Waves, Light and Sound

Types of waves: longitudinal and transverse

Waves transfer energy from one place to another. The main difference between longitudinal and transverse waves is the direction the particles move compared to the wave’s motion.

Transverse Waves

In a transverse wave, the particles move up and down while the wave travels side to side.

Examples

• light waves

• water waves

• waves on a rope

Key Parts

• crest = highest point

• trough = lowest point

Think of shaking a jump rope up and down.

Longitudinal Waves

In a longitudinal wave, the particles move back and forth in the same direction as the wave.

Examples

• sound waves

• slinky spring compressions

Key Parts

• compression = particles close together

• rarefaction = particles spread apart

Think of pushing and pulling a slinky.

Quick Comparison

A quick memory trick:

• Transverse = particles move across

• Longitudinal = particles move along the wave direction

Features of Waves: amplitude, wavelength, frequency

Here are the main features of waves you should know:

1. Amplitude

The amplitude is the height of the wave from the middle (rest position) to the crest or trough.

• Bigger amplitude = more energy

• Measured in meters (m)

2. Wavelength

The wavelength (\lambda) is the distance between two matching points on a wave.

Examples:

• crest to crest

• trough to trough

• Measured in meters (m)

3. Frequency

The frequency is how many waves pass a point in 1 second.

• Unit: hertz (Hz)

• Higher frequency = more waves each second

4. Period

The period is the time for one complete wave.

• Measured in seconds (s)

Relationship:

• period and frequency are opposites

5. Wave Speed

The speed of a wave depends on wavelength and frequency.

Where:

• v = wave speed

• f = frequency

• \lambda = wavelength

6. Crest and Trough (Transverse Waves)

• crest = highest point

• trough = lowest point

7. Compression and Rarefaction (Longitudinal Waves)

• compression = particles close together

• rarefaction = particles spread apart

Quick Summary Table

Interpreting Wave Diagrams

When interpreting wave diagrams, you usually look for the wave’s main features and what they tell you about energy and motion.

How to Read a Wave Diagram

1. Identify the Rest Position

The middle line is the rest position (where the medium would be with no wave).

2. Find the Amplitude

The amplitude is the distance from the rest position to a crest or trough.

• Larger amplitude = more energy

• Measure vertically

3. Find the Wavelength

The wavelength (\lambda) is the distance between matching points.

Examples:

• crest to crest

• trough to trough

Measure horizontally.

4. Identify Crests and Troughs

For transverse waves:

• crest = highest point

• trough = lowest point

5. Determine Frequency

If a graph shows time:

• count how many complete waves pass in 1 second

More waves in less time = higher frequency.

6. Compare Energy

• bigger amplitude → more energy

• higher frequency → more energy (for many waves like light)

Longitudinal Wave Diagrams

Instead of crests and troughs, look for:

• compressions = crowded particles

• rarefactions = spread-out particles

Wavelength is:

• compression to compression

• or rarefaction to rarefaction

Quick Example

If a wave:

• has tall crests → high amplitude

• has closely packed waves → short wavelength/high frequency

• has wide spacing → long wavelength/low frequency

Common Mistakes

• Don’t measure wavelength diagonally

• Don’t confuse amplitude with wavelength

• Amplitude is vertical; wavelength is horizontal

What Causes Sound

Sound is caused by vibrations.

When an object vibrates, it makes the particles around it vibrate too. Those vibrations travel through a medium as a longitudinal wave.

Examples

• a guitar string vibrating

• vocal cords vibrating when you speak

• a drum skin vibrating after being hit

The vibrating particles create:

• compressions → particles pushed together

• rarefactions → particles spread apart

These vibrations travel through:

• air

• water

• solids

Sound cannot travel through empty space because there are no particles to vibrate.

