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Number 257

FactsheetPhysicswww.curriculum-press.co.uk

Comparison of Light SourcesDue to its versatility, electricity is now an essential commodity, not least due to how easy it is to convert electrical energy into light energy. Just because a process is easy, however, doesn’t make it efficient or effective. The 100 years since the advent of domestic electricity has seen a range of innovations in electric lighting, and in this Factsheet we will examine some of the key types of lighting and bulbs.

Filament lamps

Bayonet cap

Fuse

Glass pinch

Glass support

FilamentInert gas filling

How it works These were the first type of electric light, developed simultaneously by Joseph Swan and Thomas Edison, who together formed the EdiSwan Lighting Company.

The bulb consists of a thin glass container with a thin coiled filament inside, designed to get hot enough to glow as a current passes through it. To glow visibly (rather than just getting hot and emitting infra-red radiation) the filament has to be at a high temperature (around 3000K or 2700°C) and therefore made from a conductive material with a high melting point. The metal tungsten is the first choice, although some lamps have carbon or graphite filaments. Being thin, this filament is then supported by a metal wire at each end which act as electrical contacts for the filament.

These design limitations give rise to a host of problems;• At such high temperatures, the tungsten or carbon filament

would quickly react with any oxygen surrounding it, so the bulb must be either evacuated of all air or filled with an unreactive/inert gas at low pressure instead, such as Argon.

• As it is, the high temperature that the filament must be at causes the atoms of tungsten to gradually ‘evaporate’ away from the filament, darkening the inside surface of the bulb and making the filament thinner over time until ultimately it snaps. This gives filament lamps a short lifetime for usage.

• The high operating temperature of the bulb also gives rise to another, less visible problem.

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4

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200 400 600 800 1000Wavelength (Nanometers)

Relative Intensity

Infrared

TF = 2850 K

TF = 3200 K

TF = 3400 K

Tungsten lamp emission spectrum

This chart compares the intensity of the different colours or wavelengths produced by a type of filament bulb. There are two key things to notice here.

Firstly, the most prominent colours are always reds, oranges, and yellows. The filament has to be at a higher temperature to produce any of the shorter wavelength (blue and violet) colours. This means the light from filament lamps seems warm, as the ‘warm’ colours of red, orange and yellow are produced in the greater amounts. The temperature of the filament can easily be adjusted by simply controlling the current through the bulb, so the bulb can be dimmed very easily.

Also, however, the area under the graph represents the energy being produced and it is immediately obvious that the vast majority of energy from filament bulbs is in the infra-red region. This means the filament lamp produces mostly heat energy rather than light energy and this makes it very inefficient, and impractical in situations where surplus heat would be a problem, e.g., in a large walk-in cooler. Typically a traditional EdiSwan bulb would have an efficiency of 10% or less.

To summarise:Filament Bulbs produce light from a hot tungsten filament. They are very inefficient, but cheap to make and create a pleasant, warm yellow light. They can easily be dimmed, and their brightness controlled.

They're ideal if you want cheap bulbs, don’t mind replacing them often, and can tolerate terrible efficiency.

With these shortcomings, however, there was much opportunity for improvement.

Halogen bulbs Reflector withdichroic coating

Filament

Halogenfilamentcapsule

Visible radiation (light)

Infraredradiation(heat)

How it worksA halogen bulb is very much an upgrade to the original filament lamp. The Argon gas inside the bulb has been replaced by a halogen such as fluorine or chlorine, meaning the filament can operate at a higher temperature, and as a consequence the filament is encased in a small quartz mini-bulb or ‘capsule’ which can withstand higher temperatures although natural oils from fingers can cause hot-spots to occur on the outer-surface of the bulb and might cause it to crack or implode, so you’re advised to wear gloves or use clean paper to handle these bulbs. Halogen bulbs tend to be smaller and therefore more versatile.

