SM

05_lecture

Chapter 5: Light and Matter

How Do We Experience Light?

  • Light is a form of energy, felt as warmth from sunlight.

  • Measured in watts: 1 watt = 1 joule/second.

Colors of Light

  • White light consists of all colors of the rainbow.

Interactions of Light with Matter

  • Emission: Process of light being produced.

  • Absorption: Matter absorbing light.

  • Transmission: Transparency of objects to light.

    • Transparent objects transmit light.

    • Opaque objects block light.

  • Reflection/Scattering:

    • Mirrors reflect light directionally.

    • Movie screens scatter light.

What is Light?

  • Light behaves as both a wave and a particle (photons).

Waves

  • Waves carry energy without transporting matter.

  • Wavelength: Distance between wave peaks.

  • Frequency: Number of wave vibrations per second.

  • Wave Speed Formula: Wave speed = wavelength × frequency.

Electromagnetic Waves

  • Light waves are vibrations of electric and magnetic fields.

  • Interaction with charged particles is influenced by these fields.

Wavelength, Frequency, and Energy

  • Relations: wavelength × frequency = speed of light (constant).

  • Photon energy: E = h × f (where h = Planck's constant).

Polarized Sunglasses

  • Polarization indicates the vibration direction of light waves.

  • Polarized lenses block reflected light off horizontal surfaces.

The Electromagnetic Spectrum

  • Ranges from gamma rays to radio waves, characterized by different wavelengths and frequencies.

  • Structure aligns different electromagnetic sources from cosmic phenomena to everyday machines.

Types of Spectra

  • Continuous Spectrum: Light from sources like light bulbs emits all visible wavelengths.

  • Emission Line Spectrum: Thin gas emits light at specific wavelengths.

  • Absorption Line Spectrum: Cloud of gas absorbs specific wavelengths of light, resulting in dark lines.

Chemical Fingerprints

  • Unique transition patterns in atoms produce specific emission or absorption lines.

  • These lines help identify atomic presence in objects.

Energy Levels of Molecules

  • Molecules have energy levels due to vibrations and rotations, leading to complex spectra, mainly in infrared.

Thermal Radiation

  • All dense objects emit thermal radiation related to their temperature.

  • Hotter objects emit more light and higher energy photons.

Doppler Shift

  • Indicates the motion of distant objects based on light wavelength changes:

    • Blueshift: When objects approach.

    • Redshift: When objects recede.

  • Measures speed through shifts in spectral line wavelengths.