Formation of Image by a Plane Mirror
Incase of the plane mirror, the distance of object and image formed are equal,but are on the opposite side of the mirror. One of the important facts observedin the plane mirror is that, when an object is placed in front of a plane mirror,the right side of the object appears to be left and the left side appears to beright. This phenomenon is called lateral inversion.
The word AMBULANCE on the hospital van is always written in the form of itsmirror image. This is done because if we are driving a car, we will be able tosee the hospital van coming behind us, in rear view mirror. We get the laterallyinverted image of the word ambulance and thus give pass to the van.
Uses of Plane Mirror
(a) It is used as a viewing mirror in our house
(b) Is used in the shop of the barber, jeweler etc
(c) It is used in periscope
Spherical Surfaces
There two types of spherical mirror:
(i) Concave
(ii) Convex
Concave Mirror: The mirrors curved inwards are called concave mirror.
Convex Mirror: The mirrors curved outwards are called convex mirror.
The spherical surfaces consist of principal axis, focus, poles, centre of curvature.
The straight line passing through the focus and the pole of the mirror is calledthe principal axis.
Focus
The point on the principal axis at which the ray of light converges after reflectionfrom the spherical surfaces is called principal focus.
Focal Length
The distance between the focus and pole is called the focal length.
Pole
The point at which the principal axis passes the mirror is called the pole.
Centre of Curvature
The centre of curvature of the mirror is the centre of hallow spherical surface,of which the mirror is the part. It lies in front of the mirror, and on the principalaxis.
Radius of Curvature
The distance between the centre of curvature and the pole is called the radiusof curvature.
Aperture
The portion of the mirrorfrom which the reflection of light actually takes place,or it is the diameter of the reflecting surfaces.
Relation between the Radius of Curvature and FocalLength
For any spherical surfaces, focus is always the mid-point of the more... 
Reflection of Light
When the light is incident on the surface of an object, it is reflected, transmitted,or absorbed. When it falls on the polished surface, it reverts back on the sameplane. This phenomenon is known as reflection of light. In other words,"Bouncing back of the light after striking a polished surface is called reflectionof light".
Silver metal is the best reflector of light, as it reflects all of the light falling onit. It does not transmit or absorb any portion of light received. So an ordinarymirror is made by polishing the surface of glass with the silver paints. If wescratch the paints from the surface, it becomes transparent and light passesthrough it.
Laws of Reflection of Light
Reflection of light from the plane or spherical surface takes place according tothe laws given below. They are known as the laws of reflection of light.
(a) It states that the incident ray, the reflected ray, and the normal, alllies on the same plane.
(b) The angle of incidence and the angle of reflection are equal.\[\angle i=\angle r\]
Here,\[i\] denotes the angle of incidence and r denotes the angle of reflection.We observe, when the ray of light falls normal to the surface, it is reflectedalong the same path. In such case, the angle of incidence and the angle ofreflection are zero. Anything, which gives off the light or reflects the light iscalled an object. An optical appearance is produced, when the light falls on anobject or reflection produced by an object. Image is of two types:
(i) Virtual
(ii) Real
Virtual Image
The image that cannot be obtained on the screen are called virtual image. Forexample, the image of our body formed by a mirror is a virtual image. Thisimage cannot be obtained on the screen. A virtual image is formed when thelight ray coming from an object appears to be produced behind the mirror.
Real Image
The image that can be obtained on the screen is called real image. A realimage is formed when the ray of light coming from an object meets at a pointafter reflection from the mirror. Real image can be formed with the help ofconcave mirror and convex lens. For example, the image formed on the screenof the cinema hall is a real image.
Transverse Nature of Light
Light always travel in a straight line. If an obstacle is placed in its path, itstops there only. For example, if we switch on the light in a room and close thedoors and windows of the room, the light will not go out side.
Experiments carried over hundred years showed us that the light has dual naturei.e. more... 
Sources of Light
A great deal of light reaches our eyes from luminous surfaces. To make thepoint clear, imagine how your room would appear if the walls and other surfacesare covered with black paint that reflects no light. White ceilings or brightwalls reflect and diffuse much of the light they receive and hence increase thebrightness of the room. In fact when we use indhrect lighting, we hide thelamps from sight and the light reach us after being reflected from the wallsand ceiling. For example, the Moon, which we often think of as a source oflight at night is actually an indirect-lightning device, reflecting sunlight.
