. Example: sound, spring toy
Transverse Wave — crests, troughs, amplitude, and wavelength labeled
Sound waves are compressional (mechanical) waves. They are produced by vibrating objects. Your vocal cords vibrate to create them; the vibrating drumhead creates compressions and rarefactions in the air. Without a medium, sound cannot travel — this is why there is no sound in outer space.
Electromagnetic waves are transverse waves produced by electrically charged particles vibrating. Unlike mechanical waves, they do NOT need a medium — they can travel through a vacuum (outer space).
Examples of EM Waves
- Radio waves (AM, FM, TV)
- Microwaves
- Infrared (heat)
- Visible light
- Ultraviolet (UV)
- X-rays
- Gamma rays
Useful Applications
- GPS satellites use EM radio waves
- Sunscreen protects against UV
- X-rays used in medical imaging & security
- Infrared = heat you feel from sunlight
- Visible light lets you see color
| Feature | Mechanical Waves | Electromagnetic Waves |
|---|---|---|
| Needs a medium? | ✅ Yes | ❌ No |
| Types | Transverse & Compressional | Transverse only |
| Travel through space? | ❌ No | ✅ Yes |
| Examples | Sound, water waves, seismic waves | Light, radio, X-rays, UV |
| Speed in air | ~340 m/s (sound) | 300,000,000 m/s (light) |
The amplitude of a transverse wave is the distance from the rest position to a crest or trough. For a compressional wave, amplitude is greater when particles are squeezed more tightly in compressions.
Amplitude = Energy: The greater the amplitude, the more energy the wave carries. Loud sounds have large amplitudes; bright light has large amplitudes. A hurricane's massive water waves have huge amplitudes and cause great destruction.
Wavelength is the distance between two adjacent crests (or two adjacent troughs) of a transverse wave. For compressional waves, it is measured from the center of one compression to the center of the next compression.
Different colors of light have different wavelengths — red light has a longer wavelength than green light, which has a longer wavelength than blue light.
Frequency is the number of wavelengths that pass a given point in 1 second. The unit of frequency is the hertz (Hz), named after German physicist Heinrich Hertz.
Frequency & Pitch: Higher frequency = higher pitch in sound. Higher frequency = color shifts toward blue/violet in light.
Wave speed depends on the medium through which it travels, not just the type of wave.
Mechanical Waves (e.g., Sound)
- Fastest in solids
- Slower in liquids
- Slowest in gases
- Sound travels ~340 m/s in air
Atoms closer together = faster energy transfer
Electromagnetic Waves (e.g., Light)
- Fastest in gases/vacuum
- Slower in solids
- Light: 300 million m/s in air
- Light is 1.5× faster in air than glass
Opposite behavior from mechanical waves!
Reflection occurs when a wave strikes an object or surface and bounces off. The wave does not pass through or change direction by changing speed — it simply reverses.
Smooth Surface
All reflected light rays go in the same direction → you see a clear, sharp image (mirror, calm pond).
Rough Surface
Reflected light scatters in many directions → no clear image visible (white stone wall, choppy water).
Examples: echoes (sound reflecting off walls), seeing your face in a mirror or still pond.
Refraction is the bending of a wave as it moves from one medium into another at an angle, caused by a change in wave speed.
- A line perpendicular to the surface is called the normal.
- Light passing from air into water slows down and bends toward the normal.
- Light passing from water into air speeds up and bends away from the normal.
- The larger the speed change, the greater the bending.
Real-World Examples
- A straw looks bent in a glass of water
- Fish appear closer to the surface than they are
- Prisms separate white light into a rainbow
- Rainbows form when sunlight refracts through raindrops
Color & Refraction
Different colors (wavelengths) refract at slightly different angles. Violet light bends more; red light bends less. This separates sunlight into the colors of the spectrum.
Diffraction is the bending of waves around a barrier or through an opening. It is a property of waves, not particles.
Sound Diffracts Easily
Sound has wavelengths of a few millimeters to ~10 m — similar to everyday objects. You can hear music around corners because sound bends around walls and doorways.
Light Barely Diffracts
Visible light has very short wavelengths (400–700 billionths of a meter), much smaller than door openings. You cannot see around corners; light travels in straight lines through doorways.
Water waves diffract around islands. Radio waves diffract around buildings. This is why AM radio reaches farther around hills than FM.
Interference occurs when two or more waves overlap and combine to form a new wave. After the waves pass through each other, they continue as if nothing happened.
✅ Constructive Interference
Crests align with crests → LARGER amplitude wave is formed. More energy.
Example: Concert hall acoustics designed to amplify sound
❌ Destructive Interference
Crests align with troughs → SMALLER amplitude (waves cancel). Less energy.
Example: Noise-canceling headphones, airplane cockpit ear protection
| Behavior | What Happens | Cause | Real-World Example |
|---|---|---|---|
| Reflection | Wave bounces off a surface | Wave strikes an object | Echoes, mirrors, radar |
| Refraction | Wave bends, changes direction | Change in wave speed (different medium) | Prisms, mirages, straw in water |
| Diffraction | Wave bends around a barrier | Opening/barrier similar in size to wavelength | Hearing around corners, water around islands |
| Interference | Two waves combine | Waves overlap | Concert acoustics, noise-canceling headphones |
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