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Senior Form Physics – Nature of Light Nature of Light Student’s Version Table of Contents Activity 1: Nature of light P.1 Activity 2: Revision: The history of the photoelectric effect P. P.3 Activity 3: Experiment 1: Generating a current using UV radiation P.6 Activity 4: Experiment 2: Generating a current from visible light, and determination of Planck’s constant P.9 Appendix 1: Proofs of law of reflection & law of refraction by geometry P.13 Appendix 2: Notes for Technician: Electroplating sodium onto the inner surface of a neon bulb Appendix 3: References P.15 P.21 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Activity 1 Nature of light Christian Huygens 1629 - 1695 Isaac Newton 1643 - 1727 Answer the following questions while listening to your teacher’s presentation about the nature of light. Q1 What is a scientific law? What is a scientific theory? What are the differences between them? Q2 If you were one of the scientists in the 17th century, would you believe the “particle theory of light” or the “wave theory of light”? Why? Hint: Which theory has a greater ability to explain the characteristics of light? P. 1 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Q3 Can the wave theory of light explain the result of the photoelectric experiment? Q4 What can we learn from the long story about the nature of light? P. 2 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Activity 2 The History of the Photoelectric Effect Thomas Young 1773 - 1829 Albert Einstein 1879 - 1955 The History of the Photoelectric effect In late 19th century it was discovered that electrons were emitted from a zinc plate when it was exposed to ultraviolet (UV) radiation. As visible light and UV are electromagnetic (EM) waves of similar nature, the result suggests that EM radiation carries energy and is capable of knocking electrons (displacing them) from metal atoms. From Young’s double slit and diffraction grating experiments, we learn that visible light behaves like a wave with wavelengths ranging from 400 nm to 700 nm. However, in the early 20th century, physicists discovered that results of the photoelectric effect cannot be explained simply by the wave nature of light. For instance, the kinetic energy of photoelectrons emitted from zinc is independent of the intensity (energy) of the incident UV radiation, but no photoelectrons are emitted if visible light, no matter how intense, is used. In 1900, Max Planck studied the radiation emitted by hot bodies. He found that the energy of vibrating molecules is an integral multiple of h, where is the frequency of vibration and h is a constant now called Planck’s constant. h = 6.626 10-34 J s Based on Planck’s work, Einstein established the quantum theory of light by assuming that light consists of particles called photons. Each photon carries an amount of energy h. In this case, increasing the frequency of the light (its colour) will increase the energy of each photon, but increasing the intensity of the light (brightness) will increase the number of photons. According to Einstein, the maximum kinetic energy of a photoelectron is given by hc KEmax h W W (1) where W is the energy required to take an electron out from a metal surface known as the work function for the metal being bombarded. So from (1), a photon can knock out an electron from the metal only if its frequency satisfies the inequality KEmax h W 0 . The energy of photons and photoelectrons is conveniently P. 3 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light expressed as units of electronvolts (eV). 1 eV is the energy gained by an electron when it is accelerated by a potential difference of 1.0 V (Figure A). + 1.0 V e0.0 V Figure A: An electron moving across a region with a potential difference of 1.0 V Revision: (i) Find the energy supplied by a dry cell of voltage 1.5 V when a charge of 1.0 C is driven across the cell in an electric circuit. (ii) Find the kinetic energy gained (J) by an electron (1.6 10-19 C) in moving from the bottom plate to the top plate in a vacuum (Fig. 1). (iii) How many electronvolts are there in 1 J? (iv) Express Planck’s constant, h, in eV s-1. P. 4 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light The following table shows the work functions of some common metals in units of electronvolts (eV). Metal Sodium Calcium Magnesium Iron Zinc Copper Chemical symbol Na Ca Mg Fe Zn Cu Work function (eV) 1.82 – 2.36* 2.52 3.66 4.67 – 4.81 3.63 4.53 – 5.10 Sodium can appear in the form of crystalline metal or polycrystalline metal. The crystal lattice of the monocrystalline sodium is continuous and unbroken to the edges of the sample. The opposite of a single crystal sample is a polycrystalline sample, which is made up of a number of smaller crystals known