Key Takeaways
- Waves transfer energy without transferring matter; mechanical waves need a medium, electromagnetic waves do not
- Wave properties: wavelength (distance between crests), frequency (waves per second, in Hz), amplitude (height/intensity)
- The wave equation: speed = wavelength x frequency (v = lambda x f)
- Sound waves are longitudinal mechanical waves; they travel fastest through solids and cannot travel through a vacuum
- Light is an electromagnetic wave that travels at 3 x 10^8 m/s in a vacuum; visible light ranges from red (longest wavelength) to violet (shortest)
- Reflection is the bouncing of waves off a surface; refraction is the bending of waves when they change medium
- Electric current is the flow of electrons through a conductor; measured in Amperes (A)
- Ohm's Law: Voltage = Current x Resistance (V = IR); power = voltage x current (P = VI)
Waves, Sound, Light & Electricity
These physics topics have numerous applications in healthcare, from diagnostic imaging (ultrasound, X-rays) to cardiac monitoring (ECG) and electrical safety.
Waves
A wave is a disturbance that transfers energy from one place to another without transferring matter.
Types of Waves
| Type | Motion | Medium Required | Example |
|---|---|---|---|
| Transverse | Perpendicular to direction of travel | Depends | Light waves, water surface waves |
| Longitudinal | Parallel to direction of travel | Yes | Sound waves, compression waves |
| Mechanical | Any | Yes (solid, liquid, gas) | Sound, ocean waves |
| Electromagnetic | Transverse | No (travels through vacuum) | Light, X-rays, radio waves |
Wave Properties
| Property | Definition | Unit |
|---|---|---|
| Wavelength (lambda) | Distance between two consecutive crests (or troughs) | Meters (m) |
| Frequency (f) | Number of waves passing a point per second | Hertz (Hz) |
| Amplitude | Maximum displacement from rest position (height of wave) | Meters (m) |
| Period (T) | Time for one complete wave cycle | Seconds (s) |
| Speed (v) | How fast the wave travels | m/s |
Wave Equation: v = lambda x f (speed = wavelength x frequency)
Relationship: As frequency increases, wavelength decreases (for a constant speed)
Sound
Sound is a longitudinal mechanical wave produced by vibrating objects:
- Requires a medium (cannot travel through a vacuum)
- Travels fastest through solids, then liquids, then gases (slowest)
- Speed of sound in air at room temperature: approximately 343 m/s
- Pitch = frequency (high pitch = high frequency)
- Loudness = amplitude (louder = greater amplitude)
- Decibels (dB) measure sound intensity
| Sound Level | dB | Example |
|---|---|---|
| Whisper | 20 dB | Library |
| Normal conversation | 60 dB | Office |
| Vacuum cleaner | 75 dB | Household |
| Pain threshold | 120-130 dB | Rock concert, jet engine |
Ultrasound uses high-frequency sound waves (above 20,000 Hz) for medical imaging:
- Safe, non-invasive, no radiation
- Used for pregnancy monitoring, abdominal imaging, echocardiography
Doppler effect: The change in frequency of a wave relative to an observer moving relative to the wave source.
- Sound from an approaching ambulance has a higher pitch; receding ambulance has a lower pitch
- Medical Doppler ultrasound uses this effect to measure blood flow velocity and detect blockages
Resonance and Standing Waves
- Resonance occurs when an object vibrates at its natural frequency in response to a matching external frequency
- Medical application: MRI uses resonance of hydrogen atoms in a magnetic field
- Standing waves form when waves reflect and interfere, creating patterns of nodes (no movement) and antinodes (maximum movement)
Light and the Electromagnetic Spectrum
Light is an electromagnetic wave that travels at 3 x 10^8 m/s (speed of light) in a vacuum.
The Electromagnetic Spectrum (from longest to shortest wavelength)
| Type | Wavelength | Medical Application |
|---|---|---|
| Radio waves | Longest | MRI imaging |
| Microwaves | Long | Diathermy (heat therapy) |
| Infrared | Medium-long | Thermal imaging |
| Visible light | Medium | Endoscopy, phototherapy |
| Ultraviolet (UV) | Medium-short | Sterilization, jaundice treatment |
| X-rays | Short | Radiography, CT scans |
| Gamma rays | Shortest | Radiation therapy for cancer |
Visible light spectrum (ROYGBIV): Red, Orange, Yellow, Green, Blue, Indigo, Violet
- Red has the longest wavelength and lowest frequency
- Violet has the shortest wavelength and highest frequency
Inverse Square Law for Radiation
The intensity of radiation decreases with the square of the distance from the source:
I = 1 / d^2 (proportional relationship)
| Distance from Source | Relative Intensity |
|---|---|
| 1 meter | 100% |
| 2 meters | 25% (1/4) |
| 3 meters | 11% (1/9) |
| 4 meters | 6.25% (1/16) |
Clinical significance: This law is critical for radiation safety. Doubling your distance from a radiation source reduces your exposure to one-quarter. Nurses working near radioactive patients or X-ray equipment use this principle: maximizing distance is one of the three cardinal rules of radiation protection (time, distance, shielding).
