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Question #43467
The Question
An LED is used as the monochromatic light source in a Young's Double Slit Experiment (YDSE) with a slit separation and a screen placed at a distance . Initially, the LED is forward biased with a current . At this current, its junction temperature is and it emits light corresponding exactly to its bandgap energy . When the forward current is increased to , 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 per of forward current increase above . The bandgap energy of the LED material decreases with temperature at a rate . Assuming the emission peak always corresponds to the bandgap energy, what is the magnitude of the shift in the position of the order bright fringe on the screen? (Take )
Options
- A.
- B.
- C.
- D.
Check Your Answer
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 ).
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 order fringe with the change in a single fringe width () leads to an off-by-a-factor-of- error (yielding ).
Quick Facts About This Question
Question Information
| Subject | Physics |
| Chapter | Semiconductor Electronics: Materials, Devices, and Simple Circuits - Class 12 |
| Concept | Photo-diode vs LED Bias |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |