This medication is used on the skin to treat the itching, scaling, and flaking due to skin conditions such as psoriasis or seborrheic dermatitis. Coal tar belongs to a class of drugs known as keratoplastics. It works by causing the skin to shed dead cells from its top layer and slow down the growth of skin cells. This effect decreases scaling and dryness. Coal tar can also decrease itchiness from these skin conditions.

(a) Coal is classified into four main types, or ranks: anthracite, bituminous, peat and lignite.

B) Fractions that are separated out include gasoline, diesel, kerosene, and bitumen. Fractional distillation allows a lot of useful products to be made from crude oil, with many environmental consequences for the use of those useful products

c. Sodium metal is kept in kerosene because sodium is very reactive metal. It is kept in kerosene to prevent it from coming in contact with oxygen and moisture. If this happens, it will react with the moisture present in air and form Sodium hydroxide. This is a strongly exothermic reaction, and lot of heat is generated. It will result out in the form of fire.

1. Metal alloys have improved and unique property of hardness and sonority. Also, alloys have excellent ductility. Therefore metal alloys are used for making bells and strings of instruments. For example, bronze is used for making bells and stringed instruments.

A n s w e r
Smooth and polished surface provides least friction. eg: polished marble, frozen river etc.
As the coefficient of friction (μ) is less for smooth surfaces 
so they produce less friction than rough surface.
 

Smooth surface produces less friction.

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Amplitude—maximum displacement from the equilibrium position of an object oscillating around such equilibrium position. Frequency—number of events per unit of time. Period—time it takes to complete one oscillation. The number of cycles that a vibrating object completes in one second is called frequency. The unit of frequency is hertz (Hz). One hertz equals one cycle per second.

When a ray of light approaches a smooth polished surface and the light ray bounces back, it is called the reflection of light. The process through which light rays fall on the surface and gets bounced back is known as a reflection of light.

Laws of reflection

The laws of reflection determine the reflection of incident light rays on reflecting surfaces, like mirrors, smooth metal surfaces, and clear water.

Following are the laws of reflection:

• The incident ray, the reflected ray and the normal to the surface of the mirror all lie in the same plane.
• The angle of incidence is equal to the angle of reflection.

• Eating carrots is good for your vision.
• Eye exercises are great when done in the morning, when your eyes feel tired and before retiring to bed.
• Closing your eyes for just a few minutes is helpful.
• Resting your eyes for a couple of minutes isn’t enough. 

Chemical Changes
The changes in which new substance are formed,are called chemical changes.

In a chemical change,the substances involved change their identity ie they get converted into entirely new substance.

For Ex:Burning of magnesium ribbon,Rusting of iron,Formation of curd from milk,Cooking of food etc.

The presence of hydrogen and helium in very high amount leads the sun to be the only source of heat and light for all planets as hydrogen and helium being a light element do nuclear fusion to form enormous energy with production of heat and light.

The major products (or fractions) of petroleum refining are:

• Petroleum gas
• Diesel
• Petrol
• Kerosene
• Lubricating oil
• Paraffin wax
• Bitumen

The kaleidoscope is an instrument or a toy containing inclined plane mirrors which produce multiple reflections of coloured glass pieces and create beautiful patterns. The coloured glass pieces act as objects and the inclined plane mirrors form multiple images of these glass pieces by repeated reflections, which look like beautiful patterns or designs.

To make a kaleidoscope, take three rectangular mirror strips about 15 cm long and 4 cm wide each and join them together to form a prism. Fix them in a circular cardboard tube or tube of a thick chart paper. Close one end of the tube by a cardboard disc having a hole in the centre, through which you can see. To make the disc durable, paste a piece of transparent plastic sheet under the cardboard disc. At the other end, touching the mirrors, fix a circular plane glass plate. Place on this glass plate several small pieces of coloured glass. Close this end of the tube by a ground glass plate. Allow enough space for the colour pieces to move around.

To make a kaleidoscope, get three rectangular mirror strips each about 15 cm long and 4 cm wide. Join them together to form a prism as shown in Fig. (a). Fix this arrangement of mirrors in a circular cardboard tube or tube of a thick chart paper. Make sure that the tube is slightly longer than the mirror strips. Close one end of the tube by a cardboard disc having a hole in the centre, through which you can see [Fig (b)]. To make the disc durable, paste a piece of the transparent plastic sheet under the cardboard disc. At the other end, touching the mirrors, fix a circular plane glass plate [Fig (c)]. Place on this glass plate several small pieces of coloured glass (broken pieces of coloured bangles). Close this end of the tube by a ground glass plate. Allow enough space for the colour pieces to move around.

1. Rods and Cones are the photoreceptors found in the eye, rods have rod-like structure and provide twilight vision, while cones are of the cone shape, fewer in number and provides the vision in the day or bright light.
2. Rods are found around the boundary of the retina, whereas cones are there in the centre of the retina.
3. Out of the 125 million photoreceptors, rods are about 120 million in the human eye and cones are 5 million photoreceptors.
4. The outer segment is cylindrical of rods which contain rhodopsin pigment, made up of Vitamin A, while the outer segment is conical of cones which contain iodopsin pigment.
5. Cones provide colour vision, which is of three types: red, blue and gree, whereas rods cells do not give colour vision, they do not have any differentiation.
6. Lack of the rhodopsin in the rods may cause night blindness, while lack of the iodopsin in the cones may cause colour blindness.
7. Rods provide the vision in the dull light (darkness or night), whereas cones are known to give the vision during the day or bright light.

