Modern Commentary

Torah & Spirit
Family Life






Shabbat - Lights

Exodus 35.3:
You shall not cause a fire to burn in all your settlements on the Sabbath day.

Stories exist in Rabbinic lore about shivering Karaites eating stone-cold food huddled on the floor in a dark room over Shabbat. While there is some historical truth to this story, there is also a great deal of myth - mostly due to the story's persistence.  The Torah does, in fact, prohibit the burning of fires on Shabbat. As above, Ex. 35.3 (this time in Hebrew): Lo'-Teva`aru Esh BeKhol Moshevoteikhem BeYom HaShabbat. 

The word 'Tav'aru' comes from the root word 'ba'ar,' which means to burn. Consequently, what we are prohibited from doing on Shabbat is burning fire - or having a fire burning.  The question now, may be raised, how does this apply to light bulbs? Those familiar with the Rabbinic prohibition not to turn on and off lights will find a different prohibition herein discussed. To best address the issue at hand, one must first understand the elements one is dealing with.  The following paragraphs will explain the elements that are factored into the discussion at hand: Fire, incandescent light bulbs, and fluorescent light bulbs. 

Fire is a chemical reaction in which a combustible fuel reacts with oxygen to release large amounts of thermal energy. Many atoms bind very strongly with oxygen atoms and these fuel atoms release energy when they bind with oxygen. Initiating these combustion reactions normally requires some thermal energy to get started. This starting energy is known as activation energy. That's why you have to light the fire--you must provide the activation energy. After that, each oxidization reaction produces the activation energy needed to start another oxidization reaction and the fire keeps itself going until it has consumed all of its fuel.  

A normal incandescent lamp contains a double-wound tungsten filament inside a gas-filled glass bulb. A double-wound tungsten filament is a very fine wire that is first wound into a long, thin spiral. This spiral, then, is wound into a wider spiral. While the final filament looks about 1 or 2 centimeters long, it actually contains about 1 meter of fine tungsten wire. When the bulb is turned on, an electric current flows through the filament from one end to the other. The electrons making up this current carry energy, both in their motion and in the forces that they exert on one another. As they flow through the fine tungsten wire, these electrons collide with the tungsten atoms and transfer some of their energy to those tungsten atoms. The tungsten atoms and the filament become extremely hot, typically about 2500° Celsius. Tungsten wire is used because it tolerates these enormous temperatures without melting and because it resists sublimation longer than any other material. Sublimation is when atoms "evaporate" from the surface of a solid. The gas, argon, inside the bulb is there to slow sublimation and extend the life of the filament. (Argon [Ar] is a monatomic, chemically inert gas composing slightly less than 1% of the air. Its gaseous specific gravity is 1.38 and its boiling point is -302.6 degrees F (-185.9 degrees C). Argon is colorless, odorless, tasteless, non-corrosive, nonflammable, and nontoxic.) Once the filament is hot, it tends to transfer heat to its colder surroundings. While much of its heat leaves the filament via convection and conduction in the gas and glass bulb, a significant fraction of this heat leaves the filament via thermal radiation. For any object that is hotter than about 500° Celsius, some of this thermal radiation is visible light. For an object that is approximately 2500° Celsius, about 10% of the thermal radiation is visible light. However, most of the filament's thermal radiation is invisible infrared light. While you can feel this infrared light warming your hand, you can't see it. Only about 80% of the electric power delivered to the bulb becomes thermal radiation and only about 12% of that thermal radiation is visible. Consequently, an incandescent light bulb is only about 10% energy efficient.  

Other types of lamps, including fluorescent and gas discharge lamps, are much more energy efficient. A fluorescent lamp tries to produce light without heat. It collides electrons with mercury atoms to produce an atomic emission of ultraviolet light. This ultraviolet light is then converted to visible light by the layer of white phosphor powders on the inside of the lamp's glass envelope. In principle, this whole activity can be performed without creating any thermal energy. However, many unavoidable imperfections cause the lamp to convert some of the electric energy it consumes into thermal energy. Nonetheless, the lamp only becomes warm rather than hot.   

At first glance, it may appear that both fluorescent light bulbs and incandescent light bulbs would be permissible for use on Shabbat. Neither produces a flame to generate light. Consequently, they would not violate Lo Tav'aru. However, the tungsten filament in an incandescent light is burning. It burns the same way a glowing coal burns. In the case of the coal, carbon monoxide and carbon dioxide inhibit it from flaming. In the case of the tungsten filament, it is the argon. However, the argon doesn't keep the filament from burning all together. In fact, when a bulb blows the filament has burned so thin that the amount of heat generated in it causes it to flame (ever so briefly) and totally burn through. The thinner the filament the less electrical energy it takes to cause it to flame. The argon only inhibits the flaming on a newer filament because the thickness of the filament in the presence of the argon doesn't allow it to get to the flash point, but as the filament burns, it gets thinner and thinner. 

In conclusion, from this we learn that incandescent lights are prohibited for use on Shabbat, but fluorescent lights are permitted.  Also, keep in mind that because we are not allowed to engage in commerce over Shabbat, so we are also prohibited from using electricity from the electric company.  Lights operated on Shabbat should be battery operated, or solar.