Lecture 2: Generation of light

Question 1

How is light emitted

Answer 1

• Heat sources
• Luminescence (no heat)
• Radioactivity (Gamma rays)

Question 2

What are the types of luminescence?

Answer 2

Chemiluminescence, Electroluminescence, Mechanoluminescence, Photoluminescence

Question 3

What is Chemiluminescence

Answer 3

Describes the emission of light that occurs from chemical reactions that produce high amounts of energy lost in the form of photons

A + B → AB* → Products + Light

Question 4

What is Electroluminescence

Answer 4

result of an electric current: LEDs, OLEDs, most lasers

Question 5

What is Mechanoluminescence

Answer 5

result of mechanical
stress: triboluminescence, piezoluminescence, sonoluminescence

Question 6

What is Photoluminescence

Answer 6

result of the absorption of
another photon: fluorescence, phosphorescence, Raman…

Question 7

What is bioluminescence ?

Answer 7

• A form of chemiluminescence
• Reactions are ATP-driven, and relate to Luciferin
• General formula:
  Luciferin + O2 –> Oxyluciferin + Light (catalyzed by luciferase)

Question 8

How does bioluminescence work?

Answer 8

ATP is hydrolysed by luciferase enzyme to oxidize luciferin (exciting it), and upon the compound’s return to its ground state, a photon is released

Question 9

What is fluorescence?

Answer 9

Emission of light from an excited state created by absorption of light. (lecture definition)

Can be described as a property of a compound to absorb light in a certain range of wavelengths and emit a different wavelength of light, usually longer. (Tutorial definition)

Question 10

Uses of chemiluminescence

Answer 10

Glowsticks, forensics (“The iron from the hemoglobin in the blood serves as a catalyst for the chemiluminescence reaction”: luminol + hydrogen peroxide –> 3-APA + light)

Question 11

Name a disadvantage of fluorescence

Answer 11

Fluorescence of a biomolecule has a variable (but usually short) mean lifetime or half-life.

Question 12

What is luciferin?

Answer 12

Light producing molecule whose structure varies from species to species

Question 13

Name fluorescence parameters

Answer 13

quantum yield (=nbr photons emitted/nbr photons absorbed), spectrum, lifetime, Stokes shift (difference between peaks of absorption and emission spectrum)

Question 14

What’s the mean lifetime ?

Answer 14

Quantity denoted by the exponential time constant, which is the time it takes for the quantity to be reduced to 1/e of its initial value. (r = 1/lambda)

Question 15

What’s phosphorescense ?

Answer 15

A phosphorescent material does not immediately re-emit the absorbed radiation. The transition between excited and ground states happens very slowly (triplet state before being emitted as light).
Light is also re-emitted at a lower intensity intensity for up to several hours after original excitation.

Question 16

What’s the photoelectric effect ?

Answer 16

The emission of electrons when EM radiation hits a material. Electrons emitted are called photoelectrons. Photoemission is most common in metals and conductors.