When a photo-diode is reverse biased, the width of depletion layer increases as compared to forward biased and a small reverse current (dark current) flows through the diode . Now , when the light is incident on the junction, electron-hole pairs are generated in depletion layer in a big amount (due to broad depletion layer) and these charge carriers can easily cross the barrier, hence contribute to current across the diod.

We can say that in reverse bias, diode changes the incident light to current , more effectively due to broad depletion layer i.e. when the diode is reversed biased , no ordinary current flows and the detection of the photo current is much easier.

The process in which a volatile liquid is first vaporised at its boiling point and the vapours are condensed to get back the liquid in pure form is called distillation.

If the difference in boiling points of the liquids in the mixture is less than 250C the method of fractional distillation is used to separate them. 

Fractional distillation method is same as simple distillation except that in the apparatus a fractionating column is fitted in between the distillation flask and the condenser.

It makes the appratus more efficient . A fractionating column is a wide tube packed with glass beads, which provides a larger surface area of cooling. Vapours of liquid having higher boiling point get condensed on the surface of these beads, while the vapours of lower boiling point liquid pass on upwards. Length of the column also increases the efficiency of the process.

Example:  In an oil refinery, the different fraction of petroleum products are separated by fractional distillation of crude petroleum. 

Burning of coal in absence of air to obtain some useful constituents is called destructive Distillation. Following constituents are obtained from this process :

(i) Coke : It is a hard porous substance which is almost the purest form of carbon. It is used in manufacture of steel and in extraction of many metals.

(ii) Coal-tar : It is a black liquid with unpleasant smell. It is a mixture of about two hundred substanes which are very useful in our day to day life. They are used in drugs, plastics, paints, photographic materials, synthetic dyes, roofing materials, perfumes, explosives etc. Insect repellants like naphthalene is also obtained from it. Earlier, it was also used for surfacing of roads.

(iii) Coal gas : This gas is also obtained during destructive distillation of coal. It is used as fuel in coal processing plants and in various other industrial units situated near coal processing plants. Coal gas was used by the Americans and the Britishers for street lighting.

• Wash your eyes with clean cold water.
• Do not use too bright or too dim light while reading or doing work. Dim light can cause eye strain and headaches. Too bright light can injure the retina.
• Avoid reading in a moving vehicle.
• Never rub your eyes if something gets inside. You should wash your eyes with clean cold water.
• Self-treatment could be dangerous to the eyes. You must consult a doctor in case of any injury to the eyes.
• Good nutrition is very important for both your general and eye health. Good nutrition keeps the eye in good condition.

  Iris: It controls the amount of light entering the eye.
 Pupil: It controls the illumination in the eye.

Retina: It is a delicate membrane having a large number of light-sensitive cells called 'rods' and 'cones' which respond to the intensity of light and colour of objects respectively.
Optic nerve: It conveys the image formed on the retina to the brain.

Eyes are the sense organs which give us the sense of vision. The human eye is almost spherical in shape. Following are the main structures in the human eye.

Cornea: The eyeball is covered with a tough layer. This layer is transparent on the front. This transparent portion is called cornea.

Iris: Iris is a thin circular structure. It works like the shutter of a camera. It controls the amount of light entering the eye. The colour of iris imparts distinct colour to the eyes of an individual. A person with blue iris has blue eyes.

Pupil: There is a hole in the centre of iris. This is called pupil. Light enters the eye through the pupil. When the light is bright, iris contracts and thus allows less light into the eye. When the light is dim, iris dilates and thus allows more light into the eye.

Lens: Lens is present behind the pupil. The lens in the human eye is a convex lens. The size and thickness of the lens change as per the distance of an object.

Retina: Retina is at the back of the eye and marks the inner layer of the eyeball. Images are formed on the retina and thus it works like a screen. There are photosensitive cells in the retina. These cells are of two types, which are as follows:

• Cone Cells: The cone cells are sensitive to bright light. They also give the sense of colour.
• Rod Cells: The rod cells are sensitive to dim light.

Optic Nerve: The optic nerve emerges from the back of the eye ball. This nerve goes to the brain.

Blind Spot: The junction of retina and the optic nerve is called the blind spot. There is no photosensitive cell at this spot, and hence no image is formed at this spot on the retina.

Carbonisation is the slow process of conversion of dead vegetation into coal. Scientists believe that about 300 million years ago, dead vegetation was overrun by soil. As more soil got deposited over the it, more pressure was exerted on this humus. Humus, at this stage, is called peat, the inferior quality of coal. As this went deeper and deeper (due to the weight from above), the peat began to experience more and more pressure from above and higher temperature from under. This compressed the peat and better varieties of coal were produced i.e. lignite and bituminous to anthracite (best) coal.

Cloning is the production of an exact copy of an animal by means of asexual reproduction. Any two animals which contain exactly the same genes are called genetically identical. An animal which is genetically identical to its parents is called clone

Animal husbandry refers to livestock raising and selective breeding. It is the management and care of animals in which the genetic qualities and behavior of animals are further developed for profit. A large number of farmers depend upon animal husbandry for their livelihood.

Animals provide us with a variety of food products which have high nutritional values. Therefore, they require a lot of care and attention.

Animals are bred commercially in order to meet the high demand for food. Dairy products from animals like cows, buffaloes, goats, are rich sources of protein. These animals are called milch animals as they provide us with milk.