Bending of light at the boundary of two medium is called refraction.
The light bending ability of that particular medium is called refractive index.
Refractive index is defined as ratio of sine of angle of incidence to sine of angle of refraction.
It is denoted by n or μ.
Thus, refractive index is given by,

Absolute refractive index: When the light is going from vacuum to another medium, then the value of refractive index is called the absolute refractive index.
Relative refractive index: When the light goes from one medium ( other than vacuum or air) to another medium then that refractive index is called relative refractive index.

Voltmeter is used to measure potential difference.It is always connected in parallel across the points where the potential difference is to measured so one end of the voltmeter is connected to the point A of a resistance AB and the other end to the point B,and also it has a high resistance so that it takes a negligible current from the circuit.

Voltmeters have high resistance, so in series, they would greatly decrease the current going to the circuit.

Ammeters are very low R, so in parallel, the current would bypass the circuit, all go through the ammeter, and blow out the ammeter. 

Voltmeter is used to measure potential difference.It is always connected in parallel across the points where the potential difference is to measured so one end of the voltmeter is connected to the point A of a resistance AB and the other end to the point B,and also it has a high resistance so that it takes a negligible current from the circuit.

Voltmeters have high resistance, so in series, they would greatly decrease the current going to the circuit.

Ammeters are very low R, so in parallel, the current would bypass the circuit, all go through the ammeter, and blow out the ammeter. 

• Electric generators are based on the principle of electromagnetic induction and converts mechanical energy into electrical energy.
• An electric generator consists of a rotating rectangular coil placed between the two poles of a permanent magnet.
• The two rings R1 and R2 are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field.
• Outer ends of the two brushes B1 and B2 are connected to the galvanometer to show the flow of current in the given external circuit.
• When the axle is rotated, arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet. Let us say that the coil ABCD is rotated clockwise. By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions AB and CD. Thus, an induced current flows in the direction ABCD. If there are more turns in the coil, the current generated in each turn adds up to give a large current through the coil.
• After half rotation, CD and AB reverse direction, and thus, CD starts moving up and AB starts moving down. As a result, the directions of the induced currents in both the arms change. Such a current which changes direction after equal intervals of time is called an alternating current. Thus, this device is called an AC generator.
• To get a direct current, a split-ring type commutator must be used. The generator is thus called a DC generator.
• The difference between direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically.
  

Spore formation is a type of asexual reproduction found among most of the non-flowering plants and eukaryotic organisms like fungi and bacteria.
Spore formation also called as sporulation involves reproduction of new plants by producing numerous microscopic spores.
These spores are present inside the spore-producing asci which break open under favorable condition to release the spores.
Sporogenesis is usually as a result of meiotic cell division.
A typical spore of a rhizobium consists of a stalk called hyphae, spore-containing sac called sporangium, and the spores inside the sporangium.
Motile spores are called as zoospores and nonmotile spores are called aplanospores.

Homologous organs are defined as the organs of different animals that are having a similar structure but differ in their functions.

Example of homologous organs are the forelimbs of frog, man, and the flippers of the whale. The structure of these animals are the same but the functions are different.

The flippers are used for swimming, the forelimbs of frogs are used for jumping and propping, and the forelimbs of man are used for writing, eating, holding things, etc. Having a similar structure shows that all these animals have a common ancestor.

Egg shell is made of calcium carbonate. We know that when calcium carbonate reacts with nitric acid, it gives carbon dioxide, calcium nitrate and water. Same reaction happens when nitric acid is poured over egg shell. Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine gas:

HNO₃ (aq) + 3 HCl (aq) → NOCl (g) + Cl₂ (g) + 2 H₂O. as evidenced by the fuming nature and characteristic yellow color of aqua regia.

Nutrition in Human Beings (Complex Multicellular Animal):

• The various organs of the human digestive system in sequence are: Mouth, Oesophagus (Food pipe), Stomach, Small intestine and Large intestine.

• The glands which are associated with the human digestive system are: Salivary glands, Liver and Pancreas.

