Optical Sources MCQ || Optical Sources and Detector Questions and Answers

1. The advantage of ILD over LED is

  1. ILD emits incoherent light whereas LED emits coherent light
  2. In ILD it is difficult to couple light whereas in LED it is easy to couple light
  3. In ILD coupling loss is more whereas in LED coupling loss is less
  4. ILD emits coherent light whereas LED emits incoherent light

Answer.4. ILD emits coherent light whereas LED emits incoherent light


ILD (Injection laser diode) produces coherent radiation in which the waves generated by the ILD are at the same frequency. ILD emits coherent light whereas LED emits incoherent light.

Comparison between ILD and LED is shown in the following table:


ILD (Injection

laser diode)

LED (Light-emitting diode)

Working operation

It emits light by stimulated emission.

It emits light by spontaneous emission.





Power output

High(few mW to GW)

Relatively low

Coupled power







Moderate to high


Fiber type

Single-mode and multimode

Multimode only


2. What does LED stand for?

  1. Light Emitting Display
  2. Low Energy Display
  3. Light Emitting Diode
  4. Light Emitting Detector

Answer.3. Light Emitting Diode


LED stands for Light Emitting Diode.

LED (light-emitting diode):

  • The LED (light-emitting diode) is a PN junction device that emits light when a current pass through it in the forward direction, i.e. when LED is forward biased, it emits light.
  • In an LED, this energy lies in the visible region of electromagnetic radiation, and the photon released is perceived as light.
  • An LED in this way converts electrical energy into light energy.
  • In reverse biased mode, it works like a normal diode and does not emit light.


3. A low resistance LDR of 20Ω, operated at a certain intensity of light, is to be protected through a series resistance in such a way that up to 12 mA of current is to flow at a supply voltage of 10 V. What is the nearest value of the protective resistance?

  1. 873 Ω
  2. 813 Ω
  3. 273 Ω
  4. 81 Ω

Answer.2. 813 Ω


According to the question, we can draw circuit as:

low resistance LDR of 20Ω, operated at a certain intensity of light

By voltage division rule, we can write:

${V_R} = \left( {\frac{R}{{R + 20}}} \right) \times 10$

VR = (12 × 10-3) × R     —(2)

From equation (1) and (2), we get:

$\left( {12 \times {{10}^{ – 3}}} \right) \times R = \frac{{R \times 10}}{{R + 20}}$

12 + 10-3 = 10/(R + 20)

12 × 10-3 R = 9.76

R = 9.76/(12 × 10-3) = 813.33Ω

∴ R = 813 Ω

Hence, the nearest value of the protective resistance should be 813 Ω to protect LDR.


4. The value of the current limiting resistor for a stack of 4 LEDs connected in series will be ______ if the LED’s are 3 V, 3 mA, and DC source is 15 V.

  1. 10 Ω
  2. 100 Ω
  3. 1 kΩ
  4. 10 kΩ

Answer.3. 1 kΩ


Source voltage = 15 V

Voltage of LED = 3 V

Current of LED = 3 mA


The given circuit can be drawn as

value of current limiting resistor for a stack of 4 LED's connected

The current through the LED circuit must be 3 mA and the voltage drop across them is 3 V each.

Therefore, by applying KVL, we get,

15 – R (3 mA) – 3 – 3 – 3 – 3 = 0

R = (15 − 12)/3  = 1 kΩ


5. When the LED lights need a range of voltages to operate, Which of the following drivers is best suited for the job?

  1. Constant frequency
  2. Constant Voltage
  3. Constant Impedance
  4. Constant Current

Answer.4. Constant Current


  • LED drivers are the driving force that provides and regulates the necessary power to make sure the LEDs operate in a safe and consistent manner. One important choice is that of choosing a constant current LED driver versus a constant voltage LED driver.
  • Constant current LED drivers are designed for a designated range of output voltages and a fixed output current (mA). LEDs that are rated to operate on a constant current driver require a designated supply of current usually specified in milliamps (mA) or amps (A).


6. The output from a laser is monochromatic, this means that it is

  1. Polarized
  2. Single-frequency
  3. narrow beam
  4. None of these

Answer.2. Single-frequency


A monochromatic laser emits only one wavelength i.e. single frequency & therefore reduces or does not contribute to chromatic dispersion.

Note: Monochromatic Dispersion results from the spectral width of the emitter i.e. the spectral width determines the number of different wavelengths that are emitted by the LED or laser. One way to reduce chromatic dispersion is to narrow the spectral width.


7. Which of the following LED colors is NOT present in an LED TV panel?

  1. Green
  2. White
  3. Red
  4. Blue

Answer.2. White


1) A LED display is a flat panel display that uses an array of light-emitting diodes as pixels for a video display. 

2) LED displays can offer higher contrast ratios than a projector and are thus an alternative to traditional projection screens.

3) LED display consists of a matrix of red, green, and blue diodes.


8. The efficiency of an LED for generating light is directly proportional to the

  1. Applied voltage
  2. Current injected
  3. Temperature
  4. Level of doping

Answer.2. Current injected


  • The efficiency of LEDs will be determined by the radiate recombination rate
  • Temperature influence the output power of an LED through its effects on the internal quantum efficiency of the device.
  • The internal quantum efficiency of LEDs decreases exponentially with increasing temperature.
  • The efficiency of LED is directly proportional to the current injected.


9. What color LED lamp is commonly used for domestic use?

  1. White
  2. Black
  3. Red
  4. Green

Answer.1. White


Light Emitting Diode:

  • A light-emitting diode (LED) is a semiconductor light source.
  • Electroluminescence is the working principle of LED.
  • A light-emitting diode emits light when a current flows through it.
  • Infrared LEDs are used in remote-control circuits.
  • Warm white lighting fixtures are often preferred in living rooms to create a peaceful atmosphere.
  • The warm white lights are a good choice to compliment earthy tones and wood furniture.
  • White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device.


10. ______ light-sensing device that is used to sense the intensity of light

  1. Laser diode
  2. Zener diode
  3. Schottky diode
  4. Photo-diode

Answer.4. Photo-diode


Photo-diode is a light-sensing device that is used to sense the intensity of light. Some of the examples are the smoke detector, a receiver in tv for converting remote signals, etc.

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