Energy Gap Using Newton Rings

Lab Experiments

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Experiment-26

S

WAVELENGTH AND ENERGY GAP DETERMINATION USING NEWTON′S RINGS MICROSCOPE IN CASE OF LIGHT EMITTING DIODES Dr S.P Basavaraju Dept. of Physics, Bangalore Institute of Technology, K R Road, Basavanagudi, Bangalore 560 004, INDIA.

Abstract Using a plano-convex lens of know radius of curvature, Newton’s Rings are observed by illuminating with lights of different wavelengths obtained by different LEDs. Each kind of light gives different set of Newton’s Rings. Using the standard procedure the wavelengths of the illuminating illuminating lights are determined. determined. The energy gaps of the materials of the respective respective LEDs are determined and the values values obtained obtained are compared compared with with the standard standard values. values.

Introduction Light emitting diodes (LEDs) are special semiconductor devices in which recombination that takes place near the p-n junction results in light emission. The emitted light though not highly monochromatic is treated monochromatic on the practical considerations, as line width is of the order of ±10%. Table-1 lists the compound semiconductors used in the fabrication of various LEDs, their dominant wavelengths, the corresponding line widths and energy gaps. Table-1 Material

Emitted Light Color

EG (eV)

λ(nm)

δλ δλ(nm)

SiC

Blue

2.86

480

50

GaP:N:Zn GaAsP:N:Zn GaAsP:N:Zn GaAl:Zn GaAsP:N GaAl:Si GaAs:Si

Green Yellow Orange Red Red Infrared Infrared

2.22 2.10 1.98 1.92 1.85 1.43 1.34

565 590 625 650 660 870 900

40 40 40 20 20 80 20

Different Different semiconductor semiconductor materials materials and their their characteristic characteristic parameters parameters One must be careful in the selection of LEDs in the market because many commercially available LEDs are encapsulated with colored caps that will give false light. But transparent capsulated LEDs are now available in all colors and only they must be used for experimental studies. Vol-2, No-2, SEPTEMBER-2002

Lab Experiments

11 Clear Newton’s rings are formed when a monochromatic light falls normally on a plano-convex lens and glass assembly [1]. Using LED light Newton’s rings are observed. Under the assumption that the LED light is monochromatic. The diameter of the nth dark ring in the reflected light is given by

dn = √(4Rnλ)

…1 th

where, dn is the diameter of the n ring R is the radius of curvature of the Plano convex lens n is the ordinal number of the ring λ is the wavelength of the monochromatic light used The equation can be rewritten as 2

dn /n λ = ---------4R

…2

If R is known, λ can be determined by measuring d n. LED

R=1K

+

-

Figure-1, Forward biased LED

Figure-1 shows a forward biased LED. A free electron leaving the negative terminal of the battery enters the LED at the anode and further travels through the body of the semiconductor. While it is near the pn junction, it is most likely to combine with a free hole at which time it transits from conduction band to valence band directly. The consequent loss of energy of the electron appears as a photon and emitted from the LED surface as light. The wavelength of the light emitted and the energy gap of the semiconductor are related through the equation, hc EG = ------

…3

λ -15

where

h, is Planks constant = 4.14 x 10 eV 8 c, is velocity of light = 3x10 meters/sec λ, is the wavelength 1240

EG = ------

…4

λ

In Equation-4 energy gap is in electron volt and wavelength is in nanometer. Hence by determining λ, energy gap of the semiconductor emitting the light is determined. Vol-2, No-2, SEPTEMBER-2002

Lab Experiments

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Figure-2, Newton’s Rings Microscope

3(a)

(b)

Figure-3(a), Bottom portion of Newton’s Ring microscope fitted with LED light source 3(b), Three different color LEDs mounted on a cabinet

To use LED light in Newton’s Rings measurements, certain modifications are done. The bottom portion of the Newton’s rings microscope [2] containing plano-convex lens and glass plate is shown in Figure-3 (a). The LEDs are mounted on a cabinet as shown in Figure 3(b) and attached to the bottom portion. The LED light is allowed to fall on the turning glass plate inclined at 45 degree and then on the plano-convex lens glass plate assembly. Newton’s rings are observed with different LED lights. The number of rings over a given radial distance is found to vary with LED light.

Apparatus Used The apparatus consists of LED light source fitted to bottom portion of the Newton’s rings microscope. Different color LEDs that can be switched on one at time by a power supply controlled by a micro switch. Plano-convex lens of 100cm radius of curvature, a plane glass plate and Newton’s rings microscope. Vol-2, No-2, SEPTEMBER-2002

Lab Experiments

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Experimental Procedure 1. The plano-convex-lens is placed inside the bottom portion of the Newton’s rings microscope. The radius of curvature is noted as R =1meter (data) 2. The LED power supply is switched on and blue color light is selected. The turning glass plate is adjusted so that light falls normally on lens assembly. 3. Newton’s rings are observed and the microscope is adjusted for proper focus. By rotating the drum, the adjustment is made such that center dark fringe at the center of the field of view. Also the cross-wire are adjusted such that their intersection is at the center of the dark spot. 4. The drum of microscope is rotated until the 6 cross wire as shown in Figure-4.

th

ring on the left-hand side coincides with the

Cross W ire coinciding 6th ring on the left of the rings set

center dark fringe Cross wires

Figure-4 Cross wire coinciding 6 the ring on the LHS

Ring No

6 5 4 3 2

Table-2 Micrometer reading (mm) d(mm) Left Right 20.78 17.48 3.30 20.65 17.51 3.08 20.49 17.73 2.76 20.33 17.94 2.39 20.13 18.14 1.99

2

-6

2

d x10 dn /n -6 m x10 10.89 1.815 9.48 1.896 7.61 1.902 5.71 1.903 3.96 1.980 Average = 1.879 Blue -λblue = 469nm, EG =2.64eV

Blue LED wavelength and energy gap determination 5.

