100 Most Important MCQ Of Illumination with answer & explanation

Ques.81. A 60 W lamp given a luminous flux of 1500 lumen. Its efficiency is

  1. 1500 lumen/watt
  2. 250 lumen/watt
  3. 25 lumen/watt
  4. 2.5 lumen/watt

Answer.3. 25 lumen/watt

Explanation:-

The luminous efficiency of an electric lamp is the ratio of the luminous flux (in lumen) emitted by the lamp to the electric power (ill watt) given to it i.e.

Luminous efficiency η = Luminious flux/Electric power

Luminous flux =1500 lumen

Electric power = 60 W

η = 1500/60 = 25 lumen/watt

 

Ques.82. When using ultra-violet lamps the reflector for maximum should be made of?

  1. Aluminum
  2. Copper
  3. Leaf
  4. Glass

Answer.1. Aluminum

Explanation:-

UV lamps are often called mercury or “amalgam” lamps because they contain solid amalgam “spots” (an amalgam is an alloy of mercury with another element, such as indium or gallium that controls the mercury vapor pressure. In addition to mercury, UV lamps also contain a starter gas, typically argon. The two most common types of UV lamps are high-intensity discharge (HID) lamps (also called high pressure or medium pressure mercury vapor lamps and low-pressure mercury vapor lamps. HID lamps contain mercury gas at a pressure of approximately 1000 torr and generate high levels of UV irradiance over a broad range of wavelengths. Low-pressure UV lamps contain mercury gas at pressures of about 10 torrs or less, and when this gas is stimulated by an electrical charge, it emits UV light in a narrow band of wavelengths centered around 254nm.

Reflectors:-  Since a UV lamp emits in all directions, a reflector is used to direct the light emitted in the “wrong direction” onto the surface of the coating. Well-designed reflectors collect and project about 75% of the radiant energy from the bulb to the surface, whereas parabolic reflectors collect no more than 55% The material of the reflector is mainly aluminum. Aluminum, however, also reflects IR, which may lead to unwanted heat up of the surface. Therefore, reflectors have been developed, which also reflect UV radiation very well, but not visible and IR radiation. Furthermore, the geometry of the reflector is responsible for the light distribution at the surface, whether homogeneously distributed or well focused.

UV reflector

 

Ques.83. The fluorescent tube of 30 W has the luminous flux of

  1. 1800 lumen
  2. 14 lumen
  3. 50 lumen
  4. 325 lumen

Answer.1. 1800 lumen

Explanation:-

Luminous efficiency is defined as the output in lumens per watt of the power consumed by the source of the light. For fluorescence lamps, luminous efficiency is 40 – 90 lumens/watt.

The luminous flux ΦV in lumens (lm) is equal to the power P in watts (W), times the luminous efficacy η in lumens per watt (lm/W):

ΦV(lm) = P(W) × η(lm/W)

So

lumens = watts × (lumens per watt)

The luminous flux between minimum to maximum range

luminous flux = watts × (lumens per watt)

luminous flux = 30 × 45 = 1350 lumen

&

luminous flux = 30 × 90 = 2700 lumen

So a 20 Watt tube should produce between 1350 – 2700 lumens.

Hence option A i.e 1800 lumens is right option

 

Ques.84. Which of the following application does not need ultra-violet lamps?

  1. Medical purposes
  2. Aircraft cockpit dashboard lighting
  3. Car lighting
  4. Blueprint machines

Answer.3. Car lighting

Explanation:-

There are various use of ultraviolet radiation such as

Car lighting does not need ultra-violet lamps.

  • The use of ultraviolet light often provides a safe, efficient, and economical means of accomplishing disinfection for small potable water systems, industrial, food, and pharmaceutical plants, as well as for wastewater effluent treatment.
  • In medicine, ultraviolet radiation is used in the treatment of rickets and certain skin conditions.
  • Ultraviolet light has been used for cockpit illumination and has a disconcerting side effect if it were to become reflected directly into the eye. These radiations produce fluorescence of the crystalline lens in the eye, giving the pilot a sensation that he is flying in a fog. Properly adjusting the ultraviolet lamps and reducing their intensity can overcome this fluorescence problem to some degree.
  • The UV light is used in the blueprint machine to dry or cure ink as it’s printed.

Note- Inscadescent Bulb is used in car lighting systems.