as crystallites. Almost all common metals are polycrystalline. For monocrystalline sodium, the work function is 1.82 eV. For polycrystalline sodium, the work function is 2.36 eV. Revision: (i) P. 5 The work function of sodium (Na) is 1.82 eV. What is the wavelength (in nm) of light required to generate photoelectrons on the metal surface? Find also the wavelength of radiation required for emission of photoelectrons from zinc with a work function of 3.63 V. © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Activity 3 Generating a current by UV radiation Generating a current by UV radiation A bulb contains neon gas in low pressure with two iron electrodes sealed inside (Figure A). The bulb glows with a distinct reddish-orange colour when the voltage across its electrodes is increased above a threshold starting level. Since the operating current is no more than 100 A, small neon bulbs are commonly used as indicators. The typical starting voltage for a neon indicator is 60 V to 80 V d.c., or 43 Vrms to 57 Vrms a.c. Once ignited, a lower voltage is needed to sustain the glow until the applied voltage drops below the “extinction level”, a few volts lower than the starting level. Figure A: Three samples of neon bulbs: the two bulbs on the left are indicators; the bulb on the right is a flicker lamp used to give an effect like a candle in the home The glowing of a gas discharge tube, including the neon bulb, can be explained in atomic terms. Electrons, produced by cosmic rays or other means, are accelerated to a high speed by the voltage applied across the electrodes. Gas atoms become excited or “ionized” when they are bombarded by fast moving electrons. Light is emitted when atoms return from an excited state to a lower energy state. P. 6 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Investigation - Generating a current by UV radiation Object: To generate a current by shining UV radiation onto a neon indicator low voltage a.c. supply Apparatus: 1 1 1 1 1 1 2 1 low voltage a.c. power supply digital a.c. voltmeter step-up transformer with turns ratio 1:20 neon indicator bulb mini torch with UV emitting diode (UVED) power torch emitting visible light leads with clips at both ends 250 k potentiometer Precaution: Prolonged viewing of the UV beam directly from the UVED may have a harmful effect on the eyes. Wear safety goggles to reduce any chance exposure (spectacle wearers are protected to some degree) Transformer with turnsratio 1:20 Neon bulb 250 k potentiometer Circuit to initiate glow discharge in a neon bulb Procedure: 1. Connect the circuit as shown in the figure. 2. Slowly increase the voltage of the power supply until the neon bulb starts glowing. Note the ignition voltage [in a.c. or (root mean square)] shown on the a.c. voltmeter. 3. Reduce the voltage across the iron electrodes of the neon bulb until extinction occurs and then increase the voltage again to a level just below the ignition threshold. (about 0.5 V) 4. Hold the UV torch about 5 cm above the neon bulb. (Precaution: If the diode is too close to the neon bulb, the change in capacitance between the UV torch and the neon bulb may initiate glow discharge in the bulb. Try this with a finger.) 5. Direct a beam of UV radiation onto the surface of the neon bulb. What happens to the bulb? (The UVED gives out not only invisible UV radiation, but also a small amount of visible light.) P. 7 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Result and conclusion: ____________ (visible light/UV radiation) can cause a current to flow in the neon bulb when the voltage applied across its electrodes is slightly below the normal ignition voltage. This suggested that___________ (photoelectrons/ thermal-electrons) are emitted from impurities such as zinc in the iron electrodes, when the surface of the electrodes is illuminated by UV radiation. As these electrons are given an initial kinetic energy by the potential difference across the transformer, a lower __________ (ignition/extinction) threshold voltage is required to start a glow discharge in the bulb. Our result is consistent with the prediction that the minimum wavelength of EM radiation required to generate photoelectrons in zinc is ______, which is longer than the minimum wavelength of 330 nm (from handbook) emitted by the UV diode (Figure B) Relative Intensity Threshold wavelength of zinc = 342 nm Visible region 400 nm to 700 nm . 