The micrometer reading is noted and recorded in Table-2. The cross wire is made coincident th with the 5 ring and the corresponding micrometer readings are noted in Table-2. The same nd measurement is continued till the 2 ring.

6.

The cross wire is made coincident with 2 ring on the right hand side and the corresponding micrometer reading is noted in Table-2. Similar observations are made for consecutive rings

nd

Vol-2, No-2, SEPTEMBER-2002

Lab Experiments

14 th

up to the 6 ring on the RHS. Diameters of the various rings are determined and the ratio 2 2 dn /n is calculated for each case. It is seen that d n /n is a constant as expected. Wavelength λ is determined using Equation-2. 2

-6

dn /n 1.879 x 10 λ = ---------- = ---------------- = 469 x10 -9 meters = 469nm 4R 4x1 7.

Energy gap is calculated using Equation – 4 1240 1240 EG = ------ = ---------- = 2.64 eV λ 469 Ring No

6 5 4 3 2


2

-6

2

d x10 dn /n m 14.98 2.49 12.53 2.50 10.11 2.52 7.56 2.52 4.88 2.44 Average = 2.51 Red-LED -λred = 627nm, EG =1.97eV

Red LED energy gap and wavelength determination

Ring No

6 5 4 3 2

Table-4 Micrometer reading (mm) d(mm) Left Right 18.37 14.65 3.72 18.21 14.81 3.40 18.03 15.00 3.03 17.81 15.20 2.61 17.58 15.45 2.13 Yellow-LED -λyellow

2

-6

2

d x10 dn /n m 13.83 2.30 11.56 2.31 9.18 2.30 6.81 2.27 4.53 2.26 Average = 2.30 = 575nm, EG =2.15eV

Yellow LED energy gap and wavelength determination 8.

The experiment is repeated with green, red and yellow lights. In each case Newton’s rings th nd are observed and diameters from 6 to 2 ring are determined. From the ring diameter and ring number, wavelength and energy gap is determined. The readings obtained are tabulated in Tables 3,4 and 5 respectively for green, orange and red LEDs.


Lab Experiments

Ring No

6 5 4 3 2

15


2

-6

2

d x10 dn /n m 13.46 2.24 11.35 2.27 9.12 2.28 6.64 2.20 4.54 2.27 Average = 2.25 Green-LED -λ reen = 563nm, EG =2.20eV

Green LED energy gap and wavelength determination

RESULTS 1. Using blue, green, yellow and red light Newton’s rings are formed and the wavelength of the light is determined in each case. 2. Energy gap of the semiconductor emitting the light is also determined in each case. The results obtained are tabulated in Table-6 Table-6 Color

Blue Green Yellow Red

λ(nm) Expt. Value 469 563 575 627

Stand. Value 480 563 589 645

EG(eV) Expt. Stand. Value Value 2.64 2.64 2.20 2.22 2.15 2.10 1.97 1.92

Wavelength and Energy gap obtained with standard values

Discussions 1.

The experiment shows the application of Newton’s rings to determine easily the value of unknown wavelength of light emitted by LEDs. LED wavelengths are required for radiometry and photometry applications. A maximum error of 2% in the wavelength measurement is noticed between the standard and experimental value.

2.

Using an LED for illumination has an advantage that the power requirement is only about 80mW (maxi). This stands very attractive against the huge power requirement by sodium vapor lamp. (Minimum is 35 watts). This is helpful in the regular lab arrangement, especially during the failure of mains electric power supply, as a 3V supply from dry cells will be enough to drive LEDs. It may be noted that it doesn’t require very high intensity light to secure Newton’s rings. Also costwise , a set of LEDs along with the power supply etc works out to be less than ¼ of the cost of a sodium vapor lamp set


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Lab Experiments

References 1. SP Basavaraju, Newton’s Rings, A detailed textbook of Engineering Physics Practicals, 1999, Page-32. 2. SP Basavaraju, Newton’s Rings, LE- Vol-2, N0-1, June 2002, Page-65

Dr S P Basavaraju Dr S P Basavaraju is professor and head, Department of Physics, Bangalore Institute of Technology, K R road Basavanagudi Bangalore. He has been in the teaching field for more than 22 years. Professor Basavaraju got his MSc degree in Physics from Mysore University and PhD degree from Bangalore University. Prof. Basavaraju has published 7 research papers in international journals and has presented

5 papers in symposia. His PhD work is on the NQR studies. He is one of familiar author of Engineering Physics Books in Karnataka. His recent editions are A detailed textbook of Engineering Physics, A detailed textbook of Engineering Physics Practicals, A textbook of fields and waves. He has also published articles on popular subjects in science.


Energy Gap Using Newton Rings

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