100 Most Important MCQ Of Illumination with answer & explanation

100 Most Important MCQ Of Illumination with answer & explanation | Important MCQ Question of Illumination for SSC JE (electrical) with explanation

Ques.1. The radiant efficiency of the luminous source depends on

  1. The shape of the source
  2. The temperature of the source
  3. The wavelength of the light rays
  4. All of the above

Answer.2. The temperature of the source


When an electric current is passed through a conductor, some heat is produced to I2R loss, which increases the temperature of the conductor. At low temperatures, the conductor radiates energy in the form of heatwaves, but at very high temperatures, radiated energy will be in the form of light as well as heatwaves.

Radiant efficiency is defined as the ratio of energy radiated in the form of light, which produces the sensation of vision to the total energy radiated out by the luminous body.

Radiant Efficiency = Energy Radiated in the form of light/total energy radiated by the body.s. It is found that maximum radiant efficiency would occur at about 62000° C and even then the value of this maximum efficiency would be 20%


Ques.2. Lightwave travel with a velocity of

  1. 3 × 1010 cm/s
  2. 3 × 1012 cm/s
  3. 3 × 1015 cm/s
  4. 3 × 1018 cm/s

Answer.1. 3 × 1010 cm/s


The velocity of the light wave is 3 × 108 m/sec or 3 × 1010 cm/sec.


Ques.3. Carbon arc lamps are commonly used in

  1. Domestic lighting
  2. Street lighting
  3. Cinema Projector
  4. Photography

Answer.3. Cinema Projector


Carbon arcs operate by ionizing air molecules (which carry the current between the carbon electrodes) to incandescence.

To ignite the lamp, the rods are touched together, thus allowing a relatively low voltage to strike the arc. The rods are then slowly drawn apart, and electric current heats and maintains an arc across the gap. The tips of the carbon rods are heated and the carbon vaporizes. The carbon vapor in the arc is highly luminous, which is what produces the bright light. The rods are slowly burnt away in use, and the distance between them needs to be regularly adjusted in order to maintain the arc.

The carbon arc lamp was mainly used in the cinema projector. A carbon electrode projector produces the best possible light, a pure white with even brightness. But the rods are consumed in less than an hour (another reason for changeovers), and their burning produces carbon monoxide. The carbon-arc played a significant role until the introduction of the short-arc xenon discharge lamp in 1951.


Ques.4. The unit of solid angle is

  1. Solid Angle
  2. Radian
  3. Steradian
  4. Candela

Answer.1. Solid Angle


When the normal (perpendicular) lines are drawn from the boundary of a portion on the surface of a sphere, Such that they meet at its center and form a cone, then the angle enclosed by the conical surface is called solid angle.

Solid Angle

Mathematically, the solid angle is the ratio of an area of the base of conical surface (portion on the spherical surface of the sphere) to the square of the radius of the sphere. The steradian (symbol: sr) or square radian is the SI unit of solid angle.


Ques.5. Candela is the unit of

  1. Luminous flux
  2. Luminous intensity
  3. Wavelength
  4. None of the above

Answer.2. Luminous intensity


Earlier “standard candle” was used to define the luminous intensity of a light source. However, the fundamental SI unit of luminous intensity is Candela. It is defined with respect to a black body that is a perfect emitter or absorber. The black body radiator is considered solidifying platinum at a temperature of 2046K (1773°C).


The candela is the base unit of luminous intensity in the International System of Units (SI) Candela (cd) is defined as luminous flux emitted per unit solid angle along normal to the surface of one-sixtieth (1/60) square centimeter area of a black body radiator (source of light) kept at a temperature of 2046K of solidifying platinum.


Ques.6. The unit of luminous flux is

  1. Steradian
  2. Candela
  3. Lumen
  4. Lux

Answer.3. Lumen


The SI unit of luminous flux is the lumen (lm). One lumen is defined as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian.


Ques.7. The illumination is directly proportional to the cosine of the angle made by the normal to the illuminated surface with the direction of the incident flux. The above statement is associated with

  1. Planck’s law
  2. Macbeth’s law of illumination
  3. Bunsen’s law of illumination
  4. Lambert’s cosine law

Answer.4. Lambert’s cosine law


Lambert’s Cosine Law

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Lambert’s Cosine Law states that when light falls obliquely on a surface, the illumination of the surface is directly proportional to the cosine of the angle θ between the direction of the incident light and the surface normal. The law is also known as the cosine emission law or Lambert’s emission law.

If Iv is the intensity at an angle θ from the normal to the surface and Inv is the intensity along with the normal, then Lambert’s cosine law states that Iv = Invcosθ.


Ques.8. Which curve represents the life of the lamp?

  1. Curve A
  2. Curve B
  3. Curve C
  4. Curve D

Ques.9. Illumination level required for precision work is around

  1. 50 lm/m2
  2. 100 lm/m2
  3. 200 lm/m2
  4. 500 lm/m2

Answer.4. 500 lm/m2


Illumination Level

The intensity of illumination required on the surface is depending on the type of work being done. For each type of work, there is a range of brightness that causes minimum fatigue and gives maximum output intends of quality and quantity. Moving objects and the objects that are seen for a longer duration require more illumination than those for the stationary object and casual work.

For the normal activity the light levels are in the range of 100 – 300 lux.  For precision and detailed works, the light level may even approach 1500 – 2000 lux.


Ques.10. Which of the following will need the highest level of illumination?

  1. Proofreading
  2. Bed Room
  3. Hospital wards
  4. Railway platforms

Answer.1. Proofreading


Proofreading is the process of reviewing the final draft of a piece of writing to ensure consistency and accuracy in grammar, spelling, punctuation, and formatting.

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