There are different sources from which we get light. For example, the Sun, thestars, candle, burning wood, electric bulb etc. Some sources have their ownlight, where as, others reflect light of different sources. There are, therefore,two types of the sources of light:
(i) Luminous
(ii) Non-Luminous
Luminous Sources
The luminous sources are those that have the light of their own. For example,the Sun, the star, etc.
Luminious Sources
These objects have their own internal sources of energy. They keep on producinglight continuously.
Non Luminous Sources
The sources that do not have the light of their own, but produces light derivingenergy from other sources. For example, the burning candle, electric bulb, etc.
Non Luminous Sources
The ability of an animal to see an object is only due to the light. Ifthere is no light, it will not be possible to see any object. Hence wecan say that light is a form of energy, which produce the sensationof_________.
(a) Vision
(b) Taste
(c) Hearing
(d) All of these
(e) None of these
Answer:(a)
Explanation
Light is a form of energy which produces the sensation of vision.
Therefore, option A is correct and rest of the options is incorrect.
There are some objects in this Universe that have the light of theirown, but there are some other objects having no light of their own.Depending on this property, there are two types of objects, luminousand non-luminous. Luminous or Non-Luminous nature of the objectdepends on which one of the following properties?
(a) Nature of material
(b) Quantity of material
(c) State of material
(d)All of these
(e) None of these
Answer: (a)
Explanation
It is the nature of the materials which determines whether the body isluminous or non luminous.
David have some objects with him. Which one of the following objectsis luminous?
(a) more... 
Introduction
The space including, the celestial bodies and galaxies is called universe. There are millions of galaxies in this universe and each galaxy has billions of stars. Like living things, these stars also have life cycle, take birth and die. The name of our galaxy is Akash-Ganga. Each galaxy consist of stars, the planet, satellites, asteroid, comets, and meteoroids. They are also known as celestial bodies or heavenly bodies. In this chapter, we will discuss about these heavenly bodies.
Stars
Starsarethe heavenly bodies having light of their own. They continuously emit heat and light. They mostly consist of hydrogen gas. Like Sun, they produce energy by the process of nuclear fusion. In this reaction, the hydrogen present inside the stars are converted into the helium molecules after fusion, with the release of large amount of energy in the form of heat and light. The Sun is also a star, which is nearest to us. It appears much bigger and brighter as compared to other stars. This is because, it is much nearer to us. The intensity of sunlight is very high during the day time. The stars are much bigger as compared to the Sun. Also they emit the light of much higher intensity than the Sun. The nearest star to the earth other than the Sun is Proximo centauri. There are about 10,000 billion stars in this universe. As the Earth moves from east to west, the stars seem to move from east to west too. But there is one star, which appears to be stationary when observed from the pole, it is known as Pole star or Dhruva tara. The pole star appears stationary, because it lies on the axis of rotation of the earth. As the pole star lies on the north pole, it is not visible from the south pole of the earth.
We know that the distance of these stars is very far from the earth, so they cannot be measured with the help of smaller units, such as km. We have to use the larger unit, i.e. light year. One light year is the distance traveled by the light in one year. One light year is equivalent to 9.46 x 1012 km. It is obtained by multiplying the speed of light with the number of seconds in one year. The light from the Sun takes 8 minutes to reach the earth. The next nearest star, Proxima centauri is 4.3 light year away from the earth. Hence, if a star is n- light years from earth, means that the light takes n years to reach the earth.
It is found that most of the stars and planets are made up of certain types of gases. These gases are found in large quantity and are lighter in nature, and helps in the more... 
Constellations
Thousands of stars appear in the sky during the night. Some of these stars form a pattern in a group, which have recognizable shape. This group of stars is called constellation. At present there are about 88 constellations. Each constellation has been given a name according to its shape. Some of the important constellations are Ursa Major, Ursa Minor, Orion, Leo Major and Cassiopeia.
Ursa Major: It is also known as Great Bear or Saptarishi. It consists of seven bright stars, which appear in the shape of a bear. It is visible during the summer season in the early part of the night and can be seen clearly during the month of April in the northern part of the sky. It also helps us to locate the position of the pole star in the sky. At night it appears to move from east to west as the pole star remains fixed in its position.
Orion: This constellation is also known as hunter. The Indian name is Mriga. It consists of seven or eight bright stars. The orientation of stars in Orion is similar to the figure of hunter. This constellation is visible in the sky during the winter season in the late evening. One of the brightest stars in the sky at night is the Sinus, which is very close to the Orion constellation.
