Chapter 10: Photonics basicssolutions

1.[3 marks](VCE 2004 Sample Q1)

The Light given off by the HeNe laser will be of one wavelength, in phase and described as coherent.

The red filter will allow more than one wavelength through, and the photons produced will not be in phase with each other. So the light produced by the incandescent lamp will not produce coherent light.

2.[3 marks](VCE 2004 Sample Q2)

The electric stove element would have a lot of infra-red light or light greater than 800nm.

 B - stove element (ANS)

The blue light will have photons around the 500nm.

A - blue light (ANS)

The sodium lamp has only two wavelengths emitted in the visible spectrum.

 C- sodium lamp (ANS)

3.[4 marks](VCE 2004 Sample Q3)

The electrons are in the conduction band and as they drop to the valence band they emit a photon.

4.[2 marks](VCE 2004 Sample Q4)

Use the average wavelength of 615nm, then,

,

(ANS)

5.[3 marks](VCE 2004 Pilot Q1)

The LASER and the Mercury vapour lamp do not have a continuous spectrum and would look like lines.

The LED has a smaller spread than the graphs that are shown. So the remaining light sources are the Sun, Incandescent globe, and a candle.

Source 1 is the Sun, since it is the only light source that has light in the UV part of the spectrum > 400nm.

Source 2 is the incandescent globe, since it is hotter than the candle and more likely to have shorter wavelengths emitted.

Source 3 must be the candle, having most of the light given off in the infra-red part of the spectrum.

Source 1 Sun (ANS)

Source 2 Incandescent globe (ANS)

Source 3 Candle (ANS)

6.[3 marks](VCE 2004 Pilot Q2)

The atoms in the laser must be pumped by an external energy source. This excites the electrons from the ground state into a higher energy level. Random emission of photons occurs as electrons randomly drop from the higher energy levels. Some of these photons collide with excited electrons from other atoms at the higher energy level. When a photon collides with an electron it stimulates the electron to drop to a lower energy level. When the electron drops to the lower energy level it emits a photon that is in phase, in the same direction, and has the same wavelength as the original photon. These photons then move off to stimulate more electrons and the process in continued. The light that is emitted is called coherent light.

7.[3 marks](VCE 2004 Pilot Q3)

Since the LED has a band gap of 1.9eV, the photons that are emitted have 1.9eV of energy.

Use ,

λ = 6.5 x 10-7 m (ANS)

8.[2 marks](VCE 2004 Pilot Q4)

The wavelength of light is determined by the band gap energy of the LED, which is determined by the materials that were used to make the LED. Since the Band gap is fixed at 1.9eV the wavelength is fixed at 6.5 x 10-7 m. Increasing the voltage of the power supply will increase the voltage across the resistance, which will increase the current through the LED.

 D (ANS)

9.[2 marks](VCE 2005 Q1)

The current in the globe causes the filament to heat up. When the filament is heated the atoms and molecules move faster.

The movement of atoms causes free electrons to accelerate and decelerate. During this process energy is given off in the form of photons.

These photons have a wide range of random frequencies and are incoherent.

This thermal/random excitation of electrons leads to the emission of a broad/continuous spectrum, as shown on the graph.

10.[2 marks](2005 Q2)

The light from incandescent sources will always be incoherent, as the EM radiations are random.

 B (ANS)

11.[3 marks](VCE 2005 Q1)

A laser produces [coherent] light. The input power to the laser produces [a population inversion] in the electron energies of the gas atoms. The atoms are stimulated to release their energy by interacting with [photons of the same] energy.

12.[3 marks](VCE 2005 Q2)

Spectrum A: sodium vapour lamp,

Since sodium only emits two wavelengths in the visible spectrum.

Spectrum B: red LED
Since an LED emits a small range of wavelengths with one of the wavelengths being the peak wavelength.

Spectrum C: red hot slab of iron.

Since most of the light that is emitted is coming from the infra red region.

Spectrum A Sodium vapour lamp (ANS)

Spectrum Bred LED (ANS)

Spectrum Cred hot slab of iron (ANS)

13.[2 marks](VCE 2005 Q3)

The diodes are made from materials that have different energy bands that the electrons are in. As the electrons move through the LED they drop from one band to another the difference in energy is known as the band gap energy. As the electrons drop from one band to another they emit light.

14.[3 marks](VCE 2005 Q4)

Use ,

λ = 5.9 x 10-7 m (ANS)

15.[2 marks](VCE 2006 Q1)

The light from a candle is a result of the thermal excitation of atoms. This causes the electrons to accelerate and decelerate. During this process energy is given off in the form of photons.

These photons have a wide range of random frequencies and are incoherent.

This thermal/random excitation of electrons leads to the emission of a broad/continuous spectrum.

16.[2 marks](VCE 2006 Q2)

Since the wavelength is.

Use

,

.

 A (ANS)

17.[2 marks](VCE 2006 Q3)

This is basically asking you to put the energy of each photon in descending order. Since blue light has the smallest wavelength, it has the most energy according to.

Followed by green photons then red.

 A (ANS)

18.[3 marks](VCE 2006 Q4)

The photons in a HeNe laser are in phase, of the same wavelength and travel in a parallel path.

19.[3 marks](VCE 2007 Q1)

A particular laser operates by using a voltage to initially excite atoms into a higher (stable) energy state. This process is known as [ population inversion ].

The excited atoms are then stimulated to de-excite by emitting [ photons ] by interacting with a photon of [ the same ] energy than the elevated electron energy level.

20.[2 marks](VCE 2007 Q2)

The voltage needs to be high enough to supply the electrons with enough energy to get to an allowable state. As the electrons return from the excited energy band they emit photons.

21.[3 marks](VCE 2007 Q3)

The energy that the voltage supplies the LED will be equal to the energy that the electrons will emit as they release a photon. So the 2.64V means there is 2.64eV supplied to each electron and this is the energy that is emitted as a photon. We then use:

 (ANS)

22.[2 marks](VCE 2007 Q4)

The voltage is less because the energy required to emit a red photo is less than the energy of a blue photon. Since there is less voltage across the LED there is more voltage across the resistor and therefore there is more current flowing through the circuit. There will be light emitted since there is a voltage and a current flowing through the LED.

 A (ANS)

23.[2 marks](VCE 2008 Q1)

The Laser has a very small spectrum (Monochromatic) so it is graph 1.

The Mercury vapour lamp has discrete wavelengths so it is graph 2.

The LED has a small range of wavelengths emitted with a peak value in the middle so it is graph 3.

 D (ANS)

24.[2 marks](VCE 2008 Q2)

 B (ANS)

25.[2 marks](VCE 2008 Q3)

The Laser has a very small spectrum (Monochromatic).

The LED has a small range of wavelengths emitted.

 c (ANS)

26.[2 marks](VCE 2008 Q4)

n2 is the refractive index of the cladding, 1.41.

n1 is the refractive index of the core, 1.48.

 C (ANS)