Physics Project

CERTIFICATE This is to certify that Siddhant Sethi of class XII “D” has worked under my supervision on the project “CALCULATION OF THE VALUE OF PLANCK’S CONSTANT USING LED” in physics laboratory and completed it to my total satisfaction.

Date:

(Ms. Suman)

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ACKNOWLEDGEMENT It gives me immense pleasure to express my deep sense of gratitude towards my eminent physics teacher Mrs. Suman who has always been there as guiding spirit behind the successful completion of the project. I am also grateful to our Lab Assistant Mr. MN Singh for his valuable guidance and encouragement throughout the course and preparation of this project.

(Siddhant Sethi)

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INDEX S.No. TOPIC 1.) AIM

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2.)

INTRODUCTION

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3.)

THEORY

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4.)

REQUIREMENTS

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5.)

PROCEDURE

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6.)

OBSERVATIONS

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7.)

CONCLUSION

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8.)

PHOTO GALLERY

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9.)

BIBLIOGRAPHY

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AIM: To measure Planck's constant using light-emitting diodes using the “turn on” voltage.

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INTRODUCTION Light-emitting diode

Red, green and blue LEDs of the 5mm type

Invented

Nick Holonyak Jr. (1962)

Electronic symbol:

Light-emitting diodes (LEDs) convert electrical energy into light energy. They emit radiation (photons) of visible wavelengths when they are “forward biased” (i.e. when the voltage between the p side and the n-side is above the “turn-on” voltage). This is caused by electrons from the “n” region in the LED giving up light as they fall into holes in the “p” region. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor.

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THEORY

Like a normal diode, the LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. The materials used for the LED 6

have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light. Conventional LEDs are made from a variety of inorganic semiconductor materials; the following table shows some of the available colors with wavelength range, voltage drop and material: Color Wavelength (nm)

Voltage (V)

Semiconductor Material Gallium arsenide (GaAs) Aluminium gallium arsenide (AlGaAs)

Infrared λ > 760

ΔV < 1.9

Red

610 < λ < 760

Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide (GaAsP) 1.63 < ΔV < 2.03 Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)

Orange 590 < λ < 610

Gallium arsenide phosphide (GaAsP) 2.03 < ΔV < 2.10 Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)

Yellow 570 < λ < 590

Gallium arsenide phosphide (GaAsP) 2.10 < ΔV < 2.18 Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)

Green

Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN) Gallium(III) phosphide (GaP) Aluminium gallium indium phosphide (AlGaInP) Aluminium gallium phosphide (AlGaP)

500 < λ < 570

1.9 < ΔV < 4.0

Blue

450 < λ < 500

Zinc selenide (ZnSe) Indium gallium nitride (InGaN) 2.48 < ΔV < 3.7 Silicon carbide (SiC) as substrate Silicon (Si) as substrate — (under development)

Violet

400 < λ < 450

2.76 < ΔV < 4.0 Indium gallium nitride (InGaN)

multiple types

Dual blue/red LEDs, 2.48 < ΔV < 3.7 blue with red phosphor, or white with purple plastic

Purple

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An LED is a two terminal semiconductor light source. In the unbiased condition a potential barrier is developed across the p-n junction of the LED. When we connect the LED to an external voltage in the forward biased direction, the height of potential barrier across the p-n junction is reduced. At a particular voltage the height of potential barrier becomes very low and the LED starts glowing, i.e., in the forward biased condition electrons crossing the junction are excited, and when they return to their normal state, energy is emitted. This particular voltage is called the knee voltage or the threshold voltage. Once the knee voltage is reached, the current may increase but the voltage does not change. The light energy emitted during forward biasing is given as, (1)

λ

Where c - Velocity of light. h -Planck’s constant. λ - Wavelength of light. If V is the forward voltage applied across the LED when it begins to emit light (the knee voltage), the energy given to electrons crossing the junction is, (2)

Equating (1) and (2), we get (3)

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The knee voltage V can be measured for LED’s with different values of λ (wavelength of light). ( )

(4)

Now from equation (4), we see that the slope s of a graph of V on the vertical axis vs. 1/λ on the horizontal axis is (5) To determine Planck’s constant h, we take the slope s from our graph and calculate Using the known value Alternatively, we can write equation (3) as λ

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REQUIREMENTS       

0-10 V power supply One way key Rheostat Ammeter Voltmeter 1 K resistor Different known wavelength LED’s (Light-Emitting Diodes)

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PROCEDURE  Connections are made as shown in circuit diagram.  Insert key to start the experiment.  Adjust the rheostat value till the LED starts glowing, or in the case of the IR diode, whose light is not visible, until the ammeter indicates that current has begun to increase.  Corresponding voltage across the LED is measured using a voltmeter, which is the knee voltage.  Repeat, by changing the LED and note down the corresponding knee voltage.  Using the formula given, find the value of the Planck's constant.

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OBSERVATIONS Color of LED

Wavelength (nm)

Knee (V)

Red

630

1.9

6.568 x10-34

Green

510

2.4

6.552 x10-34

Blue

470

2.6

6.565 x10-34

Yellow

590

2.2

6.604 x10

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Voltage Kgm2s-1

-34

CONCLUSION The actual value of Planck’s constant is 6.626 x10-34 Kgm2s-1 and the values in the above experiment are precise as well as in the close conformity with the actual value of the Planck’s constant.

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PHOTO GALLERY

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BIBLIOGRAPHY  www.google.com  NCERT Class XII

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