Home > Chemistry > Chemistry 1A Part Deux: Lecture 33 Notes

Chemistry 1A Part Deux: Lecture 33 Notes


They are having a test tomorrow.   For us, this is a lecture on the electromagnetic spectrum.   As usual, the class is about 50 minutes long.


Human beings glow, but at 98.2 degrees celsius.  Thus in kelvin we are 473.  Thus we are far in the red or low energy side of the spectrum for blackbody radiators.

In emission mode, white light is a mixture of all colors.  The primary colors are green, red, and blue.

In absorption mode, black is all colors.

Cone receptors come in 3 types.  The brain compares the signals to perceive color.  Rod cells are used at night.  4 detectors are green, blue, red cones and rods.

A graph of sensitivity versus wavelength.  Blue has the highest energy, red has the lowest energy.

E = hf

Colored solids absorb every color except for the color they transmit.  A thing may either reflect, absorb, transmit, diffract or refract incoming waves.  The color being absorbed is the complementary color.

Which substance predominantly absorbs visible light?  Al/NaCl/C/SiO2?

Well, aluminum is grey and shiny.  NaCl is white.  Graphite is black.  Glass is clear.  It must be graphite, since it absorbs all colors but reflects none.

Which substance appears white?  Ag, NaCl, Cu, SiO2?

White is the absorbance of none and the reflection of all colors.  It must be the crushed glass.

Specular reflection is internal reflection.  Diffuse reflection bends and comes out, sparkling.

She does an experiment with a blue solution.  Cu(NH3)4+2.

Absorption Spectroscopy

Light source into a slit into a prism into a spectrum, then use a slit to filter.

Light source to monochormator to sample to detector.  A phototube converts photons to electrons.  This will reveal how much, and can identify the material.


T = L out/L in

Absorbance A = -logT

Beer’s Law

A =  A l c

absoptivity/path length/concentration

Beta carotene is a long 3 cyclic molecule.  On a graph of absorbance versus wavelength, it peaks at blue, emitting an orange carrot color.

EM spectrum: gamma, xray, UV, vis, IR, micro, FM, TV, shortwave, AM

***Important demonstration at 48 minutes in***

IR photography.  Objects near room temperature emit low energy light.  A plastic garbage bag will be invisible to vis systems.  Perhaps several garbag bags would be invisible to the IR system.

Microwave ovens resonate at 12.24 cm for wavelength.  The little grid on the  microwave door is to keep the radiation inside the oven.

Electric field of microwaves couples with the dipole of water molecules causing them to rotate.  As the KE of the water increases, so the temperature increases.


G = H -TS

PV = nRT

Electronegativity: NOF Cl Kr

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