A semiconductor is not merely a passive material; under illumination, it becomes an active medium where photons create motion, currents, and radiation. This unit examines the fundamental processes governing light–matter interaction and the principles behind devices that convert optical signals into electrical ones—and vice versa.
Author: Surya Bhaskaram
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Satya–Chandra: On Accuracy and Precision
Chandra:
Satya, today my measurement matched the true value exactly. So my experiment is perfect, right?Satya:
Not so fast, Chandra. One correct answer can be luck. Tell me—would you get the same result again?Chandra:
Hmm… maybe not. Yesterday it was slightly different.Satya:
Then you may have accuracy, but not precision.Chandra:
So accuracy is closeness to truth, and precision is consistency?Satya:
Exactly.
Accuracy asks, “How close am I to reality?”
Precision asks, “How reliable am I?”Chandra:
Satya, suppose an examiner evaluates a student’s answer.
The student’s true understanding deserves 6 marks.Satya:
Good. Now tell me—how does the examiner mark?Chandra:
In one case, the examiner gives 8, 8, 8, 8 every time.Satya (raises an eyebrow):
Then the examiner is consistent… but biased.Chandra:
So the marking is precise, but not accurate?Satya:
Exactly.
Precision reflects the examiner’s habit.
Accuracy reflects the examiner’s judgement.Chandra:
What if the examiner gives 6, 6, 6, 6?Satya (smiles):
Then consistency meets truth.
Both precision and accuracy are achieved.Chandra:
And if the marks are 8, 4, 9, 3?Satya:
Then the examiner sometimes hits the truth, sometimes misses it.
Accurate on average, but lacking precision.Chandra (thinking):
So repetition alone doesn’t guarantee fairness.Satya:
No.
Without accuracy, precision becomes reliable error.
Without precision, accuracy becomes fortunate coincidence.
And remember—without precision, accuracy cannot be trusted;
without accuracy, precision is meaningless.Chandra:
That sounds like life advice too.Satya (laughs):
Physics always is. -
Dimensional Reasoning
Satya:
Chandra…I forgot the formula for the period of a pendulum.
Chandra:
Forgetting formulas is normal.
Satya:
I remember something
…. 
…..
… umm…but not exactlyChandra:
Then don’t memorize the formula. Let dimensions guide you.
Satya:
Dimensions? How?
Chandra:
The period is time period. Time has dimension of [T], length has [L], and g has [L][T]-2. Now combine them so that the final result has dimension [T].
Satya:
I got it
Chandra:
Exactly. No memorization, just logic.
Satya:
Wow.. so I can rebuild and verify formulas!
Chandra:
Correct. Now tell me quickly,
right?
Satya: Yeah.. ahh.. wait a minute
Nooo! My brain memorized, but dimensions caught the error.
and 
are [L][T]-1 ,
is [L]Therefore It must be
Now I got a spell check for equation.