The device that regulates change in intensity of illumination into change in electric current is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Photodiode
A device that turns a change in light intensity into a change in current is an optical detector.
Option (1) LED — Incorrect. An LED does the reverse job: it converts electric current into light when it is forward biased.
Option (2) Zener diode — Incorrect. A Zener diode is a voltage regulator; it keeps the voltage across a load steady when it works in reverse breakdown, and light plays no part.
Option (3) Photodiode — Correct. A photodiode works in reverse bias. Light falling on its junction creates electron-hole pairs, so the reverse current rises with the intensity of illumination.
Option (4) Transistor — Incorrect. A transistor amplifies or switches a current controlled by its base current; it does not respond to light.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Optoelectronic Junction Devices – “… A Photodiode is again a special purpose p-n junction diode fabricated with a transparent window to allow light to fall on the diode. … The magnitude of the photocurrent depends on the intensity of incident light (photocurrent is proportional to incident light intensity). … Thus photodiode can be used as a photodetector to detect optical signals.”