Question

Question 11- M2 (8 marks) 11.1) (2 marks) What is the maximum band gap that a material may exhibit and remain fully opaque? Band gap 11.2) (4 marks) The precious gem, Ruby, is bright red in colour. Ruby is made of Alumina (Al2O3 - band gap of 8eV) with a few Cr atoms for every 10,000 Al and O atoms. A semiconductor with a band gap of 1.8eV will also appear red in colour. Explain the origin of the 'red' colour of these two materials. 11.3) (2 marks) Silica glass is the type of glass we use for windows. It has a band gap of 3.9eV and is transparent. However, if we have a sheet of clean and flat window glass, if we look carefully we will be able to see our reflection in this 'transparent' material. Explain why you can see your reflection in this transparent material?

Answer 1

11.1) the maximum band gap that a material may exhibit and remain fully opaque is 3.6eV, this is because of the photo energy that get absorbed by the material. Which means that wavelength is equal to or greater than 344nm .

This can be obtained from the formula of E=hc/$\lambda$

E= energy

$\lambda$= wavelength

hc=plank's constant=6.626*10^-34 J-s

11.2) As you mention that ruby is red in colour because of the minerals like corudum. when chromium is replaced with aluminum in the corudum ,then chromium absorbs certain wavelength of light and reflects the red part of the visible spectrum.

Similarly , the colour of absorbed and emitted light depends on the band gap of semiconductor. The semiconductor starts appearing colour from the band gap of 1.8 to 3.1eV. The colour of emitted light from the semiconductor corresponds to band gap. the colour of absorbed light depends on the band gap energy ,because of the electron excited from the valency band to the conduction band. Thus the semiconductor with the band gaps in the infrared appears RED because they absorb the red colour of the visible light. for these kind of semiconductor the wavelength ranges from 620nm-800nm.

11.3) the reflection in the transparent materials is due to the reflection passes through the transparent objects which makes the light to bend at its surfaces. The reflection on this transparent material is distorted image , we can't see the exact image on it because of the glass properties.

basically glass is made of molecules and these molecules gets charged particles called electrons. light has the ability to oscillate these electrons , this makes the electrons to give out light. Part of this light comes out in the reverse direction causes to reflect the image.