300 350 400 450 500 550 600 / nm Figure B: Spectrum of UV diode P. 8 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Activity 4 Generating a current by visible determination of Planck’s constant light and The neon bulb with sodium coating (Na-Ne bulb) will be used to study the photoelectric effect induced by visible light. These bulbs can be purchased from the Faculty of Education, CUHK. Since normal indoor illumination can cause sodium to emit photoelectrons, a local blackout is required to find the blackout ignition voltage in the following investigations. Investigation - Generating a current by means of the photoelectric effect using visible light Object: To find a relation between KEmax of photoelectrons and the intensity of light Apparatus: 1 low voltage a.c. power supply 1 digital a.c. voltmeter 0-200 Vrms 1 step-up transformer with turns’ ratio 1:20 1 Na-Ne bulb 1 mini torch with 970 nm IRED 1 mini torch with white LED 1 piece of black paper with a small hole at the center 1 piece of plastic red filter Several leads fitted with plugs at both ends 1 250 k potentiometer as coarse adjustment 2 50 k rheostat as fine adjustment Procedure: 1. Connect the circuit as shown in Figure A, or use the built-in circuit provided in the photoelectric kit (Figure B). Cover the neon bulb with the black paper. low voltage a.c. supply Transformer with turnsratio 1:20 50 k rheostat 250 k potentiometer Na-Ne bulb Figure B: A photoelectric kit with coarse & fine adjustment V Figure A: Circuit to initiate glow discharge in Na-Ne bulb P. 9 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light 2. Slowly increase the voltage of the a.c. power supply until the Na-Ne bulb starts glowing, detectable through a small hole in the black paper. Note the ignition voltage shown on the a.c. voltmeter in the blackout condition. 3. Reduce the voltage across the Na-Ne bulb until extinction and increase the voltage again to a level just below ignition. 4. Direct a beam of infrared light onto the surface of the Na-Ne bulb. What happens to the bulb? Compare the ignition voltage of the Na-Ne bulb in blackout and under infrared light. 5. Repeat step 3 using a more intense infrared beam. Can we reduce the ignition voltage by increasing the intensity of infrared light? 6. Repeat step 4 using a mini-torch that emits white light. No blackout is required if the illumination by the intended source is brighter than the background light. Starting with an a.c. voltage about 1.5 V (by adjusting the 250 k potential divider) below the ignition level, increase the voltage in steps of 0.1 V (by adjusting the 50 k rheostat) until a glow discharge is observed. The glowing of the Na-Ne bulb can be detected more easily under white light by viewing through a red filter. Note the new ignition threshold of the Na-Ne bulb under white light. 7. Repeat step 6 using a more intense white light beam. Is there any significant change in the ignition threshold? Result and conclusion: Complete the following table from what you have observed. Condition of illumination Approximate ignition voltage (a.c.) Voltage reduction under illumination Blackout Infrared More intense infrared White light More intense white light Relation between ignition voltage and intensity of incident light The ignition threshold of the Na-Ne bulb under ___________ (white light/infrared light), no matter how intense, is the same as that in the blackout condition. This suggested that __ photoelectron is emitted from sodium when illuminated by _________ (white light/infrared). On the other hand, the ignition threshold of the Na-Ne bulb is (lowered /raised) when illuminated by _________ (white light /infrared light). However, no significant change in the ignition level is observed by increasing the intensity of __________ (white light /infrared light). The result is consistent with Einstein’s theory that the ___________ (maximum/minimum) kinetic energy of photoelectrons emitted from a metal surface is independent of the intensity of the incident light. P. 10 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Investigation - Determination of Planck’s constant Object: To determine Planck’s constant using photoelectric effect Apparatus: 1 low voltage a.c. power supply 1 digital a.c. voltmeter 0-200 Vrms 1 step-up transformer with turns’ ratio 1:20 1 Neon bulb 1 laser pointer 1 power torch with at least 0.5 W UVED 1 power torch with at least 0.5 W green LED 1 power torch with at least 0.5 W blue LED 1 piece of black paper with a small hole at the center 1 piece of plastic red filter, several leads with plugs at both ends 3 250 k potentiometer as coarse adjustment 4 50 k rheostat as fine adjustment Procedure: 1. Connect the circuit in exactly the same way as in Figure A. 2. Repeat the experiment with the Na-Ne bulb illuminated in turn by a laser pointer, power LED’s of different colours, and UV light. The distance between the light source and the Na-Ne bulb should be about 5 cm. 3. Check the ignition voltage before and after illumination by each light source. Record the reduction in ignition level, V, in Table V if the blackout ignition level is the same (within + 0.1 V) before and after illumination. In this way, the change in ignition voltage due to changes in temperature of the Na-Ne bulb can be taken into account. 