Light absorption

Question 1

What are reasons for light attenuation?

Answer 1

• absorption
• scattering

Question 2

Explain light absorption on the atomic scale.

Answer 2

E of the absorbed light photons is used for the excitation of the outer shell e^-

Question 3

What is the formula of the general attenuation law?

Answer 3

J = J₀ * e^-μ*x

• J = attenuated intensity
• J₀ = incident intensity
• μ = attenuation coefficient (here: absorption coefficient)
• x = layer thickness

⇒ attenuation is dependent on layer thickness

Question 4

What is absorbance?

Other names?

Formula?

Answer 4

quantity characterizing the extent of absorbed light

also: extinction (E), optical density (OD)

A = lg J₀/J = lg e * μ * x

• J₀ = intensity of perpendicularly incident light
• J = intensity of perpendicularly transmitted light
• x = layer thickness

Question 5

What is transmittance?

Answer 5

quantity characterizing the extent of transmitted light (in percent)

T = J/J₀ * 100%

Question 6

What is an absorption spectrum?

What can you read from the peaks?

Answer 6

absorbance vs. wavelength

⇒ height of the curves proportional to molar extinction coefficient

⇒ locations of the maxima related to e^- excitation energies characteristic for the molecule structure

BUT: height depends on concentrations

Question 7

Why do absorption spectra of solutions never have spectral lines of discrete peaks of quanta, but always wide broadened bands?

Answer 7

• molecules of the solution are spatially close to solvent molecules
• extensive thermal motion of environment (at room temperature)
• E levels of single molecules are slightly changed by interactions with the environment (solvent)

⇒ band spectrum

Question 8

What is the energy of a photon?

Answer 8

E = h*f = h * c/λ

• h = Planck’s constant
• f = frequency
• c = speed of light
• λ = wavelength

Question 9

What does the Beer-Lambert law state?

Formula.

Answer 9

valid for dilute solutions, stating that absorbance of the solution is proportional to concentration and to the thickness of the material

lg (J₀/J) = ε(λ) * c * x

• lg J0/J = A = absorbance
• ε(λ) = molar extinction coefficient
• c = concentration of the solution
• x = thickness

Question 10

The molar extinction coefficient and absorption coefficient depend on which parameters … ?

Answer 10

• type of material
• wavelength of light

Question 11

What does a spectrophotometer do?

Answer 11

compares absorption spectra of a reference solution and sample solution

→ concentration determination

Question 12

What are complementary colors?

Answer 12

a pair of colors that gives white light as a result of additive color mixing

examples:

• yellow-blue
• green-red

⇒ color of a material apparent will be the complementary color of the absorbed one