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
Light demonstrates wave characteristics when it is:
a. reflected.
b. refracted.
c. diffracted.
d. All of the above.
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.
As frequency increases:
a. energy increases.
b. energy decreases.
c. energy stays the same.
d. wavelength increases.
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
Distance to the moon:
Approx. 380,000 km to m: 380,000,000 m.
Time for light:
Time = distance / speed.
Find frequency if wavelength is 7.3 × 10^-4 m.
Find wavelength for frequency of 7.00 × 10^16 Hz.
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
Match descriptions with terms:
a
e
a
f
d
c
Which uses radio waves:
a. sonar
Frequency of light with f = 5.21 x 10^14 Hz:
Result: Yellow
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
True/False Statements:
a. False
b. False
c. True
d. True