4. Plot a best-fit straight line graph with KEmax (KEmax in eV is numerically the same as V in V) against frequency (in s-1) of light. Determine Planck’s constant and the work function of sodium from the slope and the y intercept of the line. Result and conclusion: Source of illumination Minimum Maximum wavelength of frequency / light or UV /nm 1014 s-1 Red laser light 660 + 10 Green LED 530 + 20 Blue LED 430 + 20 UVED 330 + 10 P. 11 Voltage reduction V/V Approximate KEmax of photoelectrons in eV © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Relationship between KEmax of photoelectrons and frequency of light 2 KEmax/eV 1 0 1 -1 2 3 4 5 6 7 8 9 10 Frequency (104s-1 -2 P. 12 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Appendix 1: Proof of the Law of Reflection and the Law of Refraction by Geometry Reflection In Figure A, the wavefront AB is incident on the reflecting surface and A has just reached it. To find the new position of the wavefront when B is about to be reflected at B’, a secondary wavelet is drawn with center A and radius BB’. The tangent B’A’ from B’ to this wavelet is the required reflected wavefront. In ∆ AA’B’ and ∆ ABB’ AA’B’ = ABB’ = 90º AB’ is common AA’ =BB’ (by construction) Therefore ∆ AA’B’ = ∆ ABB (R.H.S.), hence BAB’ = A’B’A These are the angles made by the incident and reflected wavefronts AB and A’B’ respectively with the reflecting surface. The incident and reflected rays, e.g. CA and B’D, are at right angles to the wavefronts as are the normals to the surface (the dotted lines at A and B’) and so the angle of incidence i equals the angle of reflection r. This is the law of reflection. A Figure A B’ Huygens’ construction for reflection of a straight wavefront Refraction Figure B shows the end A of a plane wavefront AB about to cross the boundary between media 1 and 2 in which its speeds are v1 and v2 respectively. The new position of the wavefront at time t later when B has traveled a distance BB’ = v1t and reached B’ is found by drawing a secondary wavelet with center A and radius AA’ = v2 t. the tangent B’A’ from B’ to this wavelet is the required wavefront and the ray P. 13 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light AA’, which is normal to the wavefront, is a refracted ray. If v2 is less than v1 the refracted ray is bent towards the normal in medium 2, as shown. When v2 is greater than v1 refraction away from the normal occurs. In ∆ BAB’ and A’AB’ sin i1 sin CAN sin BAB' sin i2 sin A' AN ' sin A' B' A BB' /AB' BB' v1t v1 AA' /AB' AA' v2 t v2 But v1 and v2 are constants for given media and a particular wavelength, sin i1 a constant sin i2 This is Snell’s Law of Refraction and it holds for electromagnetic, sound and water waves. The constant is called the refractive index 1n2 for waves passing from medium 1 to medium 2. Hence we can write 1 n2 v1 sin i1 n2 v2 sin i2 n1 where n1 and n2 are the absolute refractive indices of media 1 and 2. Huygens’ construction thus explains the refraction of, for example, light, on the assumption that the speed of light decreases when it passes into a medium in which the refracted ray is bent towards the normal, i.e. when it enters an optically denser medium. This assumption is verified by experiment. Figure B Huygens’ construction for refraction of a straight wavefront P. 14 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Appendix 2: Notes for Technician: Electroplating sodium onto the inner surface of a neon bulb Electroplating sodium on the inner glass surface of a neon bulb To observe the photoelectric effect using visible light, highly reactive metals with work functions below 3.1 eV (threshold wavelength 400 nm) like sodium, potassium and cesium can be used. However, these metals oxidise rapidly in air. They are generally coated on the cathode of a photomultiplier tube for special applications such as solar cells. The work function of sodium (1.82 eV) is lower than that of zinc (3.63 eV). What is the advantage and disadvantage of using sodium to study the photoelectric effect? Advantage: sodium emits photoelectrons when illuminated by light nearly within the whole visible spectrum, say any wavelength shorter than 683 nm (red). Zinc can respond only to UV with wavelengths shorter than 342 nm. Disadvantage: Sodium metal is dangerously reactive especially in damp air. It reacts with oxygen in seconds. A high vacuum is often required if sodium is used to study the photoelectric effect. It was discovered accidentally that when red light (650 – 680 nm) from a laser pointer falls on one of the electrodes, glow discharge is initiated in a new neon bulb when the voltage across the electrodes is only 0.1 V below the blackout ignition level. As