Light, the essence of photochemistry, exhibits a fascinating dual nature as both waves and particles. This duality explains phenomena like interference patterns and the photoelectric effect, bridging classical and quantum physics in our understanding of electromagnetic radiation.

The electromagnetic spectrum spans from long-wavelength radio waves to high-energy gamma rays. Visible light, crucial in photochemistry, occupies a narrow band between 380-750 nm, triggering electronic transitions in molecules and driving processes like photosynthesis and vision.

Electromagnetic Radiation and Light

Wave-particle duality of light

• Wave-particle duality describes light exhibiting properties of both waves and particles simultaneously
• Wave nature manifests through interference patterns in double-slit experiments, diffraction around obstacles, and refraction when passing through different media
• Particle nature demonstrated by photoelectric effect where light ejects electrons from metal surfaces and Compton scattering of X-rays by electrons
• Photons represent discrete packets of energy with $E = h\nu$ where $h$ is Planck's constant and $\nu$ is frequency
• Historical development progressed from classical wave theory (Maxwell's equations) to quantum theory (Planck's quantization, Einstein's light quanta)

Regions of electromagnetic spectrum

• Radio waves span > 1 m wavelength with < 1.24 µeV energy used in telecommunications (AM/FM radio)
• Microwaves range 1 m - 1 mm wavelength, 1.24 µeV - 1.24 meV energy utilized in cooking and radar
• Infrared covers 1 mm - 750 nm wavelength, 1.24 meV - 1.65 eV energy detected as heat (thermal imaging)
• Visible light occupies 750 nm - 380 nm wavelength, 1.65 eV - 3.26 eV energy perceived by human eyes
• Ultraviolet spans 380 nm - 10 nm wavelength, 3.26 eV - 124 eV energy causing sunburns and sterilization
• X-rays range 10 nm - 0.01 nm wavelength, 124 eV - 124 keV energy used in medical imaging
• Gamma rays have < 0.01 nm wavelength, > 124 keV energy emitted in radioactive decay

Wavelength, frequency, and energy relationships

• Speed of light equation $c = \lambda\nu$ connects wavelength $\lambda$ and frequency $\nu$
• Energy of photon $E = h\nu$ relates energy to frequency
• Combining equations yields $E = \frac{hc}{\lambda}$ linking energy to wavelength
• Inverse relationship between wavelength and frequency as one increases, the other decreases
• Direct relationship between frequency and energy higher frequency means higher energy (gamma rays)
• Inverse relationship between wavelength and energy shorter wavelengths have higher energy (X-rays)

Visible light in photochemistry

• Visible spectrum spans 380 nm - 750 nm wavelength perceived as colors (violet 380-450 nm, blue 450-495 nm)
• Absorption of visible light by molecules triggers electronic transitions in chromophores
• Photosynthesis relies on chlorophyll absorbing red and blue light for energy conversion
• Vision process involves rhodopsin in retinal cells activated by visible light
• Photocatalysis uses visible light to activate materials (titanium dioxide) for chemical reactions
• Solar radiation peaks in visible region driving solar energy applications (photovoltaics)
• Photochemical reactions initiated by visible light include photoisomerization (retinal in vision) and photooxidation
• Spectroscopy techniques like UV-Visible and fluorescence utilize visible light for molecular analysis