Unit 6 Notes 5

LIGHT WAVES

• Type: Electromagnetic waves caused by vibrations of electrons.

  • Do not need a medium; travel by transferring energy between electric and magnetic fields.

• Shape: Transverse waves.

• Interaction: Light waves interact like all other waves.

LIGHT AS A PARTICLE

Photoelectric Effect

• Certain colors of light can "eject" electrons from a metal.

• Minimum energy value needed to eject electrons varies by metal type.

• Brightness (amplitude) does not influence results.

Dual Nature of Light

• Light exists as photons (massless packets of energy).

• Photons typically interact like waves.

• Energy of a photon is proportional to frequency:

  • High frequency → High Energy.

Reflection and Refraction

• Example: Not enough energy from red light to eject electrons, but purple light has sufficient energy.

• Higher frequency correlates with more energy.

WAVE/PARTICLE MODEL

Comparisons

• Wave Model:

  • Can explain:

    • Interference

    • Reflection

    • Refraction

    • Diffraction

  • Cannot explain: Photoelectric Effect

• Particle Model:

  • Can explain:

    • Reflection

    • Refraction

    • Photoelectric Effect

  • Cannot explain:

    • Interference

    • Diffraction

Intensity of Light

• Measures amount of light on a surface.

• Depends on the number of photons passing through an area per second.

• Intensity decreases with distance from the light source as light spreads out.

PRACTICE QUESTIONS

1. Light demonstrates wave characteristics when it is:

   • a. reflected.

   • b. refracted.

   • c. diffracted.

   • d. All of the above.

2. Light demonstrates particle characteristics when it:

   • a. knocks electrons off a metal surface.

   • b. passes through a narrow opening.

   • c. forms standing waves.

   • d. All of the above.

3. As frequency increases:

   • a. energy increases.

   • b. energy decreases.

   • c. energy stays the same.

   • d. wavelength increases.

4. The rate of energy flow through space is described by light as:

   • a. resonance.

   • b. pitch.

   • c. intensity.

   • d. interference.

SPEED OF LIGHT (c)

• Speed depends on the medium; decreases in denser media.

• In a vacuum, light travels at 300,000,000 m/s or 3 × 10^8 m/s.

Example Calculation

• If blue light has wavelength of 4 x 10^-7 m:

  • Frequency (f):

    • Using formula: c = λ × f

    • Therefore, f = c/λ = 3 × 10^8 m/s / (4 × 10^-7 m)

    • Result: f = 7.5 × 10^14 Hz.

PRACTICE PROBLEMS

7. Distance to the moon:

   • Approx. 380,000 km to m: 380,000,000 m.

   • Time for light:

     • Time = distance / speed.

8. Find frequency if wavelength is 7.3 × 10^-4 m.

9. Find wavelength for frequency of 7.00 × 10^16 Hz.

10. Compare frequencies: 521 nm vs 605 nm.

• 521 nm has higher frequency.

THE ELECTROMAGNETIC SPECTRUM

• Components: Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma Ray.

• Wavelength Ranges:

  • Radio: 0.1 m to 100,000 m

  • Microwaves: 0.1 m to 0.001 m

  • Infrared: 0.001 m to 0.000001 m

  • Visible light: ~ 700-400 nm

  • Ultraviolet: ~ <400 nm

  • X-ray: ~ <10^-10 m

  • Gamma Ray: ~ <10^-12 m

VISIBLE LIGHT SPECTRUM

• Color Wavelengths:

  • Red: ~ 700-635 nm

  • Orange: ~ 635-590 nm

  • Yellow: ~ 590-560 nm

  • Green: ~ 560-490 nm

  • Blue: ~ 490-450 nm

  • Violet: ~ 450-400 nm

TYPES OF ELECTROMAGNETIC WAVES

RADIO WAVES

• Longest wavelengths in the spectrum (0.1 m - 100,000 m).

• Examples: AM/FM radio, Radar, MRI.

MICROWAVES

• Wavelength: 0.1 m to 0.001 m.

• Uses: Microwave ovens, satellite communications, GPS, cell phones.

INFRARED WAVES

• Carry thermal energy (heat).

• Examples: Remote controls, thermal photographs.

ULTRAVIOLET WAVES

• Can cause sunburn; essential for Vitamin D production.

• Also used for sterilization (kills bacteria).

X-RAYS

• Can penetrate soft tissue; stopped by bones for imaging.

• Used in medical imaging.

GAMMA RAYS

• Ultra-high frequency; can break molecular bonds.

• Used in radiation therapy; can kill cancerous cells (care needed to protect healthy cells).

PRACTICE QUESTIONS

11. Match descriptions with terms:

• 1. a

• 2. e

• 3. a

• 4. f

• 5. d

• 6. c

12. Which uses radio waves:

• a. sonar

13. Frequency of light with f = 5.21 x 10^14 Hz:

• Result: Yellow

14. Consider properties of the electromagnetic spectrum:

• a. Lowest frequency: Radio waves

• b. Shortest wavelength: Gamma Rays

• c. Most energy: Gamma Rays

• d. Least energy: Radio Waves

15. True/False Statements:

• a. False

• b. False

• c. True

• d. True