Welcome to the World of Waves!
Hi there! In this chapter, we are going to explore Waves. Waves are everywhere—from the ripples on a pond to the light allowing you to read these notes. The most important thing to remember is that waves transfer energy and information from one place to another without transferring matter.
Don't worry if this seems a bit abstract at first; we will break it down into simple steps!
1. Transverse and Longitudinal Waves
There are two main ways that waves can travel. We distinguish them by looking at how the particles in the wave move compared to the direction the energy is going.
Transverse Waves
In a transverse wave, the vibrations (oscillations) are at right angles (perpendicular) to the direction of energy transfer.
Example: Think of a "Mexican Wave" in a stadium. The people move up and down, but the wave itself moves sideways around the stadium.
- Ripples on a water surface are transverse.
- All Electromagnetic waves (like light) are transverse.
Longitudinal Waves
In a longitudinal wave, the vibrations are parallel to the direction of energy transfer. These waves look like they are pushing and pulling.
Example: Pushing a Slinky spring back and forth. You will see areas where the coils are bunched up and areas where they are spread out.
- Sound waves travelling through air are longitudinal.
- They have compressions (where particles are close together) and rarefactions (where particles are spread out).
Did you know?
When you hear a sound, it’s the wave that travels to your ear, not the air itself. If the air moved from the speaker to your ear, it would create a constant wind whenever music played!
Quick Review:
- Transverse: Vibrations at 90° to energy (Up and Down).
- Longitudinal: Vibrations parallel to energy (Back and Forth).
2. Properties of Waves
To describe waves, we need to use four specific "labels." Here is the easiest way to visualize them:
- Amplitude: The distance from the undisturbed position (the middle line) to the peak (top) or trough (bottom) of a wave. It tells you how much energy the wave has.
- Wavelength (\(\lambda\)): The distance from one point on a wave to the exact same point on the next wave (e.g., peak to peak). It is measured in metres (m).
- Frequency (\(f\)): The number of waves passing a point each second. It is measured in Hertz (Hz).
- Period (\(T\)): The time it takes for one full wave to pass. It is measured in seconds (s).
The Magic Formulas
You need to be able to calculate the Period and the Wave Speed.
To find the Period:
\( T = \frac{1}{f} \)
To find the Wave Speed (\(v\)), which is the speed at which energy is transferred:
\( v = f \lambda \)
v = wave speed (m/s)
f = frequency (Hz)
\(\lambda\) = wavelength (m)
Key Takeaway:
Higher frequency means more waves pass you every second. If you keep the speed the same and increase the frequency, the wavelength must get shorter!
3. The Electromagnetic (EM) Spectrum
Electromagnetic waves are special transverse waves that can travel through a vacuum (empty space) at the same speed: 300,000,000 m/s.
We group them into a "spectrum" based on their wavelength and frequency. Here they are, from Longest Wavelength (Lowest Frequency) to Shortest Wavelength (Highest Frequency):
- Radio waves - Used for TV and radio.
- Microwaves - Used for satellite communication and cooking food.
- Infrared - Used for electrical heaters and infrared cameras.
- Visible light - The only part our eyes can see! Used for fibre optics.
- Ultraviolet - Used in energy-efficient lamps and sun tanning.
- X-rays - Used for medical imaging.
- Gamma rays - Used for medical treatments (killing cancer cells).
Memory Aid:
Use this mnemonic to remember the order:
Raging Martians Invaded Venus Using X-ray Guns.
Did you know?
Gamma rays come from changes in the nucleus of an atom, while others usually come from changes in the electron levels!
4. Hazards and Doses
Not all EM waves are harmless. The danger depends on the frequency. High-frequency waves carry more energy and can be "ionising" (they can damage your DNA).
- Ultraviolet (UV): Can cause skin to age prematurely and increase the risk of skin cancer.
- X-rays and Gamma rays: These are ionising radiation. They can cause gene mutations and cancer.
Radiation Dose: This is a measure of the risk of harm to your body. It is measured in Sieverts (Sv). Since a Sievert is very large, we often use millisieverts (mSv).
1000 mSv = 1 Sv.
5. Refraction (Higher Tier Only)
When a wave moves from one substance (medium) to another, it can change direction. This is called refraction.
- If a wave slows down, it bends towards the normal (the imaginary 90° line).
- If a wave speeds up, it bends away from the normal.
Analogy: Imagine a car driving from a paved road onto sand at an angle. The wheel that hits the sand first slows down, while the other wheel stays fast for a moment. This "tug" causes the whole car to turn. That is exactly how light refracts!
6. Magnetism and Electromagnetism
This part of the physics content links waves and forces. All magnets have a North Pole and a South Pole.
- Like poles repel: North and North push away.
- Unlike poles attract: North and South pull together.
Magnetic Fields
A magnetic field is the area around a magnet where it can exert a force.
Trick: Field lines always go from North to South.
Electromagnetism
When a current flows through a wire, a magnetic field is produced around it. If you wrap that wire into a coil, it is called a solenoid.
To make a solenoid stronger:
- Increase the current.
- Increase the number of turns in the coil.
- Add an iron core (this makes it an electromagnet).
(HT Only) The Motor Effect
When you put a wire carrying a current inside another magnetic field, the two fields interact and the wire moves. This is the basis of electric motors.
You can find the direction of the force using Fleming’s Left-Hand Rule:
- Thumb: Movement (Force).
- First Finger: Field (North to South).
- Second Finger: Current (+ to -).
Summary Checklist
1. Can you define Transverse and Longitudinal? (Remember the Mexican Wave vs. Slinky).
2. Do you know the Wave Speed formula? (\( v = f \lambda \)).
3. Can you list the EM Spectrum in order? (Raging Martians...).
4. Do you know the risks of UV, X-rays, and Gamma? (Cancer and mutations).
5. (HT) Can you use Fleming's Left-Hand Rule? (Thumb = Force).