Waves 🌊

  • The Electromagnetic Spectrum (EMS)

The electromagnetic spectrum consists of waves arranged in order of increasing frequency (and decreasing wavelengths) as follows:

  1. Radiowaves

  2. Microwaves

  3. Infrared Radiation

  4. Visible Light

  5. Ultraviolet Radiation

  6. X-Rays

  7. Gamma Rays (Y-rays)

Speed of electromagnetic waves:

All electromagnetic waves travel at the same speed in a vacuum at the speed of 3.0 × 10⁸ m/s. In air, they travel approximately the same speed as in a vacuum.

Properties of each wave in the EMS:

  1. Radiowaves

    Properties- Long wavelength, low frequency, low energy

    Uses- Transmitting information over long distances (eg- communication in broadcasting)

  2. Microwaves

    Properties- Shorter wavelength and higher frequency than radiowaves, low energy (but still slightly more than radiowaves)

    Uses- Satallite communication, microwave ovens for heating food

  3. Infrared Radiation (IV)

    Properties- Wavelength is shorter than microwaves, but longer than visible light.

    Uses- Used in vision and photography, fiber optic communication

  4. Visible Light

    Properties- The only part of the EMS that is visible to the human eye.

    Uses- Fiber optic communication

  5. Ultraviolet radiation (UV)

    Properties- Higher frequency and shorter wavelength than visible light, high energy

    Uses- Sun tanning lamps, sterilizing equipment

    Danger- Exessive exposure to UV (eg- from the sun) can cause sun burns, skin cancer, and eye damage.

  6. X-rays

    Properties- Very short wavelength and high energy

    Uses- Medical imaging (X-ray imaging), scanning luggage at an airport

    Dangers- Overexposure to x-rays can cause DNA damage, which can lead to cancers, so protective measures (eg- lead shielding) are necessary.

  7. Gamma rays (y- rays)

    Properties- Highest frequency, wavelength, and highest energy

    Uses- Medical treatment (eg- radiotherapy to kill cancer cells) and sterilize medical equipment

    Dangers- Highly penetrable and causes DNA damage, which can lead to cancers.

Dangers and safety issues:

  1. Microwaves

    Danger- Can cause body tissues to become hotter and cause damage with prolonged exposure.

    Safety- Use proper shielding (eg- microwave oven protection)

  2. X-rays

    Danger- Are ionizing radiation and can cause DNA damage and increase skin cancer risk.

    Safety- Use lead shielding and minimize exposure

  3. Ultraviolet Radiation (UV)

    Origin- Comes from the sun and tanning lamps (can also be produced artificially)

    Danger- Excessive exposure can cause eye damage, sunburns, sun poisening, skin cancer.

    Safety- Use sunscreen, sun glasses, and minimize exposure

  4. Gamma rays

    Same as X-rays, but more severe

Key Facts

Electromagnetic waves are transverse waves.

They travel at 3.0 × 10⁸ m/s in a vacuum.

Frequency increases with decreasing wavelength (inverse relationship)

  • General Wave Properties

Key Concepts:

  1. Waves transfer energy, without transferring matter

  2. This is because the particles oscillate (move back and forth) but do not move with the wave, so only energy is transferred through the medium.

Wave Motion:

Can be demonstrated through these 2 experiments:

  1. Vibrations in ropes and strings.

    When a pulse or disturbance travels along the rope/string the rope itself stays in place.

  2. Water Waves

    Dropping a stone into water, creates ripples (waves) that spread out, and spread energy across the water surface.

Wave Terminology:

Speed (v)- How fast the wave travels through the medium (measured in m/s)

Frequency (f)- The number of waves passing a point per second (measured in Hz)

Wavelength (Lambda)- The distance between two consectutive points in a wave (crest to crest, trough to trough) (measured in m)

Amplitude (A)- Height of crest or trough at their deepest/highest point.

Transverse vs Longitudinal Waves:

  1. Transverse Waves

    Behaviour- Oscillations (vibrations) are perpendicular to the direction of energy transfer.

    Examples- Water waves, light waves, waves on a rope

  2. Longitudinal Waves

    Behaviour- Oscillations are parallel to the direction of energy transfer.

    Examples- Sound waves, compressions in springs.

Wave Behaviors:

NB- Reflection and Refraction or not limited to light waves, but can also happen with other wave like sound waves.

  1. Reflection

    Definition- When waves bounce back when they hit a plane surface.

    Example- Light reflecting a mirror, sound waves echoing

    Law of reflection- Angle of incidence equals angle of reflection

  2. Refraction

    Definition- When a wave changes direction when it passes from one medium to another due to the change in speed.

    Key points:

    When a wave slows done (eg- air to glass) they bend towards the normal line.

    When a wave speeds up (eg- glass to air) they bend away from the normal line.

Wave Equuation:

v= flambda

speed= frequency x wavelength

  • Light Waves

Formation of an optical image by a plane mirror:

When a light wave is reflected off a plane mirror, they form an image that has these characteristics:

  1. Virtual- Cannot be projected onto a screen, because it is formed by the apparent intersection of reflected rays.

  2. Upright- The image is always the right the way up.

  3. Same Size- The image is always the same size as the object.

  4. Same distance behind mirror- The image appears to be as far behind the mirror, as it is in front of the mirror.

Law of Reflection:

Law= Angle of reflection= angle of incidence

Definition of angles:

Both angles are measured relative to the normal line, which is an imaginary line perpendicular to the surface of the mirror that is being hit by the ray of light.

Ray of reflection= The ray of light that is reflected off the mirror.

Ray of incidence= The ray of light that is pointed towards the mirror.

Ray Diagram

Simple constructions, measurements, and calculations for reflection by plane mirrors:

Ray Diagram Construction

  1. Draw the ray of incidence hitting the mirror at a specific point.

  2. Draw the normal line, which is perpendicular to the mirror.

  3. Reflect the ray, so that the angle of incidence equals the angle of reflection (law of reflection).

  4. Extend the reflected ray behind the mirror, to show the virtual image.

Calculations

  1. Use a protractor to measure i (angle of incidence) and r (angle of reflection).

  2. Confirm i=r for accurate reflection.

Experiment for Refraction of Light:

Materials you need:

Glass or perspex block

Ray box (and material to set it up)

Protractor

Pencil

Ruler

Procedure

  1. Shine a ray of light into the block

  2. Observe how the ray of light bends as it enters the block (refraction occurs because the speed of light is decreased)

  3. Mark the path of the incident and refracted ray (inside the block), and emergent ray (when it leaves the block)

  4. Measure the angles of incidence and refraction

Observations

  1. When light enters a denser medium (air to glass) it bends towards the normal line

  2. When light exits to a less dense medium (glass to air) it bends away from the normal line

  • Key Summary Points

  1. A plane mirror produced a virtual, upright, laterally inverted image the same size as the object.

  2. The law of reflection states that i=r, measured relative to the normal line.

  3. Reflection can be demonstrated using ray diagrams, protractors, and mirrors

  4. Refraction occurs because light changes speed as it travels between materials of different densities

  • Sound Waves

Production of Sound:

Description- Sound is produced when objects vibrate, these vibrations disturb the sorrounding medium creating regions of compression (high pressure) and rarefaction (low pressure).

Examples- When a guitar string is plucked, it produces sound. A tuning fork produces sound, as it vibrates, which disturbs the air molecules.

Longitudinal Nature of Sound Waves:

Definition- Sound waves are longitudinal waves, this means that the particles in the medium vibrate parallel to the direction of energy transfer

Key Feature- The wave will consist of alternating compressions (particles close together) and rarefactions (particles spread out).

Range of Audible Frequencies for humans:

A healthy human ear can hear sound waves within 20Hz (low pitch sounds) to 20kHz (high pitch sounds).

Sound below 20Hz are called infrasounds, and sounds above 20kHz are called ultrasounds.

Understanding the Need for a Medium:

Key Point- Sound waves require a medium (solid, liquid, or gas) to travel, because they depend on particle vibrations to propagate.

Vacuum Experiment- If a bell jar is placed in a vacuum and the air is removed, no sound can be heard because there are no particles to transmit the sound waves.

Speed of Sound in Air:

Experiment:

Materials

A pistol (or a loud clap)

Two people

Stopwatch

A large open space

Procedure

  1. One person fires a pistol or a loud clap

  2. The other person (who has a distance between the other person) starts the stopwatch when they see the pistol fire or clap clap, and stop the stopwatch when they hear the sound (Best works when they have a large distance between them, and they use a light flash to start the stopwatch)

  3. Measure the distance between the two people.

Calculation:

Formula- Speed of Sound= distance/time

Distance= speed x time

Time= Distance/speed

Common result- the speed of sound in air= 340 m/s (usually)

Sound of speed in different media:

  1. Gases (slowest)

    Sound travels slowest in gas, because the particles are farther apart (in air 340 m/s)

  2. Liquids (faster)

    Sound travels faster in liquids than gases, because the particles are closer together (in water 1500 m/s)

  3. Solids (fastest)

    Sound travels the fastest in solids, because the particles are close together (in steel 5000 m/s)

Loudness and pitch of Sounds:

Loudness= Determined by the amplitude of the wave. Large amplitude= louder sound.

Pitch= Determined by the frequency of the wave. Higher frequency= Higher pitch

Reflection of Sound Waves:

Reflection= When sound waves hit a hard smooth surface, they reflect back, creating an echo.

Conditions for an echo;

  1. The reflected sound must take 0.1 seconds to reach the ear for it to be heard as an echo.

  2. This requires the reflecting surface to be at least 17 meters away.

Example- Echoes heard in large empty halls, caves and mountains.

Key Points:

  1. Sound waves are produced by vibrating sources and require a medium to travel.

  2. Sound waves are longitudinal, consisting of compressions and rarefactions.

  3. Humans can hear sounds between 20Hz to 20kHz.

  4. Sounds travel fastest in solids, slower in liquids, and slowest in gases .

  5. Loudness related to amplitude, and pitch related to frequency.

  6. Sound reflection causes echoes, which require sufficient distance.