 

Ques.85. LED lamp has a general life expectancy of

  1. 1000 hrs
  2. 5000 hrs
  3. 15000 hrs
  4. 50000 hrs

Answer.4. 50000 hrs

Explanation:-

Light-emitting diode (LED) lamps arc fairly new to the general illumination scene. As technology continues to improve, they will become a great option.

  1. LED lamps are available in a wide spectrum of colors (2700 being the closest to incandescent.
  2. LED lamps are cool to the touch.
  3. LED lamps are high efficiency.
  4. LED lamps have the longest life expectancy (50,000 hours).
  5. LED lamps are expensive.

 

Ques.86. Sodium vapor lamps need  ionization potential of about

  1. 5 volts
  2. 50 volts
  3. 100 volts
  4. 112 volts

Answer.1. 5 volts

Explanation:-

The ionization potential is one of these and may be defined as the energy required to remove an electron from an atom or ion. For e.g. The ionization energy of hydrogen is -13.6 eV.  A free-electron would have to move through a potential difference or voltage of 13.6 V to gain enough kinetic energy (i.e. 13.6 eV) to ionize a hydrogen atom by colliding with it.

The low-pressure sodium lamp is that the sodium atom has low values of first excitation and ionization potentials when compared with other gases or vapors. The ionization potential in the case of sodium vapor it is 5.12 volts.

 

Ques.87. The flicker effect of the fluorescent lamp is more pronounced at

  1. Lower voltages
  2. Higher voltages
  3. Lower frequencies
  4. Higher frequencies

Answer.4. Lower frequencies

Explanation:-

Light flicker is a phenomenon of rapid, quick, and repeated changes in the brightness of light over time. All lamps vary their light intensity at twice the mains (line) frequency since the power being delivered to the lamp peaks twice per cycle at 100 Hz or 120 Hz.

The flicker, when perceived, may cause considerable discomfort. In interior lighting, flicker from defective fluorescent lighting may cause the most nuisance when the frequency is close to the flicker-fusion frequency. The flicker fusion threshold for humans is conventionally taken to be 16 Hz.

 

Ques.88. When a sodium vapor lamp is switched on, initially the color is?

  1. Pink
  2. Yellow
  3. Green
  4. Blue

Answer.1. Pink

Explanation:-

Initially, the sodium is in the form of a solid, deposited on the walls of the inner tube. When sufficient voltage is impressed across the electrodes, the discharge starts in the inert gas, i.e., neon; it operates as a low-pressure neon lamp with pink color.

The temperature of the lamp increases gradually and the metallic sodium vaporizes and then ionizes thereby producing the monochromatic yellow light. This lamp takes 10-15 min to give its full light output. The yellowish output of the lamp makes the object appears gray.

 

Ques.89.  In a mercury vapor lamp light red objects appear black due to

  1. High wavelength of red objects
  2. Color mixing
  3. Absence of red light from lamp radiation
  4. Absorption of red light by the lamp radiation

Answer.3. Absence of red light from lamp radiation

Explanation:-

The mercury-vapor lamp gives off no red rays, and red objects appear black under its illumination.

 

Ques.90. Arc can be produced by?

  1. AC current only
  2. DC current only
  3. Either AC or DC current
  4. All option are correct

Answer.3. Either AC or DC current

Explanation:-

An electric arc is a visible plasma discharge between two electrodes that is caused by electrical current ionizing gasses in the air. Electric arcs occur in nature in the form of lightning. With proper control, electric arcs can be harnessed and used industrially for welding, plasma cutting, and even certain types of lighting such as fluorescent lighting where a high voltage ionizes the inert gas within a glass tube; the flow of current across the ionized gas liberates visible light.

An arc is generated between two conductors of electricity, cathode, an anode (Either AC or DC) when they are touched to establish the flow of current and then separated by a small distance. An arc is a sustained electric discharge through the ionized gas column called plasma between the two electrodes.

The electron that is negatively charged are attracted towards the positive contact(i.e anode) with a high velocity and on the way they detach more electrons by impacts. The positive ions are4 attracted towards the negative contacts (i.e cathode), but as they comprise almost the entire weight of the atom, they move towards it relatively slowly. This current flow is caused due to the movement of electrons.

In order to produce the arc, the potential difference between the two electrodes (voltage) should be sufficient to allow them to move across the air gap. The larger air gap requires a higher potential difference.  If the air gap becomes too large for the voltage, the arc may be extinguished.

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