Leo Major: This constellation mainly consists of 9 or 10 stars. Its orientation is similar to that of lion, hence its name is Leo. It is visible during the summer in the early part of night.
Cassiopeia: It mainly consists of five stars, which are arranged in the form of letter W or M depending on its position in the sky. It is supposed to represent an ancient queen named Cassiopeia. It is visible during the winter in the early part of the night.
Constellation is formed by the group of stars forming different pattern. These groups are named according to the shape of pattern they resemble. Which one of the following is also the name of the constellation Ursa Major?
(a) Mriga
(b) Leo
(c) Night Queen
(d) Saptarishi
(e) None of these
Answer: (d)
Each constellation is made up of different number of stars arranged in different pattern. Orion is one of the constellations. The number of stars of which Orion is made up of is_________.
(a) 5 or 6
(b) 7 or 8
(c) 9 or 10
(d) 100
(e) None of these
Answer: (b)
Explanation
The number of stars in Orion is 7 to 8.
Life of Star
The stars are formed from the existing gas and dust in the outer space. This gas and dust is called interstellar matter. When more... 
Solar System
The family of eight planets along with the Sun is called so/or system. In solar system, the Sun is at the nucleus, and all the other planets revolve around it in a fixed orbit. There are many small celestial bodies such as Asteroid, Comets and Meteoroids. The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The ninth planet Pluto has just been removed from the family of the solar system. The smaller heavenly bodies, which revolves around the planets, as the planets revolve around the Sun, is called satellites. Asteroids are small rocky bodies, which revolve around the Sun between the orbits of the planets Mars and Jupiter. The gravitational force of the Sun keeps the solar system together, and controls the movement of planets and other members of the solar system.
The Jupiter is the biggest planet and Mercury the smallest planet. The nearest planet is the Mercury and the farthest planet is Neptune. The planets do not have the light of their own. However, they shine because they reflect the light of the Sun falling on them. The best way to differentiate between the planet and stars is that, the stars have the light of their own and hence they twinkle, and the planets do not have the light of their own so do not twinkle. Also the planets keep changing their position with respect to stars in the night. Each planet has its own orbit,along which it revolves around the Sun. The first four planets Mercury, Venus, Earth and Mars are called the inner planets. The rest of the planet are called the outer planets. The four outer planets are mainly made up of hydrogen and helium gas. The inner planets are made up of rocks and metals.
Solar System
Some Facts about the Planets
| Name of planets | Diameter of planets | Distance from sun | Period of revolution | Period of rotation |
more...
 Artificial Satellites
Satellites are the objects orbiting a planet. For example the Moon, which is a natural satellite. Artificial satellites are those which are put into orbit by man. The first artificial satellite was Sputnik 1, launched by the Soviet Union in 1957. At present there are thousands of satellites orbiting the Earth at various heights. Satellites are of different types. The communication satellites is used to relay telephone, messages, radio and television signals. It is such a satellite, which have made it possible for us to see on TV, live cricket and tennis matches being played in any parts of the world. Earlier one had to wait till the recorded version or forthe news in the paper. Some artificial satellites carry instruments, that gather and send back information about the earth and its surroundings. This information is of immense value for weather forecasts, scientists, military planners, farmers and fishermen, just to name a few. Satellites can provide ecologists with detailed images of every square meters of the earth's surface for study.
 Today we can easily access any part of the world on our computer, sitting at our home, through internet. This was not possible in the early part of 19th century. It made possible only after artificial satellites are launched into the space. The first artificial satellite launched into the space by any nation was______.
(a) Sputnik 1
(b) INSAT-A
(c) INSAT-A1
(d) INSAT-A2
(e) None of these
Answer: (a)
Explanation
The first artificial satellites was launched by USSR in the space. It was Sputnik I.
Before 1957, communication was a very difficult task. This problem has been solved by launching satellites into the earth's orbits. What is the name of the satellite, which is used for weather forecasting purpose?
(a) Communication satellites
(b) Geostationary satellites
(c) Polar satellites
(d) Geosynchronous satellites
(e) None of these
Answer: (b)
Explanation
The satellites, which is used for weather forecasting is Geostationary satellites.