Sound Wave Formula

The speed of sound follows the wave equation:

Where:

• v = wave speed

• f = frequency

• \lambda = wavelength

Pitch and Volume

• Pitch depends on frequency

  • high frequency = high pitch

  • low frequency = low pitch

• Volume depends on amplitude

  • bigger amplitude = louder sound

Quick Summary

Describing Sound Waves: Loudness and Pitch

Sound waves can be described by their loudness and pitch.

Loudness

Loudness depends on the amplitude of the sound wave.

• Large amplitude = louder sound

• Small amplitude = quieter sound

Examples:

• shouting → large amplitude

• whispering → small amplitude

Loudness is measured in decibels (dB).

Pitch

Pitch depends on the frequency of the wave.

• High frequency = high pitch

• Low frequency = low pitch

Examples:

• a whistle → high pitch

• a bass drum → low pitch

Frequency is measured in hertz (Hz).

Comparing Sound Waves

Quick Memory Trick

• Amplitude → loudness

• Frequency → pitch

The Ear

The ear has three main sections: the outer ear, middle ear, and inner ear. Each part helps sound travel to the brain.

Main Parts of the Ear

1. Outer Ear

Collects sound waves.

Parts:

• Pinna (the visible ear)

  • collects sound waves

• Ear canal

  • carries sound to the eardrum

2. Middle Ear

Transfers vibrations.

Parts:

• Eardrum

  • vibrates when sound waves hit it

• Ossicles (tiny bones)

  • hammer (malleus)

  • anvil (incus)

  • stirrup (stapes)

These bones amplify vibrations.

3. Inner Ear

Changes vibrations into nerve signals.

Parts:

• Cochlea

  • filled with fluid and tiny hair cells

  • converts vibrations into electrical signals

• Auditory nerve

  • carries signals to the brain

• Semicircular canals

  • help with balance

How Hearing Works

1. Sound waves enter the pinna

2. Waves travel through the ear canal

3. The eardrum vibrates

4. Ossicles pass on and amplify vibrations

5. Vibrations enter the cochlea

6. Hair cells create nerve signals

7. The auditory nerve sends signals to the brain

Simple Summary Table

How We Hear

Hearing happens when sound vibrations travel through the ear and are turned into signals for the brain.

Steps of Hearing

1. Sound Waves Enter the Ear

The pinna collects sound waves and funnels them into the ear canal.

2. The Eardrum Vibrates

Sound waves hit the eardrum, causing it to vibrate.

3. Tiny Bones Amplify Vibrations

The three small bones in the middle ear (ossicles):

• hammer

• anvil

• stirrup

make the vibrations stronger.

4. Vibrations Reach the Cochlea

The vibrations enter the cochlea, a fluid-filled spiral structure in the inner ear.

Inside the cochlea are tiny hair cells that move with the vibrations.

5. Hair Cells Create Electrical Signals

The moving hair cells convert vibrations into electrical nerve impulses.

6. Signals Travel to the Brain

The auditory nerve carries the signals to the brain, where they are interpreted as sound.

Simple Flow Chart

Sound waves → pinna → ear canal → eardrum → ossicles → cochlea → auditory nerve → brain

Important Idea

Sound is a longitudinal wave, meaning particles vibrate back and forth in the same direction the wave travels.

Hearing Range

The hearing range is the range of sound frequencies humans can hear.

Human Hearing Range

Most humans can hear sounds between:

20\text{ Hz} \le f \le 20\,000\text{ Hz}

• 20 Hz = lowest pitch humans can hear

• 20,000 Hz (20 kHz) = highest pitch humans can hear

Types of Sound by Frequency

Examples

• thunder → low frequency

• whistle → high frequency

• dog whistles → often ultrasonic

Animal Hearing

Some animals hear frequencies humans cannot:

• dogs hear higher frequencies

• bats use ultrasound for echolocation

• elephants can hear low-frequency infrasound

Important Notes

• As people age, the upper hearing limit often decreases.

• Frequency affects pitch:

  • low frequency = low pitch

  • high frequency = high pitch