Living Under the Sun: Challenges

Interactions of light and matter

Emission TransmissionAbsorption Reflection or Scattering

Light is an electromagnetic wave*.

Wave Characteristics

Frequency (ν): number of waves that pass a point in a given period of timeTotal energy is proportional to amplitude and frequency of waves. Because speed of light (c) is a constant (3 x 108 m/s),wavelength and frequency of electromagnetic waves are inversely proportional: E = hν; c = νλ

White light is composed of all colors which can be separated into a rainbow, or a

spectrum, by passing the light through a prism.

Each color light has a different wavelength, and, therefore, frequency.

Color

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Amplitude & Wavelength

Are there other “Colors”?

The Electromagnetic Spectrum• Visible light comprises only a small fraction of all the

wavelengths of light – called the electromagnetic spectrum.

• Short wavelength (high frequency) light has high energy.

- Gamma ray light has the highest energy.

• Long wavelength (low frequency) light has low energy. - Radiowave light has the lowest energy.

Electromagnetic Spectrum

Each chemical element produces its own unique set of spectral lines.

Oxygen spectrum

Neon spectrum

Emission and absorption spectraare inversely related.

Spectra of Mercury

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Light can induce electron transitions.

• To transition to a higher energy state, the electron must absorb energy equal to the energy difference between the final and initial states.

• Electrons in high energy states are unstable. They will transition to lower energy states and emit light.

Light induces electron transitions in molecules.

O O······ ··

absorbs UV light “colorless”

Visible light does not have enough energy to move electrons to next energy

level.

absorbs visible light“colored”

Energy levels are closer in “conjugated” system. Visible light is energetic enough to

induce transition!

Light induces other transitions in molecules.

Energy can also be absorbed and emitted in other “modes” including vibration and

rotation.

OH

Light can induce photochemical reactions.

• Photolysis O2(g) 2 O(g)

O3(g) O2(g) + O(g)

• Conformational change

Visible

Greenhouse gases!

Ozone!

Sunburn, vitamin D

UV Absorbing Molecules

Skin Pigments – absorb and dissipate energy

http://www.clinuvel.com/

How about those beads?Photochromic Pigments!

Higher energy formLower energy form

Sunscreen Ingredients

Homosalate Octyl methoxycinnamate

Inorganic compounds, e.g. ZnO and TiO2, reflect or scatter light.

Organic compounds absorb light and dissipate energy as heat.