Category Technology

   To understand refrigeration, pour a small quantity of petrol in your hand and expose it to the atmosphere. The petrol evaporates leaving a cooling sensation. This is because the petrol has taken the heat from the hand and transformed it into vapour. (This is known as latent heat of vaporization).

            The working substance in the refrigerator is called a refrigerant. It compressed to a high pressure and then pressure and then allowed to condense by rejecting its heat to the atmosphere surrounding it. The liquid is then allowed to expand at a low pressure. During this process it absorbs heat from the spaced inside the refrigerator and become a vapour. The above cycle is repeated in vapour-compression refrigerator which differs fundamentally from vapour absorption system.

            The major compounds in a refrigerant are halocarbon compounds (which includes halogens: chlorine, fluorine and bromine); hydrocarbons (such as methane, ethane and propane); inorganic compounds (such as ammonia, water and air) and unsaturated organic compounds (hydrocarbon with ethylene and propylene base).

            Refrigerant is the working fluid in a fridge. It should have a low boiling point so that it vaporizes at low temperatures and absorbs the heat from a substance. Some of the common refrigerants are ammonia, carbon-dioxide and Freon. Ammonia is often used as it is cheap and easily available. But it is toxic and explosive at high temperatures. Carbon-dioxide was one of the first refrigerants used but presently its use is limited. It is non-toxic and non explosive but causes suffocation when its concentration is high. Freon 12 has a boiling point of -30 degree Celsius. This refrigerant is fluorocarbon of methane and ethane series. It is a combination of halogens, chlorine and fluorine. It is the most commonly used refrigerant. Its chemical name is dichloro-difluoro-methane. It condenses at a moderate pressure under normal atmospheric temperature. This property makes it the most suitable refrigerant.

   Our retina can only respond to stimuli in one way: by sending a signal along optic nerve to the brain, which interprets this signal as light. Normally, the stimulus is light impinging on the retina, but other stimuli work too.

            One of the commonest is a mild trauma to the eyeball. If you close your eyes in a dark room and press gently on the front of your eye will see a faint flow in your peripheral vision.

            This can also happen when a blow to the head causes you to close your eyes hard for a moment. You sometimes see stars or flashes when you sneeze, which is again the result of eyeball compression as the eyelid is forced closed. The retina will also send a light signal response to the brain if it becomes torn or detached. This is a serious condition requiring treatment by an ophthalmologist. Another potential source of problem is the jelly-like vitreous humour that fills the rear two-thirds of the eye. It is attached to the retina at several points, and tugging on it can result in the sensation of light when, for example, you move your eyeball around rapidly under your lid.

            These attachments can fail as we age, which is normally not a problem unless the vitreous pulls some retina with it. This, incidentally, is the cause of floaters, the little bits of matter you can sometimes see floating about inside your eye. Light flashes can also be associated with migraine headache and central nervous system disorders, which can act on the vision centres of the brain to produce the sensation of light.

            This effect is the result of a marked increase in intra-ocular pressure (IOP) caused by the blow. This pressure is exerted on the eye by the vitreous humour – the fluid within the eye that gives it its shape.

            The increase in IOP triggers the rods and cones that make up the retina, sending an electrical signal via the optic nerve to the visual cortex at the back of the head. The visual cortex is where we perceive what we see, and hence we think we are seeing a star for a few second nicely matches the timescale of the temporary rise in IOP.