Exam Tip: Remember that wavelength and frequency are inversely proportional. If the wavelength increases, the frequency must decrease to keep the speed of light constant.
Worked Example: Calculating Frequency
If a certain color of light has a wavelength of 600 nm (which is 600×10−9 m), what is its frequency?
- Identify the knowns: λ=6.00×10−7 m and c=3.00×108 m/s.
- Rearrange the formula: f=λc
- Calculate:
ff=6.00×10−7 m3.00×108 m/s=5.00×1014 Hz
The Electromagnetic Spectrum
Not all light is visible to the human eye. The "light" we see is only a tiny fraction of a much larger range known as the Electromagnetic (EM) Spectrum. This spectrum organizes all electromagnetic radiation based on their wavelengths and frequencies.
The spectrum ranges from extremely high-energy, short-wavelength waves (like Gamma rays) to low-energy, long-wavelength waves (like Radio waves). As you move from radio waves toward gamma rays, the frequency and energy of the radiation increase significantly.
| Type of Radiation | Wavelength Range | Relative Energy |
|---|
| Radio Waves | >1 m | Lowest |
| Microwaves | 1 mm−1 m | Low |
| Infrared | 700 nm−1 mm | Moderate |
| Visible Light | 400 nm−700 nm | Moderate |
| Ultraviolet | 10 nm−400 nm | High |
| X-rays | 0.01 nm−10 nm | Very High |
| Gamma Rays | <0.01 nm | Highest |
The Particle Nature: Photons and Energy
While the wave model explains how light travels and interferes, it fails to explain certain phenomena, such as the photoelectric effect (where light hitting a metal surface ejects electrons). To solve this, physicists introduced the concept of the photon.
A photon is a discrete "packet" or quantum of electromagnetic energy. Unlike a continuous wave, light arrives in these individual units. The energy (E) of a single photon is directly proportional to its frequency. This is expressed by the Planck-Einstein relation:
E=h⋅f
In this equation, h is Planck's constant, approximately 6.626×10−34 J⋅s.
This relationship is crucial: it means that high-frequency light (like Ultraviolet) carries much more energy per photon than low-frequency light (like Red visible light), even if the intensity (number of photons) is the same.
Key Concept Summary:
- Wave Model: Best for explaining reflection, refraction, and diffraction.
- Particle Model: Best for explaining how light interacts with matter and transfers energy via photons.
- The Link: Frequency determines both the wave's timing and the photon's energy.
