PHYSICS (ELECTROMAGNETIC WAVES)

SCIENCE 9 — SHORT REVIEWER SCIENCE 9 — SHORT REVIEWER

1. Electromagnetic Radiation

• Form of energy that travels as waves.

• Made of electric and magnetic fields.

• Can travel through a vacuum.

• Travels at 3.00 × 10⁸ m/s in vacuum.

2. Electromagnetic Spectrum

Order from lowest → highest frequency/energy:

Radio → Microwave → Infrared → Visible → UV → X-ray → Gamma

• Frequency ↑ → Energy ↑

• Frequency ↑ → Wavelength ↓

• Wavelength ↓ → Energy ↑

Uses

• Radio – communication

• Microwave – ovens, radar, communication

• Infrared – heat, thermal cameras

• Visible – seeing, lighting

• UV – sterilization, vitamin D

• X-ray – medical imaging

• Gamma – cancer treatment, sterilization

3. High Frequency vs. Low Frequency

Remember: Remember:

High frequency → Short wavelength → High energy

Low frequency → Long wavelength → Low energy

Frequency and wavelength are inversely related, while frequency and energy are directly related.

4. Photon Energy

E = hf

• E = photon energy

• h = Planck's constant

• f = frequency

Higher frequency = higher photon energy.

5. Ionizing vs. Non-Ionizing

Ionizing

• Enough energy to remove electrons.

• Can form ions.

• Examples: high-energy UV, X-rays, Gamma rays

Non-ionizing

• Not enough energy to remove electrons.

• Can cause molecular vibration/rotation.

• Examples: Radio, Microwaves, Infrared, Visible light

6. Molecular Vibration & Rotation

Low-energy radiation can make molecules vibrate or rotate instead of removing electrons.

Example: Microwaves → water molecules move → heat → food cooks.

7. Electromagnetic Fields

• Electric field (E) – electric force around a charge.

• Magnetic field (B) – region where magnetic forces act.

• E and B are perpendicular (90°) to each other and to the direction of travel.

Poynting Vector

• Shows the direction and rate of electromagnetic energy flow.

• Stronger fields → greater energy transfer.

8. How EM Waves Travel

Changing E-field → Changing B-field → Changing E-field

The fields continuously generate each other, allowing EM waves to travel through a vacuum.

Faraday's Law: Changing magnetic field → electric field
Ampère-Maxwell Law: Changing electric field → magnetic field

9. Mechanical vs. Electromagnetic Waves

Mechanical waves

• Need a medium.

• Example: sound

Electromagnetic waves

• Don't need a medium.

• Can travel through vacuum.

• Example: light

10. Speed of Light in a Medium

In vacuum:

c = 3.00 × 10⁸ m/s

When light enters a material:

• Speed decreases

• Frequency stays the same

• Wavelength decreases

Refractive Index

Vp = c/n

n = c/Vp

The equation $V_p = \frac{c}{n}$ relates the speed of light in a medium to the speed of light in a vacuum and the refractive index of the medium. Where:

• $V_p$ =

• $c$ = speed of light in vacuum (approximately $3.00 \times 10^8 m/s$)

• $n$ = refractive index of the medium. For any given medium, the refractive index indicates how much light will slow down; for example, a higher refractive index means light travels slower in that medium.

11. Refraction & Dispersion

• Refraction – bending/change in direction of light when it enters another material.

• Dispersion – separation of light into different colors, like in a rainbow.

12. Conservation of Energy & Inverse-Square Law

EM energy doesn't disappear as it spreads.

Same energy → Larger area → Lower intensity

A = 4πr²

I ∝ 1/r²

So:

Distance ↑ → Intensity ↓

13. Power & Intensity

P = IA

• P = power (W)

• I = intensity (W/m²)

• A = area (m²)

As area increases, intensity decreases, while total power remains constant in ideal free space.

FORMULAS TO MEMORIZE

• E = hf

• P = IA

• Vp = c/n

• n = c/Vp

• A = 4πr²

• c = 3.00 × 10⁸ m/s