photoelectrons are ejected by red light, the mysterious sodium is of monocrystalline structure and has a lower work function of only 1.82 V than normal sodium. The same effect is observed using bulbs from different providers. These results suggest that at least one of the electrodes is contaminated by sodium during the manufacturing process. The following paragraph is a summary of what we learn from the Wikipedia website and related links. In making neon bulbs, a pair of electrodes is first melted to one end of the bulb with the other end attached to a manifold connected either to a neon gas source or a vacuum pump. Before sealing, the bulb is filled with neon gas, heated to cause a vacuum (evacuation) again and refilled with neon. During evacuation, a high voltage is applied to one electrode while the other is earthed, allowing an electric arc to be struck through the interior of the bulb. The arc heats the glass to a temperature near melting and impurities are driven off in vapour form and removed from the bulb by suction. We believe that the arc discharge not only gets rid of most impurities, but also introduces a new impurity in one of the electrodes, i.e. metal sodium, reduced from sodium silicate (Na2SiO3), a common constituent of glass. Our discovery suggested that we may repeat a similar heating-and-electrifying process to obtain more metal sodium inside the neon bulb. In this section, we introduce two simple methods which can be used to coat a thin layer of sodium over the inner surface of the glass envelope of a neon bulb. The working principle of electroplating sodium using sodium silicate as electrolyte is similar to that of P. 15 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light electroplating copper using copper sulphate solution as electrolyte (refer to the figure below). Copper (sodium) contained in the positive electrode changes into copper (sodium) ions and these diffuse into the solution (glass). Copper (sodium) ions in copper sulphate (sodium silicate) picks up electrons from the negative electrode and is reduced back to copper (sodium). low voltage a.c. supply Transformer with turnsratio 1:20 Positive copper electrode To EHT 2500 V Negative electrode to be electroplated Copper sulphate solution Electroplating Copper Neon bulb Heating coil A 1.0 A Fibre-glass mesh tubing Low voltage d.c. supply Electroplating Sodium The figure ‘Electroplating Sodium’ shows one of the circuit-systems that can be used. The neon bulb and heater are actually immersed in a salt bath (fused NaCl) not shown in the circuit diagram. When the neon bulb is glowing, the neon gas becomes conductive due to the presence of gas ions and electrons. Glass is a supercooled liquid containing sodium ions from sodium silicate [Na2SiO3 2 Na+ + (SiO3)-]. When the heater is on, glass is heated to several hundred degrees Celsius and becomes slightly conducting. Also, a small amount of sodium chloride is vaporized by the heater. A strong electric field is set up when a high voltage of several kilovolts is applied across the heater and the electrodes of the bulb. Due to electrostatic repulsion, the inner surface of the neon bulb is bombarded by negative ions and electrons and becomes negatively charged. Sodium ions from the salt vapour then diffuse into glass and migrate towards the inner surface, whereupon they pick up an electron and are reduced to metal sodium. P. 16 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Method One: Electrolysis of sodium silicate by EHT Object: To obtain metal sodium by electrolysis of sodium silicate in glass using an EHT Apparatus: 1 neon indicator bulb 1 heat resistant fibre-glass mesh tubing (1.5 cm long and 5 mm bore) 1 0-6 V low voltage d.c. power supply 1 0-6 V low voltage a.c. power supply 1 step-up transformer with turns ratio 1:20 1 coil of swg 22 nichrome wire with 6 turns and resistance about 1 1 0 – 5000 V Extra high tension (EHT) supply 1 0 -5.0 A d.c. ammeter 2 salt bath (a ceramic crucible containing 10 ml of common salt) Procedures: 1. Insert the neon bulb into the fibre-glass mesh tubing, which is wrapped round by a heating coil. 2. Place the neon bulb, fibre-glass tubing and heater combination into a crucible. 3. Add common salt to the crucible until the heating coil is immersed in salt. 4. Connect the circuits as shown in Figure 7. All electrical power should be off at the start. 5. Increase the voltage from the a.c. power supply from 0 V until the neon bulb glows at its normal rating. 6. Adjust the low voltage d.c. supply so that a current about 1.0 A passes through the heating coil. Both a.c. and d.c. supply can be derived from the same power supply. Independent adjustment can be done with the help of a rheostat. 