257. Comparison of Light Sources Physics Factsheet

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The key question is why does a more reactive gas inside the bulb improve its performance? The clever thing here is that as the tungsten filament gets hot it reacts with the halogen molecules to form a protective layer on the outside of the filament. This stops the tungsten atoms from evaporating away so quickly and if any do, they bind to the halogen atoms and fix back onto the hot filament. The upshot of this is that the filament doesn’t get thinner as quickly and can therefore last two to three times longer before snapping. Spectral distribution of Tungsten-Halogen lamps and blackbody radiators

500 1000 1500 2000 2500Wavelength (nm) Wavelength (nm)

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flux

200 300 400 500 600 700 800 900 1000

Tungsten-Halogenlamp (3200 K)5300K Blackbodyspectral distributionSolar spectrum

2800 K3300 K

increase invisable lightoutput withcolour temperature

Additionally, because the filament can be used at a higher temperature without snapping, a greater proportion of the energy it produces is in the visible part of the spectrum (making the bulb more efficient) and a greater proportion of colours are from the green, blue and purple part of the spectrum. This gives a ‘crisp’ or ‘clean’ brilliant white light compared to the warm, yellow tint of a filament lamp. The graph shows how the relative brightness of colours compares between a normal (cooler) 2800K filament lamp and a hotter (3300K) halogen bulb.

[HINT: The shaded area shows the additional visible light produced, representing additional efficiency]

Question 1Use the data in the graph to verify the statement that “a greater proportion of the energy it produces is in the visible part of the spectrum”. (3 marks)Try to estimate the efficiency of each temperature of bulb. (3 marks)

AnswerThe energy is represented by the area under the graph line (P). For a lower-temperature filament ( shown b y t he y ellow l ine) t he area under the line is quite a small proportion of the area under the whole curve(P) – a small proportion of the light produced is visible light(P). [The opposite could be argued to gain these two marks]

For the yellow curve (filament lamp), assuming the curve is symmetrical, the section of the graph shown is approximately half of the full plot. The area under this line in the visible region is approximately a quarter of the total area seen(P) and therefore approximately one eighth of the total area, giving an estimated efficiency of 12% (any reasonable estimation would be 5%-20%)(P)

For the red curve (halogen lamp), assuming the curve is symmetrical, the section of the graph shown is more than half the full plot; it could be approximately two-thirds of the total(P). The area under the line in the visible region is approximately half of the total area seen and therefore one third of the total area, giving an estimated efficiency of 33% (any reasonable estimation would be 20%-40%)(P). [Max 3 marks]

To summarise:Halogen Bulbs are an improvement on filament lamps, using a reactive halogen gas to protect the filament giving it a hotter operating temperature, more brilliant light and longer lifetime. Ideal for bright spotlight applications.

Question 2 For several years, car manufacturers have used halogen bulbs instead of filament lamps for the main headlights in cars. Considering the necessary properties and advantages, outline why they might have done this. (4 marks)

Answer 2The main headlights in cars are to light up the road ahead (P) (rather than sidelights which just enable other motorists to see you). The bulb needs to produce a bright light and a long lifetime as replacing the bulb often will be inconvenient and difficult (P either point) as well as unsafe. Normal filament bulbs produce a less-bright light and have a shorter lifetime so Halogen bulbs would be preferred as they produce brighter light(P) and last longer(P).

CFL (Compact Fluorescent Lamps)

Base

Ballast housing

Ballast

ArgonMercury vapour

Phophor coating

Cover

Lamp

A CFL bulb is exactly what you’d expect – a compact version of the long fluorescent ‘strip-lights’ that have been used for many years, although these became notorious for problems in their infancy, such as making a low buzzing noise, taking a minute or two to get up to full brightness, and producing light with an insipid, sickly green tinge. These things have since been improved upon.

How it worksInside the lamp are two small beads of mercury and a pair of electrodes together with low-pressure argon gas. As the lamp is turned on, the beads of mercury are vapourised. Due to the low-pressure of the argon gas this mercury vapour spreads rapidly through the bulb cavity and allows a current to flow through the gas/vapour, energising the electrons in the mercury vapour and releasing certain, fixed wavelengths of light as these electrons return to their usual energy-level (ground state). Since specific wavelengths of light are produced, wastage through heat/infra-red can be reduced considerably, so this type of bulb is about four times more efficient than its filament lamp predecessors.

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257. Comparison of Light Sources

The energy levels in mercury vapour are such that most of the light produced is in the ultra-violet part of the spectrum, so a coating of white phosphor powder on the inside surface of the bulb glass is used to turn this ultra-violet light into specific visible colours. By adjusting the types of phosphors used, the colour of the bulb can be modified and improved, meaning the sickly green tinge can be limited or reduced. To get the CFL bulb to light requires an initially large amount of energy but once the mercury has been vapourised, current flows very easily through the gas and the bulb requires a comparatively small power-input to keep it running. This requires circuitry (or a ballast unit) to control the current and prevent a sudden surge once the bulb has started and its resistance is low. The ‘ballast’ units originally used electromagnets to do this which caused the low buzzing and the slow start-up time. More recently electronic ballast units have been developed which have overcome these issues.

To summarise:CFL bulbs are more efficient, especially when kept on for a long time, and now compare favourably with alternatives in terms of colour and start-up time. Ideal for applications where the bulb isn’t going to be switched on and off all the time, and where the bulb doesn’t need to be dimmed, such as for offices and businesses or for night-lighting in a car-park.

[HINT: The efficiency of CFL bulbs is greatly reduced by constantly turning them on and off – in buildings or rooms where people are likely to go in after you, it’s actually more energy-efficient to leave the lights on for them rather than switching them off as you leave!!!]

Question 3Filament lamps produce a full spread of colours so the colour-profile graph looks like the smooth curve in Fig.2. How would you expect the colour profile graph of a CFL bulb to look? (3 marks)

Answer 3 Distinct bands of separate colours (P) including at least red, green and blue (to make white) (P) in roughly equal amounts, or at least in proportions to give white light. (P) A diagram showing this could gain some or all of the marks.

Question 4 Explain if a CFL bulb need to be handled using gloves or clean paper, like a halogen bulb? (2 marks)

Answer 4 No; a CFL bulb is more efficient an d therefore produces less heat energy(P). This means the glass gets less hot and would not be prone to the hot-spots that cause breakage(P).

Led (Light-Emitting Diode) BulbsThese work in quite a similar way to a CFL bulb in that they produce light at specific wavelengths or colours by exciting electrons into higher energy levels then emitting light as the electrons return to their ‘ground-state’. The difference is that they use a thin piece of a semi-conductor material to do this rather than mercury vapour, through a process called ‘electroluminescence’. Today’s LED bulbs are a huge step forward compared to the originals from the 1960s, where the LED was so dim it was only any use as an indicator or part of a numerical display. Advances now mean that the semiconductor can be manufactured to produce specific colours of visible light, so the phosphors in a CFL bulb (that make visible light from the UV emitted by the mercury vapour) are not needed, the toxic mercury vapour isn’t needed, and the LED generates very little heat.

(+) positive

(-) negative

anvil

semi conductor

lens

whisper

high impact plastic

Other advantages are that as well as being very efficient, they also have an impressively long lifetime – anything up to 35,000 hours of usage – meaning they rarely need replacing. They also tend to fail gradually,getting dimmer and dimmer, rather than burning out suddenly. Theyare, admittedly, more expensive to buy, but the cost of running andreplacing cheaper, less-efficient bulbs soon outweighs the initial outlayfor the high-efficiency LED.

And since the LED semiconductors are so small they can even be fashioned into filament-shaped strips to mimic those nostalgic old retro carbon-filament bulbs from decades ago, without wasting most of their energy as heat.

Laser-lightAlthough not strictly speaking a common source of lighting, lasers are still a light-source. Lasers work very much in the same way as LED lights or CFL bulbs, where an electric current momentarily excites electrons from their lowest energy-level (their ground-state) to a higher energy-level; as the excited electrons return to their ground state, distinct quantities (or quanta) of energy are released corresponding to the gaps in the electron’s energy-levels. Laser, in fact, is an acronym and stands for Light Amplification by Stimulated Emission of Radiation.

+electrical coilis wound around the tube

mirror

gas moleculesfloat inside the tube

electricitymakes the gas give off light

partially silveredmirror reflects some light and letssome escape

laser beampasses out of the endof the tube

Physics Factsheet257. Comparison of Light Sources

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Lasers were originally made of inert gases like Helium and Neon (your school might well have a Helium-Neon laser, which produces light at a single wavelength of 623nm) although developments in technology now mean laser light can be emitted from special glass or semiconductors. Due to the way the light and excitation happens, with a standing wave forming inside the laser cavity, laser light is not only one single wavelength or colour, but also coherent (all the light has a fixed phase-difference) and directional. This means laser-light can be focused over huge distances, even to the moon and back. This technique is actually used to measure the distance to the Moon with an admirable degree of accuracy – a powerful laser is shone directly at the Moon’s surface at a place where a mirror was left during one of the Apollo landings. The mirror is angled exactly back toward Earth, so the journey of a laser burst from Earth to the Moon and back can be timed and used to determine how far away the Moon’s surface is.

To summarise:As a form of lighting lasers are of limited use, but as a light-source they are very useful due to the coherence and the single, specific wavelength of the colour they produce, the sharpness of their direction, and the light intensity they can be built to produce.

Typical Exam Questions1. Threelaser-pointersareboughtandfittedwithtwoidenticalAAcellswhichareeachequally-wellchargedup.Oneofthelaser-

pointers produces a red beam, the second produces a green beam, and the third a violet/ultra-violet beam. Eachlaser-pointerisswitchedonandusedequallyoftenandforthesameperiodoftime.Whichonestopsworkingfirst?Explain.

Theviolet/UVone.Violet/UVphotonsareofhigherenergythanthoseofgreenorredvisiblelight.Asaresult,electronsinthevioletlaser-pointerrequireagreateramountofenergytoexcitethemintotheirhigherenergy-level.Asthecellsrundown,thep.d.theyprovidewilldecreasequickestinthevioletlaserastheelectronsneedmoreenergybutalsotheminimumactivationp.d.toexciteelectronstoahigherenergylevelwillbegreatestinthevioletlaser,soasthep.d.getsprogressivelylessitwillgobelowthislimitfirstinthevioletlaser with the limit being so high.

2. Ifthewavelengthsofthethreecoloursare650nm(red),532nm(green),and405nm(violet),calculatetheminimump.d.neededtoproduce these colours.

Energyofexcitedelectron=e×VPhotonenergy=h×f=h×c/l=e×V

Soforred(650nm)V = h × c = 6.63 ×10-34 × 3 ×108 =1.91Ve × l 1.6 ×10-19 ×650×10-9

Similarlyforgreen(532nm)V=2.33VAndforviolet(405nm)V=3.07V

outboundpulses

inboundpulses

retroreflector array

1. Outbound pulses start out 3.5 metres in diameter, 2 cm thick.2. Atomsphere causes beam to diverge by one arcsecond or more.3. At the moon, 1 arcsecond is 1.8 km, so beam at moon is about 2 km across.4. Only about 1 in 30 million photons in this 2 km beam hit the suitcase-sized mirror.5. Each outgoing laser pulse contains 300 quadrillion photons,

6. Returning beam expands due to corner-cube diffraction.7. Returning beam divergence is about 8 arcseconds.8. Return beam footprint on earth is about 15 km across.9. About 1 in 30 million of the returning photons hit 3.5 in mirror.10. Apollo lauches 29 pulses per second.11. The round-trip time is about 2.5 seconds.12. There are about 50 pulses en-route at any moment in time.

Acknowledgements: This Physics Factsheet was researched and written by Chris Hoyle and published in September 2016 by Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU. Physics Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form or by any other means, without the prior permission of the publisher. ISSN 1351-5136