The speed of the light in different transparent substances is different. For example speed of light in air is 3×10⁸ m/s and that of in glass is 2×10⁸ m/s. It is clear that the two speeds of the light are different. When light enters from air to glass the speed of the light gets decreased at the junction of the two media. This change in speed of the light affects the speed the of the various segments of the beam of the light. This can be concluded by the image. This change in speed of the light causes refraction.

(1) The reflecting surface of the concave mirror is bulged inwards. But the reflecting surface of the convex mirror is bulged outwards.
(2) Concave mirror produces real and inverted images (except when the object is placed between pole and focus. When the object is placed between F and P, concave mirror produces virtual and erect image). But the convex mirror always produces virtual and erect images.
(3) The image formed by the concave mirror may be enlarged or diminished, according to the position of the object. The image formed by the convex mirror will be always diminished. 
LENSES

Convex lenses are curved outward and they cause light that goes trough it to converge or concentrate on one point. Think of a magnifying glass that is used to burn something. The light that goes through it concentrates on a point so you can burn things. Concave lenses are curved inward and when light goes through it diverges or spreads out. Think of a flashlight. The light comes from a small bulb goes through it and spreads out to a bigger beam on the other side.

Gene is a unit of heredity which is transferred from a parent to offspring and is held to determine some characteristic of the offspring.

Combustion is the process of burning something.

Combustion  or burningis a high-temperature exothermic redox chemical reactionbetween a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke.

Sodium metal reacts rapidly with water to form a colourless solution of sodiumhydroxide (NaOH) and hydrogen gas (H2).

Na + H2O →NaOH + H2

Now as per the law of conservation of mass, mass is neither created nor destroyed in a chemical reaction. So, the equation should be balanced.

Balancing the equation we have

2Na+ 2H2O → 2NaOH + H2

Braille symbols are formed within units of space known as braille cells. A fullbraille cell consists of six raised dots arranged in two parallel rows each having three dots. The dot positions are identified by numbers from one through six. Sixty-four combinations are possible using one or more of these six dots.

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heart is a muscular organ that pumps blood to your body. Your heart is at the center of your circulatory system. This system consists of a network of blood vessels, such as arteries, veins, and capillaries. These blood vessels carry blood to and from all areas of your body.

Eutrophication, or more precisely hypertrophication, is the enrichment of a water body with nutrients, usually with an excess amount of nutrients.

electroplating is  used to coat a relatively mundane metal, such as copper, with a thin layer of another, more precious metal, such as gold or silver. Electroplating has lots of other uses, besides making cheap <a href="http://www.explainthatstuff.com/introduction-to-metals.html">metals</a> look expensive. We can use it to make things rust-resistant, for example, to produce a variety of useful <a href="http://www.explainthatstuff.com/alloys.html">alloys</a> like brass and bronze, and even to make plastic look like metal.

Petrochemical is related to or denoting substances obtained by the refining and processing of petroleum or natural gas.