The various steps of nutrition in human beings are as follows:

1. Ingestion:

• In human beings, food is ingested through the mouth. The food is put into the mouth with the help of hands.

2. Digestion:

• The digestion of food begins in the mouth itself.

• The teeth cut the food into small pieces, chew and grind it. (Physical digestion)

• The salivary glands in our mouth produce saliva (watery liquid) which contains an enzyme salivary amylase which digests the starch (carbohydrate) present in the food into sugar. (Chemical digestion)

• Our tongue helps in mixing this saliva with food.

• The digestion of food remains incomplete in mouth.

3. Oesophagus:

• The slightly digested food in the mouth is swallowed by the tongue and goes down the food pipe called oesophagus.

• When the slightly digested food enters the food pipe, the walls of food pipe start contraction and expansion movements called as peristaltic movement.

• This peristaltic movement of food pipe pushes the slightly digested into the stomach.

4. Stomach:

• The stomach is a J-shaped organ present on the left side of the abdomen.

• The stomach walls contain s three tubular glands in it walls which secrete gastric juice.

• The gastric juice contains three substances: Hydrochloric acid, the enzyme pepsin and mucus.

• The hydrochloric creates an acidic medium which facilitates the action of the enzyme pepsin i.e. digestion of protein.

• The mucus helps to protect the stomach wall from its own secretions of hydrochloric acid.

• The partially digested food then goes from the stomach into the small intestine.

5. Small intestine:

• • From the stomach, the partially digested food enters the small intestine.

  • The small intestine is the largest part (about 6.5m) of the alimentary canal.

  • The small intestine is very narrow and arranged in the form of a coil in our belly.

  • The small intestine in human beings is the site of complete digestion of food (like carbohydrates, proteins and fats)

  • The small intestine receives the secretion of two glands: Liver and Pancreas.

  • Liver secretes bile (greenish yellow liquid made in the liver and stored in gall bladder).

  • Bile performs two functions:

    1. Makes the acidic food coming from the stomach alkaline so that pancreatic enzymes can act on it.

    2. Bile salts break the fats present in the food into small globules making it easy for the enzymes to act and digest them.

  • The pancreas secretes pancreatic juice which contains enzymes like pancreatic amylase for breaking down starch, trypsin for digesting proteins and lipase for breaking down emulsified fats.

  • The walls of the small intestine contain glands which secretes intestinal juice. The enzymes present in it finally convert the proteins into amino acids, complex carbohydrates into glucose and fats into fatty acids and glycerol.

  • In this way the process of digestion converts the large and insoluble food molecules into small water soluble molecules.

6. Absorption:

• The small intestine is the main region for the absorption of digested food.

• The inner surface of the small intestine has numerous finger-like projections called villi which increase the surface area for rapid absorption of digested food.

• The digested food which is absorbed through the walls of the small intestine goes into our blood.

7. Assimilation: The blood carries digested and dissolved food to all the parts of the body where it becomes assimilated as part of the cells and is utilised for obtaining energy, building up new tissues and the repair of old tissues.

8. Egestion: The unabsorbed food is sent into the large intestine where more villi absorb water from this material.

The cause of refraction of light is that light travels with different speeds in different media. This Change in the speed of light when it moves from one medium to another causes it to bend.

Alkalis are bases which are completely soluble in water.
Sodium oxide and potassium oxide are alkalis which dissolve in water to form soluble bases sodium hydroxide and potassium hydroxide respectively.

The magnesium ribbon should be cleaned before burning in the air because the layer of magnesium oxide (which is formed due to the reaction of magnesium with air ) can be removed in order to get the desired chemical reaction. When the magnesium metal burns it reacts with oxygen found in the air to form Magnesium Oxide. ... After it burns, it forms a white powder of the magnesium oxide. Magnesium gives up two electrons to oxygen atoms to form this powdery product. This is an exothermic reaction.

Magnesium ribbon must be cleaned before burning in air so that the the layer of magnesium oxide (which is formed due to reaction of magnesium with air) can be removed in order to get the desired chemical reaction.