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Question #43467

Difficulty: 8/10 | 🟢 Free

The Question

An LED is used as the monochromatic light source in a Young's Double Slit Experiment (YDSE) with a slit separation d=0.10 mmd = 0.10 \text{ mm} and a screen placed at a distance D=1.0 mD = 1.0 \text{ m}. Initially, the LED is forward biased with a current I1=10 mAI_1 = 10 \text{ mA}. At this current, its junction temperature is T1=300 KT_1 = 300 \text{ K} and it emits light corresponding exactly to its bandgap energy Eg=2.000 eVE_g = 2.000 \text{ eV}. When the forward current is increased to I2=50 mAI_2 = 50 \text{ mA}, the junction temperature increases due to Joule heating. The thermal characteristics of the LED setup are such that the junction temperature rises at a constant rate of 1.0 K1.0 \text{ K} per mA\text{mA} of forward current increase above 10 mA10 \text{ mA}. The bandgap energy of the LED material decreases with temperature at a rate dEgdT=−4.0×10−4 eV/K\frac{dE_g}{dT} = -4.0 \times 10^{-4} \text{ eV/K}. Assuming the emission peak always corresponds to the bandgap energy, what is the magnitude of the shift in the position of the 10th10^{\text{th}} order bright fringe on the screen? (Take hc=1240 eV⋅nmhc = 1240 \text{ eV}\cdot\text{nm})

Options

  • A.

    0.50 mm0.50 \text{ mm}

  • B.

    0.00 mm0.00 \text{ mm}

  • C.

    0.05 mm0.05 \text{ mm}

  • D.

    0.25 mm0.25 \text{ mm}

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Multi-Concept_Bridge

Master problems that require linking physics and math concepts to solve complex competitive questions.

Concepts Behind This Question

Photo-diode vs LED Bias|Intensity Distribution in YDSE|Phase Difference vs. Path Difference

Why This Question Is Tricky

Common Mistakes

This question tests the intersection of solid-state semiconductor physics and classical wave optics.

The primary trap lies in the assumption that an LED's emission wavelength is strictly constant and that increasing forward bias only increases intensity (leading to an incorrect answer of 0.00 mm0.00 \text{ mm}).

Students must recognize that increased forward current leads to Joule heating, which increases the junction temperature.

This temperature rise causes bandgap narrowing (Varshni effect), resulting in a red-shift of the emission spectrum.

By failing to account for this dynamic spectral shift, students misapply the static YDSE formula.

Furthermore, confusing the shift of a specific nthn^{\text{th}} order fringe with the change in a single fringe width (β\beta) leads to an off-by-a-factor-of-nn error (yielding 0.05 mm0.05 \text{ mm}).

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterSemiconductor Electronics: Materials, Devices, and Simple Circuits - Class 12
ConceptPhoto-diode vs LED Bias
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree