Spectra

White Light

• Light from many colors mixes to form white light.

• Different colors have different wavelengths.

• Shorter wavelengths bend more through a prism.

• A diffraction grating combines many edges and acts like a prism.

Sources of Photons

• Accelerated electrons and atoms emit photons.

Moving electron

Emitted photon

• Photons can be reabsorbed as well.

Temperature

• Higher temperature means more energy.

• Temperature is measured in kelvin (K).

• Absolute zero: 0 K = -273 ºC = -459 ºF.

• Room temperature: 300 K 20 ºC = 68 ºF.

• Temperature of the Sun: 5800 K 6100 ºC 11,000 ºF

• At high temperatures kelvin and centigrade are about the same.

Radiation from Heat

• Heated objects give off electromagnetic waves.– Higher temperature has

more radiation

• A hot object gives off a spectrum of frequencies.– Shifts based on

temperature

low energy

high energy

frequency

intensity

Continuous Spectrum

• Radiation due a heated object is called blackbody radiation.

• Wien’s law relaters the temperature to the peak wavelength.

low energy

high energy

frequency

intensity

nmK109.2 6

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Atoms and Light

• An atom is a nucleus surrounded by electrons.

• An electron can be in specific energy states.

• Change from a high to low energy state produces a photon.

• Atoms can also absorb a photon to excite an electron.

Discrete Spectrum

• Each atom has its own set of energy levels.

• Each atom generates photons at specific frequencies.

• The pattern of frequencies (colors) identifies the atom.

• Examples include neon or mercury lights.

helium

neon