7. Switch on the EHT and adjust the voltage slowly from 0 V to -2500 V. Precaution: High voltage! Do not touch any exposed part of the circuit. To avoid sparking, the earthed heater and the electrodes at a negative high potential should be separated by the fibre-glass tubing and be kept at least 5 mm apart. 8. When all power sources are switched on, electrolysis of sodium silicate is taking place in the glass envelope. The process should be closely monitored for the first 30 minutes to ensure that all meter readings are stable and no sparking occurs between the electrodes and the heater. Reduce the voltage of the EHT if sparking happens or the EHT voltage P. 17 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light falls gradually. The later is an indication of overloading which happens if the salt melts or there is a flaw in the circuit. 9. Disconnect the circuit and remove the neon bulb after 24 hours. If the experiment is successful, a layer of metal sodium can be seen as a translucent brown deposit on the inner glass wall. Note: Using a large salt bath (Figure A), several neon bulbs can be electroplated at the same time by connecting the bulbs in parallel (Figure B) and the heating coils in series (Figure C) with the same high voltage applied across the heater and the electrodes. Figure A A crucible made of ceramic tiles glued together by epoxy putty Figure B Bulbs connected in parallel Figure C Heating coils in series wound round fibre-glass mesh tubings P. 18 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Method Two: Electrolysis of sodium silicate by an induction coil Object: To obtain metal sodium by electrolysis of sodium silicate in glass using an induction coil Reed relay Apparatus: 1 1 1 1 neon indicator bulb power signal generator 0-12 V d.c. power supply 1100 turns induction coil with soft iron core 1 reed relay Several leads with clips ~ Signal generator 50 Hz, 6 V 0-12 V d.c. Neon bulb L 5.0 H 1100 turns Procedure: 1. Connect the circuit as shown in Figure D. Figure D: Electrolysis of Na2SiO3 by the emf induced in an induction coil 2. Set the low voltage d.c. supply at about 2.0 V. 3. Switch on the signal generator. Set the output at 50 Hz and 6.0 V,. At these settings, the neon bulb should glow with a normal reddish-orange tint. 4. Slowly increase the voltage of the d.c. supply and/or decrease the frequency of the signal generator until the negative electrode of the neon bulb flashes brightly with a light yellow colour. Danger: Do not touch the terminals of the induction coil. The coil generates pulses with a voltage over 1,000 V at moments when the relay is switched off. In the absence of the neon bulb, the voltage of the induction coil may even build up to 10,000 V. 5. Under the conditions described in step 4, the glass envelope is heated and electrified by the discharging current and electrolysis of sodium silicate takes place inside the glass. Sodium ions in the glass are pulled out by the strong electric field generated by the coil to the inner glass surface and reduced back to metal sodium. The process is allowed to proceed for 2 hours. 6. Disconnect the circuit and examine the neon bulb. If the experiment is successful, a layer of metal sodium, appearing as a shiny grayish-brown layer, has deposited on the inner glass surface near the negative (glowing) electrode. P. 19 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light You may also try this circuit for obtaining the sodium metal in the neon bulb: Points to note: The primary coil of the transformer is connected to an a.c. power supply. To obtain sodium metal from electrolysis of glass, we increase the a.c. voltage of the power supply (primary) until the half-wave rectified output current reads about 8-10 mA and let the neon bulb overrun for 12 hours, or until a partially reflecting deposit is formed on the inner glass wall. P. 20 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Physics – Nature of Light Appendix 3: References Electrolysis of sodium silicate in glass Adlam GHJ 1931 The Science Masters’ Book Series 1 Part 1 (London: John Murray), pp207-217 Mak, Se-yuen 2006 Neon bulbs test photoelectricity, Physics Education (UK), May 2006, pp213-216 History of photoelectric effect http://www.colorado.edu/physics/2000/quantumsone/photoelectric.html http://www.britannica.com/nobel/micro/465_57.html http://www.galileo.phys.virginia.edu/classes/252/photoelectric_effect.html UV diode http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=40230679 http://www.brl.ntt.co.jp/E/activities/file/report99/E/qe/qe2.htm Work function of Metals McGraw-Hill Encyclopedia of Science and Technology Glow discharge of neon bulb http://en.wikipedia.org/wiki/Neon_lamp Manufacturing neon bulbs http://en.wikipedia.org/wiki/Neon_sign http://www.madehow.com/Volume-2/Neon-Sign.html P. 21 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved.