152.Light Sources

Physics Factsheet www.curriculum-press.co.uk

Number 152

Light Sources However tungsten filament lamps used in homes actually operate at about 3300K.

For thousands of years, the only significant light source available to us came from a bright sphere that can occasionally be seen in the sky. At the individual level, lamps containing vegetable or mineral oils (or more recently, gas) could be burned to provide lighting for the home or street.

Example 2 : Why is the tungsten filament restricted to 3300K? Answer: Tungsten melts at a temperature lower than 4000K.

The advent of electricity led to the creation of a whole new family of light sources available to us today. They work on a variety of Physics principles, and each has its advantages and disadvantages. This factsheet will take a look at some of these sources.

The graph shows how inefficient incandescent lamps are. Only a fraction of the emitted radiation falls within the visible region; most emitted radiation is in the infrared spectrum. The lamp would be most efficient at 6600K, but the filament would melt.

Incandescent lamps

Incandescent lamps can be only 7% efficient in turning electrical energy into light energy.

These were the first mass-produced electric lamps. Any heated material emits electromagnetic radiation. A “blackbody” emits (and absorbs) all wavelengths perfectly. Its name comes from the fact that it absorbs all radiation incident upon it, reflecting none, and so should appear black.

Example 3: Find the input power required for a filament lamp to emit visible light at a power of 12W. Answer:

A lamp filament is made from resistance wire, and glows brightly when current is passed through it. Electrical Energy

Thermal Energy

12 =

Electro-magnetic Energy

7 × P 100

therefore P =

100 × 12 = 170 W 7

The lamp in more detail

If we assume that the filament acts approximately as a blackbody, the radiation emitted would depend on the temperature of the filament.

filament

argon glass

6000 K

Intensity of emitted radiation

supports 4000 K base 2000 K contacts

uv

visible

IR

• wavelength of emitted radiation

• •

Wien’s Law tells us that the peak wavelength emitted is inversely proportional to the temperature of the filament.

λ peak =

W where W = 2.9×10-3 mK T

•

Example1 : From the graph, estimate a reasonable filament temperature for a lamp designed (a) as a visible light source (b) as an infrared light source Answer: (a) 6000K

Tungsten is generally used as the filament. It has a high melting point and reasonable mechanical strength All oxygen must be removed from the glass bulb. At high temperatures the filament would react with the oxygen (combustion), and would quickly burn out. The oxygen is replaced with an inert gas e.g. argon. As tungsten atoms evaporate from the surface of the filament, many will rebound off the argon atoms and rejoin the filament, extending its life. The filament is wound into a coiled coil. This greatly increases the surface area emitting radiation. In a domestic lamp, the length of the filament wire could be stretched out to over a metre.

coiled coil filament

(b) 3000K

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Physics Factsheet

152. Light Sources Halogen lamps

Cold Cathode Dischage Tube

These are incandescent lamps where the filament can operate at a higher temperature. A halogen, such as iodine, surrounds the filament. The envelope is quartz (or hard glass). As tungsten atoms evaporate from the filament and reach the quartz, they react with the halogen to form tungsten halide. When these halide molecules touch the very hot filament, the molecule splits apart and the tungsten recombines with the filament.

Low pressure Gas

High Voltage supply

Incandescent lamps are very inefficient. The vast majority of the radiation is emitted in the infrared region.

The fluorescent tube is the most common gas discharge lamp, but others are used e.g. for street lighting (where the typical sodium yellow/orange light is emitted).

Gas discharge lamps A gas discharge lamp emits a line spectrum from the excitation of gas atoms in the lamp. The atoms are excited from their ground state to higher levels.

Fluorescent tube A fluorescent tube provides a source of light, arising from spontaneous emission of electromagnetic radiation from excited mercury atoms.

As they spontaneously drop back (in one or more steps) to the ground state, they emit photons whose frequency is dependent on the magnitude of the gap between allowed energy levels (E = hf). Different gases will emit different line spectra.

The tube contains a small amount of mercury in a low-pressure inert gas, such as argon. As the argon is bombarded by electrons from the electrodes, it is ionised, forming a plasma of positive ions and electrons. These are accelerated by the potential difference across the electrodes. As they collide with the mercury atoms, they excite them to a higher energy level.

Energy (eV) (0.0) ionisation level

The mercury atoms spontaneously drop back to their ground state, emitting energy as ultraviolet radiation (as we have shown). This is absorbed by the fluorescent paint on the inside of the glass tube, and visible light is emitted.

(-5.74) first excited state

visable light (-10.38) ground state mercury energy levels uv light

Example3 : Find the wavelength of the light emitted from a transition from the first excited level to the ground state with mercury.

mercury atom

As less heating occurs in a fluorescent lamp, it can be four or five times more efficient than a filament lamp in generating light from electricity. Even so, many fluorescent lamps are still less than 50% efficient.

Answer: hc E= λ 4.64 × 1.6 × 10-19 =

6.6 × 10-35 × 3.0 × 108 λ

Example 4: Suggest two advantages of fluorescent tubes over filament lamps.

l = 2.7 × 10-7 m (ultraviolet region)

Answer: They last several times longer than filament lamps. They light the room more evenly because of the length of the tube. They save money and make the room more comfortable because less heat is being produced.

Gas discharge emission spectrum Exam Hint:- Exam questions often require knowledge of both strengths and weaknesses. Some of the strengths of fluorescent lamps are detailed above. However fluorescent lamps do not produce a broad spectrum of light, and some people are sensitive to the flicker associated with them (as they are operated from the mains a.c. supply). This can lead to headaches and problems with vision.

