100 Most Important MCQ Of Illumination with answer & explanation

Answer.3. 4 to 5

Explanation:-

The effect of Voltage Variation

The variations in normal supply voltages will affect the operating characteristics of incandescent lamps. 

With an increase in the voltage owing to the increase in the temperature, the luminous output of the incandescent lamps, and the efficiency and power consumption, its lifespan decreases.

The depreciation in the light output is around 15% over the useful life of the lamp. The above-stated factors are related to the variations of voltage are given as:

Lumens output ∝ (voltage)^3.55 (For tungsten)

Power consumption ∝ (voltage)^1.55.

Luminous efficiency ∝ (voltage)².

Life ∝ (voltage)⁻¹³ (for vacuum lamps).

Life ∝ (voltage)⁻¹⁴ (for gas-filled lamps).

Answer.3. When tube operates on dc supply

Explanation:-

The fluorescent lamp is essentially a gas discharge tube. A voltage is applied between two electrodes of a tube containing gas. Once the voltage exceeds the critical voltage, ionization voltage, gas is ionized and the ionized gas emits radiation, partly in the visible range and partly in the ultraviolet or infrared range. 

Like all arc discharge lamps, the fluorescent lamp requires a ballast in its circuit.  It consists essentially of an iron core on which is wound a coil. Its primary function is to limit the current in the arc circuit. For this reason, simple ballasts are referred to as chokes, since they are no more than a choke coil. Without the ballast in the circuit, the lamp would draw excessive current, and the fuse or circuit breaker would open. 

Fluorescent Lighting with Electronic Ballasts—

Fluorescent lamps can run directly from a d.c. supply of sufficient voltage to strike an arc. A choke has very low impedance with dc. supply. Hence, resistance in series with the choke must be used. This would consume about as much power as the lamp. When operated from d.c. supply, the starting switch is often arranged to reverse the polarity of the supply to the lamp each time it is started; otherwise, the mercury accumulates at one end of the tube. Fluorescent lamps are (almost) never operated directly from d. c. supply for those reasons. Instead, an inverter converts the d.c. into a.c. and provides the current-limiting function as described below for electronic ballast. Its primal function is to limit the current in the arc circuit. Without the ballast in the circuit, the lamp would draw excessive current, and the fuse or circuit breaker can be open.

The efficiency of fluorescent ballasts can be improved beyond the use of energy-efficient magnetic ballasts. When fluorescent lamps are operated by an electronic ballast at high frequency, they convert the input power to light output more efficiently. The lumens per watt (LPW) of the lamp/electronic ballast combination is increased, which means either producing more light for the same power or producing the same light with lower power. 

Answer.3. Unity

Explanation:-

A filament lamp is considered a resistive load. So the power factor of it is 1 (unity).

Answer.1. Sodium vapor lamp

Explanation:-

SODIUM VAPOR LAMP

A sodium vapor lamp is low pressure and cold cathode lamp i.e its electrode is not electrically heated by a filament. A sodium vapor discharge lamp consists of a U-shaped tube enclosed in a double-walled vacuum flask, to keep the temperature of the tube within the working region. The inner U-tube consists of two oxide-coated electrodes, which are sealed with the ends. These electrodes are connected to a pin-type base construction of sodium vapor lamp is shown in Fig.

This sodium vapor lamp is a low luminosity lamp so the length of the lamp should be more. In order to get the desired length, it is made in the form of a U-shaped tube. This long U-tube consists of a small amount of neon gas and metallic sodium. At the time of start, the neon gas vaporizes and develops sufficient heat to vaporize metallic sodium in the U-shaped tube.

Answer.3. Assists in developing enough heat to vaporize the sodium

Explanation:-

Working of Sodium Vapor Lamp

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.  At the time of start, the neon gas vaporizes and develops sufficient heat to vaporize metallic sodium in the U-shaped tube. 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.

