Photonics basics

• describe the production of incoherent light from wide spectrum light sources, including the Sun, light bulbs, and candles, in terms of the random thermal motion of valence electrons when atoms collide;

• explain light emission from light emitting diodes (LEDs) as the energy emitted when electronsmove from the conduction band of a semiconductor to the valence band (knowledge about n and p type materials is not required);

• analyse the effect of band gap energy on LED colour, Eg= hf = hc/λ;

• describe the production of light by coherent light sources (lasers), in terms of light amplification via stimulation from external photons;

• describe laser light in terms of coherence, wavelength and phase;

1.[3 marks](2004 Sample Q1)

One way of producing an intense narrow beam of red light is to use a Helium-Neon (HeNe) laser. Alternatively, an incandescent lamp coupled with a red filter and a series of lenses arranged to focus the beam can also produce an intense beam of light. Compare the characteristics of the red light in these two cases by describing how the light is produced. Use each of the following keywords in your answer.

i. Wavelength

ii. Coherence

iii. Phase

2.[3 marks](2004 Sample Q2)

The spectrum for sunlight is shown below. Use this as a reference to identify which of the spectra (A–C) best matches the following sources of light.

Electric stove element Blue theatre light Sodium vapour lamp (yellow)

3.[4 marks](2004 Sample Q3)

Briefly, with a diagram, describe how the light is produced in a LED.

4.[2 marks](2004 Sample Q4)

A recent invention is that of the super-bright indium-gallium-aluminum-phosphide (IGAP) Light Emitting Diode (LED). These LEDs are now finding wide application in many devices.

The IGAP LED glows predominantly red, with wavelengths in the range 610–620 nm. Use this information to estimate the band-gap energy in the LED.

(h = 4.14 × 10-15eV s )

5.[3 marks](2004 Pilot Q1)

Visible light is typically in the range 400–700 nm. Shown below are the individual wavelength properties of three different light sources. The dashed-vertical lines indicate the range of visible wavelengths.

Identify each source of light from the six options below, and write your choice clearly in the boxes.

• candle• 100 W incandescent globe

• laser• LED (Light Emitting Diode)

• mercury vapour lamp• sunlight

source 1

source 2

source 3

6.[3 marks](2004 Pilot Q2)

Describe how light is produced in a laser. Include in your description a concise term for the type of light that is produced by a laser.

7.[3 marks](2004 Pilot Q3)

The light output of an LED of band gap energy 1.9 eV is being investigated with a variable DC power supply. The power supply voltage is slowly increased and once the potential across the LED reaches 1.9 V, a current of 5 mA flows, and the LED begins to glow.

Calculate the wavelength of the light emitted by this LED.

8.[2 marks](2004 Pilot Q4)

The power supply voltage applied is now increased slightly.

Which one or more of the following statements is/are true?

A. The wavelength of the light emitted increases.

B. The wavelength of the light emitted decreases.

C. The wavelength of the light emitted remains the same.

D. The current flow through the LED increases.

E. The current flow through the LED decreases.

F. The current flow through the LED remains the same.

9.[2 marks](2005 Q1)

The spectrum of wavelengths produced by a particular incandescent light globe is shown below.

Describe the mechanism by which light is produced in an incandescent light globe.

10.[2 marks](2005 Q2)

The light produced by an incandescent light globe can best be described as

A. / coherent. / B. / incoherent. / C. / monochromatic. / D. / in phase.

11.[3 marks](2005 Q1)

In the sentences below, options are given within the brackets. Only one of the options will be correct. Circle the best option.

A laser produces [coherent / multi-modal / wide spectrum] light. The input powerto the laser produces [coherence / a population inversion / ionisation] in the electronenergies of the gas atoms. The atoms are stimulated to release their energy byinteracting with [electrons of the same / photons of the same / photons of higher]energy.

12.[3 marks](2005 Q2)

The spectra of wavelengths produced by three different light sources are shown below.

A selection of light sources is listed below.

sodium vapour lamp red hot slab of iron

incandescent globe red LED

blue laser candle

For each spectrum (A - C), identify the most likely light source. Write the corresponding light source in theappropriate box below.

Spectrum A

Spectrum B

Spectrum C

13.[2 marks](2005 Q3)

Explain how light is produced in a LED (Light Emitting Diode). Your explanation should include reference to the band gap.