Usually the excitation is provided by an electrical source. A lowpressure inert gas in the tube becomes ionised. The accelerating ions and electrons then collide with the atoms of the emission gas, exciting the atoms as described.

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Physics Factsheet

152. Light Sources Advantages of LEDs include: o High efficiency o Small size o Mechanical strength o Life span (50,000 - 60,000 hours) o Full dimmability (unlike fluorescent lamps) o Mercury-free

Compact fluorescent lamp (CFL) Incandescent lamps are now being replaced by CFLs. These “energy efficient” lamps are direct replacements for incandescent lamps, and work in the same way as fluorescent tubes. They should last longer and be less wasteful of energy than incandescent lamps, but they suffer from the same problems as fluorescent tubes. Many people dislike them intensely. In the future they will probably be replaced by lamps based on LEDs (light emitting diodes).

And some disadvantages: o Low power (producing high power LEDs is difficult and expensive) o Directional (not ideal for domestic lighting) o Compatibility with a.c. mains is difficult

Light emitting diode An LED is a semiconductor diode that emits visible light when a current flows through it. When a standard p-n diode is forward-biased, current flows through it, and holes and electrons combine in the depletion layer at the junction of the two materials. As they combine, energy is released - this energy may be emitted as a photon. For certain materials, the photon will be in the visible light region, and the diode will be an LED. p-type

Lasers A laser is a device that emits light through “light amplification by stimulated emission of radiation”. The light beam emitted is narrow, monochromatic (single wavelength), and usually intense. This intensity is a result of the emitted light beam being coherent – the photons are all in phase.

n-type

Energy input (pumping) _

+

Light Laser medium

For a given material, this photon would be at a specific wavelength related to the drop in energy levels as the electron and hole combined. To make light of a different colour, different materials would have to be used in the LED. The diagram shows light emitted from three different types of LED – notice the very narrow range of wavelengths emitted by each sort.

The emitting material may be solid (ruby) or gas (helium-neon). In either case the atoms are excited (pumped) into a higher energy state, often by a bright flash of light. As they drop down to the ground state they give off photons (electromagnetic radiation). Some of these photons will reflect back and forth inside the laser, stimulating the emission of more photons from other excited atoms. These photons will be emitted in phase. The resulting output is an intense, highly directional beam of light.

yellow red Intensity

Partial reflector

Total reflector

e.m. radiation

blue

Laser light is very different from natural light. Here is a summary of the three important differences: 1. The laser light is monochromatic. It is comprised of one specific wavelength, dependent on the drop between energy levels in the atom. 2. The laser light is coherent. All the photons released are travelling in phase with one another. 3. The light is very directional. The beam is very narrow and intense. It does not spread out significantly as it travels – it remains intense over a distance.

wavelength

LEDs are low-voltage devices, and have a very small forward resistance, with the same voltage drop (perhaps 7 volts d.c.) for widely varying currents. Consequently they cannot connect directly with mains voltage without becoming damaged. The mains voltage is both far too high, and is alternating. To make it possible to use LEDs with mains voltage: • the supply must be full wave rectified and smoothed • a string of LEDs must be wired in series, to share the mains voltage

Depending on the solid or gas selected, the wavelength of the laser light can vary from the infrared through to the ultraviolet. Some examples are shown in the table: Laser type Argon fluoride (UV) Argon (blue) Helium neon (green) Ruby (red) Carbon dioxide (IR)

A single series string would work, but one LED failure would extinguish the whole string. Parallel strings increase reliability. In practice usually 3 strings or more are used. LEDs come in many colours, depending on the crystals used to manufacture them. Combinations of LEDs can be used to produce white light.

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Wavelength (nm) 193 488 543 694 10600

152. Light Sources Uses of lasers • • • • •

surgery cutting materials in industry laser printers light displays marking targets (weapons)

Exam Hint:- For devices like lasers, you should be able to quote a number of uses, and also be able to discuss difficulties or problems. For lasers, one problem is that the light is so intense it can damage the retina.

Prcatice Questions: 1. An incandescent lamp operates at 5500K. Find the peak wavelength of emitted light. 2. An incandescent lamp is 7% efficient. A mains supply (230Va.c. rms) pushes a current of 1.4A rms through the filament. (a) find the input power (b) find the power of the visible light emitted (in Watts) (c) find the wasted energy (in kWh) if the lamp is run for 6.0 hours. 3. A mains filament lamp (230V a.c. rms) draws an rms current of 0.84A. It emits visible light at a power of 23W. Calculate the efficiency of the lamp, to the nearest percentage. 4. Give two advantages and two disadvantages of using a fluorescent lamp rather than an incandescent lamp. 5. A laser beam of power 0.30mW emits light at wavelength 620nm. Calculate the number of photons emitted each second (to 2s.f.) (Make use of the expression E=hf, with h=6.6×10-34Js.)

Answers 2.9 × 10-3 = 5.3 × 10-7 m 5500 2. (a) 322W (b) 22.5W (c) (322 – 22.5) / 1000 × 6 = 1.8kWh

1. λ peak =

3. Power in = 230 × 0.84 = 193W Useful power out = 23W Efficiency = (23 / 193) × 100 = 12% 4. Advantages: • longer life • more efficient • less heating of the room • lower operating cost • more even light across room Disadvantages: • higher initial cost • mercury (poison) used in manufacture • disposal of lamp difficult (because of mercury) • flicker • do not produce broad spectrum of light 5. Photons per second = Energy per second / Photon energy Number =

energy/sec 0.3 × 10-3 = 9.4 × 1014 = hc/λ 6.6×10-34 × 3.0×108/620×10-9

Number of photons per second = 9.4×1014.

Acknowledgements: This Physics Factsheet was researched and written by Paul Freeman The 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

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