Week 7 - Optical Properties Of Materials Flashcards
Why does CFL have a band gap of 3.9eV not 9.3eV?
CFL light operates by UV light emitted from the ahh atoms, hitting the phosphor and exiting photons of visible light. UV light is damaging to humans and for the CFL to be really safe there needs to be some safeguards in place to ensure the Uv light doesn’t get out of the globe.
The glass bulb is supposed to be the safety barrier, the UV light has a wavelength around 4.88eV. It is 3.9eV so all good. If it had a band gap of 9.3eV than the UV light would be able to pass through the glass.
The optical properties of nonmetallic materials depend strongly in their band gap. What is the minimum band gap that a material must have for it to appear transparent and colourless?
All of the light of the visible spectrum must be able to pass through it. This means none can be absorbed by the material. The lowest wavelength of the light in the visible spectrum corresponds to 400nm with an energy of 3.1 eV. So long as the material has a band gap larger than 3.1eV there is a chance that it could be transparent because light can go through.
What is the maximum band gap that a material may exhibit and remain opaque?
To be opaque nine of the light of the visible spectrum must be able to pass through. That means even light of the highest wavelength must be absorbed. Red light of energy 1.77eV is the lowest energy light in the spectrum. If this can be absorbed than all other colours with higher energies can also be absorbed. Therefore we would expect the maximum band gap an opaque material may exhibit is 1.77eV.
Cadmium Sulfide is an important semiconductor material with a band gap of 2.4eV. If no visible light is emitted as a consequent of electron relaxations in the material, what colour do you expect CdS to be?
Yellowish orange red colour
Ruby and sapphire are both minerals of Al2O3. This has a large band gap of 8eV and one could reasonably expect them to be transparent or translucent. Yet they are red and blue. Explain why ruby and sapphire are coloured.
This is because of the impurities in the materials. They are not 100% Al2O3. The impurities introduce special energy levels within the band gap. The red and blue colours arise from the relaxation of electrons from these special energy levels back to the valence band, and the selective absorbance of certain frequencies of viable light in the excitation of valence electrons to these special acceptor sites introduced by the impurity atoms.
Metals tend to reflect almost all of the visible light shone on them. However, different metals can still exhibit different colours. Silver and gold are 2 good examples. One is silvery in colour and the other is orange red. Explain how metals that reflect almost all their light can have different colours.
The reason is that reflection is actually a process of absorption and then almost immediate re-emission of the light energy. In the case of something like Ag, the spectrum of energies emitted during relaxation of the excited electrons is basically the same as that which was absorbed. For Au and Cu, some of the wavelengths of light give rise to excitations whose subsequent relaxation follows a multi-step path, i.e they don’t relax following a Single jump back to the valence band and an emission of a light photon of energy similar to that which cause the excitation. These multi step relaxations can cause energy dissipation of heat, or other lower energy light photons.
In the case of Au and Cu, these multi step relations occur preferentially for higher energy light, and as a result it is the longer wavelength light that is emitted during the relaxations characterising the reflection process.
What are the three basic processes for electromagnetic radiation when it hits a piece of matter?
Reflected
Absorbed
Transmitted
What is the difference between transparency and translucency? Explain two ways that normal window glass could be made translucent and explain the reasoning behind your suggestions.
Both translucency and transparency require the light to be transmitted through the material. However, for transparency we are able to see a clear image on the other side of the glass and in the case of translucency we cannot- the light is transmitted but the image is blurred.
Normal transparent window glass can be made translucent by roughing one of the surfaces, such as by sand blasting and this is what is done commercially to make frosted glass. The idea is that the reflection that occurs at the rough surfaces causes the light to lose its organisation and hence the image is lost even though the light is still transmitted.