In order to start the lamp, 380-450 V of striking voltage is required for 40- and 100-W lamps. These voltages can be obtained from a high reactance transformer or an autotransformer. The operating power factor of the lamp is very poor so a capacitor is placed to improve the power factor to above 0.8. More care should be taken while replacing the inner tube, if it is broken, then sodium comes in contact with the moisture; therefore, fire will result. The lamp must be operated horizontally or nearly so, to spread out the sodium well along the tube.

Answer.2. 3500°C

Explanation:-

• Carbon has a high melting point of 3,500°C; even though its melting point is high, carbon starts disintegration at a very fast rate beyond its working temperature of 1,800°C.
• Its resistance decreases with an increase in temperature, i.e., its temperature coefficient of resistivity is negative, so it draws more current from the supply. l he temperature coefficient (α) is -0.0002 to -0.0008.
• The efficiency of the carbon filament lamp is low; because of its low operating temperature, large electrical input is required. The commercial efficiency of carbon lamps is 3 – 4.5 lumens/W approximately.

Answer.2. Increasing filament temperature

Explanation:-

The lumen output of the discharge lamp is given by the following expression

Lumens output ∝ (voltage)^3.55

Lumens output ∝ L.d

Where

L = length of filament

D = Diameter of filament

The maximum temperature at which the filament can be worked without oxidization is 2,000°C, i.e., beyond this temperature, the tungsten filament blackens the inside of the bulb. The tungsten filament lamps can be operated efficiently beyond 2,000°C, it can be attained by inserting a small quantity of inert gas nitrogen with a small quantity of argon. 

Answer.4. Any of the above

Explanation:-

The materials commonly used as the filament for incandescent lamps are carbon, tantalum, tungsten, and osmium.

The materials used for the filament of the incandescent lamp have the following properties.

• The melting point of the filament material should be high.
• The temperature coefficient of the material should be low.
• It should be high resistive material.
• The material should possess good mechanical strength to withstand vibrations.
• The material should be ductile.

Answer.3. Vapor pressure inside

Explanation:-

The rate of evaporation of tungsten filament in a lamp depends on Vapor pressure inside.

The color temperature of a normal filament lamp is typically between 2800 K and 3000 K. At the extremely high temperature of the filament, tungsten tends to evaporate. This leads to the familiar blackening of an incandescent lamp envelope. The evaporation of the tungsten filament can be reduced by filling the lamp envelope with a suitable gas that does not chemically attack the filament.

However, gases also cool the filament by conducting heat away from it, and they decrease lamp efficiency. The gas used must, therefore, be carefully chosen. It should adequately suppress tungsten evaporation without overcooling the filament. In addition, it should not readily pass an electric current, for otherwise arcing may occur which would destroy the lamp.

The evaporation of the tungsten filament means a steady decrease in the light output, because the tungsten vapor condenses on the bulb wall, turning it black and absorbing light. In order to increase the efficiency of the incandescent lamp without at the same time shortening filament life, one must aim a reduce the evaporation rate of the filament. The rate of evaporation is controlled by the gas filling, usually a mixture of argon and nitrogen. Evaporated tungsten settles on the inner surface of the glass bulb, discoloring it and reducing the amount of light emitted by the lamp throughout its life. 

Argon and nitrogen are the gases most commonly used. Nitrogen will minimize the risk of arcing but will absorb more heat than argon. Argon is used by itself in general service lamps. A mixture of the two gases is used in incandescent lamps where the tendency for arcing is more likely, such as in projector lamps. In this case, the amount of nitrogen present is kept very small as little as 5% in order to obtain optimum lamp efficiency. Lamps filled with krypton gas have a longer life than argon and nitrogen lamps and cost more.

Answer.3. 314 lumens

Explanation:-

The mean spherical candlepower is the average of the candlepowers in all directions from the lamp. 

This relationship between candlepower and lumens can be expressed mathematically as follows:

L = 12.57 CM

Where

L = Lumen

CM = mean spherical candlepower

L = 12.57 × 25 = 314.25