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Echo
Echo
A sound that reaches listeners more than 0.05 seconds after an initial sound has reached them directly. (See also: Reverberation.)
Electric light
Light produced when electrical energy is converted into light energy. This can be done using
a spark (as in lightning or an
arc lamp), by heating a filament (as in an ordinary incandescent electric light bulb), or by making special chemicals glow (as in a fluorescent tube).
Halogen light
 Electric light – We use many forms of electric light, including the light from hot filaments: incandescent, halogen lights, the light from the reaction of charged gases with phosphors – fluorescent lights – and the glowing of gases when an electric charge passes through them (neon lights, mercury vapour lights, sodium lights, and so on).
Incandescent light: This light bulb consists of a coiled filament of wire that is suspended inside a glass bulb. The glass bulb is filled with
a gas like argon or nitrogen. When electricity flows through the filament, the filament gets hot and gives out light. The gas in the bulb is designed to keep the filament from burning away, as it would if it was in air.
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Eye, eyesight
The organ that collects light and converts it into electrical signals that are then sent to the brain. The eyesight of all living things works in basically the same way. The eyes contain special cells that are sensitive to light and nerves that connect these cells directly to the brain.
The simplest ‘eyesight’ is found in some worms. They have light- sensitive cells on parts of their skins. Insects have lots of small eyes that cannot change their focus. They are called compound eyes. More developed animals have eyes that can focus light and alter the amount of it reaching the eye. In advanced animals the back of the eye may have millions of light- sensitive cells. Cells near the centre of the eye are cone-shaped (cones), while those near the edge are rod- shaped (rods).
The human eye is contained in
an outer protective coat. The front of this coating is transparent and is called the cornea. On the inside at the back are the light-sensitive cells. This region is called the retina.
Light passing through the cornea travels through the pupil of the eye, which changes size depending on the amount of light. It then passes through the lens and travels through the fluid inside the eye before reaching the retina.
The cornea, lens and liquid
all have the same light-bending powers, so light does not get bent
or distorted as it passes from one to another. Most of the light-bending happens at the outside of the cornea, while only fine-tuning and focusing of the light are done by the lens.
To work properly, the cornea has to be kept moist, which is why we have tear ducts and why we blink often.
The lens is made of elastic, transparent fibres. Over time these fibres become less elastic, and it becomes harder for the eye to focus. This usually causes problems focusing on objects that are close to the eye.
The iris is a thin, circular disc with a hole in the centre called the pupil. The iris contains a layer of coloured cells. They give the eye its