Types of Lenses and Mirrors
| Type | Shape | Image | Clinical Application |
|---|---|---|---|
| Convex lens | Thicker in middle | Can produce real or virtual images | Corrects farsightedness (hyperopia) |
| Concave lens | Thinner in middle | Always produces virtual, upright, smaller images | Corrects nearsightedness (myopia) |
| Concave mirror | Curves inward | Can produce real or virtual images; magnifies | Dental mirrors, examination lights |
| Convex mirror | Curves outward | Always produces virtual, upright, smaller images | Security mirrors, car side mirrors |
Key terms for optics:
- Focal point — where parallel light rays converge (convex) or appear to diverge from (concave)
- Real image — formed where light rays actually converge; can be projected on a screen
- Virtual image — formed where light rays appear to diverge from; cannot be projected
- Total internal reflection — light reflects entirely within a medium when hitting the boundary at a steep angle; used in fiber optic endoscopy
Light Behaviors
| Behavior | Description | Example |
|---|---|---|
| Reflection | Light bounces off a surface | Mirror image |
| Refraction | Light bends when changing medium | Straw appears bent in water |
| Absorption | Light energy is taken in by material | Dark clothing absorbs light/heat |
| Diffraction | Light bends around obstacles | Light spreading through a narrow slit |
Electricity
Basic Electrical Concepts
| Quantity | Definition | Unit | Symbol |
|---|---|---|---|
| Voltage (V) | Electrical "pressure" (potential difference) | Volts (V) | V |
| Current (I) | Flow of electric charge (electrons) | Amperes (A) | I |
| Resistance (R) | Opposition to current flow | Ohms (Omega) | R |
| Power (P) | Rate of energy use | Watts (W) | P |
Ohm's Law
V = I x R (Voltage = Current x Resistance)
- Rearranged: I = V/R and R = V/I
- Example: A device uses 2 A of current with 120 V. What is the resistance?
- R = V/I = 120/2 = 60 Ohms
Electrical Power
P = V x I (Power = Voltage x Current)
- Example: A monitor uses 0.5 A at 120 V. What is the power?
- P = 120 x 0.5 = 60 W
Circuits
| Type | Current Path | Failure Effect |
|---|---|---|
| Series circuit | One path; components connected end-to-end | If one component fails, entire circuit breaks |
| Parallel circuit | Multiple paths; components on separate branches | If one component fails, others continue working |
Electrical Safety in Healthcare
- Grounding protects against electrical shock
- GFCI outlets (Ground Fault Circuit Interrupters) are required near water sources
- Wet skin has lower resistance → higher risk of electrical shock
- Microshock hazard: Very small currents (as low as 100 microamperes) can cause cardiac arrest if they reach the heart directly (via catheters or pacemaker leads)
- All medical equipment must be properly maintained and inspected for electrical safety
Magnetism
Magnetism is closely related to electricity — both are aspects of electromagnetism:
| Concept | Description |
|---|---|
| Magnetic field | Region around a magnet where magnetic forces act |
| Magnetic poles | North and south; like poles repel, opposite poles attract |
| Electromagnet | Magnet created by running electric current through a coil of wire |
| Ferromagnetic | Materials strongly attracted to magnets (iron, nickel, cobalt) |
Medical application — MRI:
- MRI uses powerful electromagnets and radio waves to create detailed images
- The magnetic field aligns hydrogen atoms in body tissues
- Radio pulses cause hydrogen atoms to emit signals that are detected and processed
- MRI safety: Ferromagnetic objects (metal implants, pacemakers, oxygen tanks) are DANGEROUS near MRI machines
- Patients must be screened for metal implants before MRI
Sound travels fastest through which medium?
Using Ohm's Law, what is the current in a circuit with 240 V and 80 Ohms of resistance?
Which type of electromagnetic radiation is used in MRI imaging?
The bending of light as it passes from one medium to another is called:
The wave equation states that wave speed equals wavelength times _____.
Type your answer below
Which of the following are TRUE about electromagnetic waves? (Select all that apply)
Select all that apply
In visible light, which color has the LONGEST wavelength?
Arrange these types of electromagnetic radiation from LONGEST wavelength to SHORTEST wavelength.
Arrange the items in the correct order