14.[3 marks](2005 Q4)

The band gap in a LED is 2.1 eV. Calculate the average wavelength of light emitted by this LED.

15.[2 marks](VCE 2006 Q1)

The light from a candle can best be described as

A. coherent, arising from the vibrations of electrons.

B. incoherent, arising only from the transition of electrons in excited energy levels falling to lower energy levels.

C. coherent, arising only from the transition of electrons in excited energy levels falling to lower energylevels.

D. incoherent, arising from the vibrations of electrons.

16.[2 marks](2006 Q2)

An LED emits light of wavelength 5.8 × 10-7m.

Which one of the following (A - D below) is the best estimate of the band gap of the semi-conductor materialin this LED?

A. / 2.14 eV / B. / 21.4 eV / C. / 214 eV / D. / 2.14 keV

17.[2 marks](2006 Q3)

Which one of the following (A - D below) indicates the correct sequence of LED colours in decreasing value of band gap?

A. / blue, green, red / B. / red, blue, green
C. / green, red, blue / D. / blue, red, green

18.[3 marks](2006 Q4)

The light produced by a red LED and a red HeNe laser is being compared.

Identify three features of the red HeNe laser light that are superior to the light produced by the red LED.

19.[3 marks](2007 Q1)

In the paragraph below, options to complete each sentence are given within the brackets. Circle the correct option in each case.

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

The excited atoms are then stimulated to de-excite by emitting [ X-rays / electrons / photons ]by interacting with a photon of [ lower / the same / higher ] energy than the elevated electronenergy level.

Use the following information to answer Questions 17 - 19.

20.[2 marks](2007 Q2)

Chris is testing LEDs (Light Emitting Diodes).

She has a LED which emits blue light (blue LED) and another which emits red light (red LED).

Blue light has a higher frequency than red light.She uses the circuit shown below.

With the blue LED in the circuit the supply voltage is gradually increased. The LED does not emit light until the voltage reading on the voltmeter is 2.64 V. At this time there is a current of 5.00 mA read on the ammeter, A.

Explain why the blue LED needs 2.64 V to emit light.

21.[3 marks](2007 Q3)

Assuming an ideal diode, calculate the wavelength of the blue light emitted by the blue LED.

22.[2 marks](VCE 2007 Q4)

The blue LED is now replaced with the red LED. The supply voltage is left unchanged.

Which one of the following sets of observations (A - D) will now describe the situation?

Red LED / Voltage across LED / Current through A
A. / Light emitted / Less than 2.64 V / Greater than 5.00 mA
B. / Light emitted / Greater than 2.64 V / Less than 5.00 mA
C. / No light emitted / Less than 2.64 V / Greater than 5.00 mA
D. / No light emitted / Greater than 2.64 V / Less than 5.00 mA

23.[2 marks](VCE 2008 Q1)

The spectra in the visible region produced by three light sources are shown in graphs 1, 2 and 3 below.

The light sources are a laser, a LED and a mercury vapour lamp (not in this order).

Which one of the following boxes correctly matches each graph with its source?

LED / laser mercury / vapour lamp
A. / graph 1 / graph 3 / graph 2
B. / graph 2 / graph 1 / graph 3
C. / graph 3 / graph 2 / graph 1
D. / graph 3 / graph 1 / graph 2

The band gap of a LED is 1.80 eV.

24.[2 marks](VCE 2008 Q2)

Which one of the following best gives the wavelength of light emitted by this LED?

A. 110 nm

B. 690 nm

C. 6.90 × 10-7 nm

D. 1.10 × 10-16 nm

25.[2 marks](VCE 2008 Q3)

Comparing light from a laser and from a LED, which one of the following statements is true?

A. Light from a LED is coherent but light from a laser is incoherent.

B. Light from both a LED and a laser is coherent.

C. Light from a laser has a narrow range of wavelengths (more monochromatic) than light from a LED.

D. Light from a LED is pulsed but light from a laser is continuous.

26.[2 marks](VCE 2008 Q4)

Which one of the following statements best describes stimulated emission in a laser?

A. Atoms are raised to a metastable state.

B. A population inversion is created.

C. Photons interact with atoms in a metastable state causing them to release their energy as photons.

D. Photons interact with the atoms in a metastable state to cause emission of electrons.