Asteroids
The asteroids are the small heavenly bodies that revolve around the Sun, between the orbit of Mars and Jupiter. They are also called small planets and are made up of rocks of various sizes. The biggest asteroids is hundred kilometers in diameter, whereas the smallest ones are only few hundred meters in diameter. There are millions of such small and big asteroids orbiting the Sun. Sometime these asteroids collide with each other, break into small pieces and form meteoroids. The largest asteroid by far is 1 Ceres. Its diameter has been 974 km and has contained about 25% of the mass of all the asteroids combined together. The next largest is 2 Pallas, 10 Hygiea and more...  Introduction
When a boatman rows the boat in the river, he has to apply force to push the boat in the forward direction. Or, if a person pulls a box from one place to another he is applying force. A women cutting grass in the field and binding it with the rope; here also she is applying the force. For any type of work , force has to be applied. In thischapter, we will discuss the various kinds of force and its applications. The force can be defined as the push or pull of an object. It is also the influence, which can change the state of an object.
 In a tug of war, when two teams pull the rope, a stage comes when the rope does not move to either side at all. What can you say about the magnitude of force being applied to the rope by the two teams at that stage?
(a) Equal
(b) Greater on first side
(c) Greater on second side
(d) All of these
(e) None of these
Answer: (a)
Explanation
If there is no movement on either side, the force applied form both side is equal in magnitude.
It the two forces of equal magnitude are acting on an object in opposite directions, the net force acting on the object is equivalent to the _________.
(a) Unity
(b) Zero
(c) Infinity
(d) All of these
(e) None of these
Answer: (a)
Explanation
If the two forces of equal magnitude are acting on the object then the net force on the object is zero..  Types of Forces
There are different types of forces:
(i) Applied force (ii) Gravitational force
(iii) Normal force (iv) Frictional force
(v) Tension force (vi) Muscular force
(vii) Electrostatic force (viii) Magnetic force
 Applied force
It is the force, which is applied to an object by a person or by another object. If a person is pushing a bed across the room, there is a applied force acting on the bed.
Gravitational Force
The force, which exists between any two planets, or any two objects in this universe, is called gravitational force. This force always attracts the object towards each other. All object on the surface of earth experience a force of gravity towards the centre of earth, which is equivalent to the weight of the object. Numerically it is given by,
\[F=m\times g\]
Here, m is the mass of the object and g is the acceleration due to the gravity.
The numerical value of g is 9.8 m/sec2.
 Two objects of mass 10 kg and 35 kg are 50 m apart from each other. The force acting between two objects at any instant of time is _________.
(a) 2x 10–11 N
(b) 1.2x 10–11 N
(c) 2.2x 10–11 N
(d) 0.934 x 10–11 N
(e) None of these
Answer: (d)
Explanation
The net force between any two objects can be calculated by using the relation, \[F=\frac{G\times M\times m}{{{R}^{2}}}\]. On calculation it was founded that its value is 0.934 x 10–11N.
According to the universal law of gravitation, the value of gravitational constant remains same at all places in this universe. But, the acceleration due to the gravity changes from one place to another on the surface of the planet. The reason for this change in the value of acceleration due to gravity on the planet is________.
(a) Volume
(b) Density
(c) Mass
(d) Surface area
(e) None of these
Answer: (c)
Explanation
The acceleration due to the gravity depends on the mass of the planet.
Normal Force
The normal force is the force, which is exerted upon an object in contact with another stable object. For example, if a plate is resting on the table, the surface of the more...  Tension Force
When an object is hanged with the help of a rope, a force is transmitted through the rope, called tension force. It always acts along the length of the rope, and is pulled equally on the objects in the opposite direction. Let us consider the surface of water. If you observe the upper surface of water carefully, you will find that, it behaves like an elastic surface. This is because of the fact, that there is intermolecular force of attraction between the molecules of water, which holds the molecules together. This force is called surface tension.
 Thomas gently places a coin on the surface of water in the bucket, and finds that it does not sink in the water. It shows that the surface is behaving like a elastic surface. The reason for this behavior of the surface is _________.
(a) Nuclear force
(b) Electrostatic force
(c) Intermolecularforce
(d) Gravitational force
(e) None of these
Answer: (c)
Explanation
The surface of water behaves like an elastic surface, because there is intermolecular force between the molecules of water.
David mixes a drop of water in the cup containing mustard oil. He observes that it changes into the shape of a spherical ball. What could be the possible reason for this?
(a) Surface tension
(b) Viscosity
(c) Density
(d) Surface area
(e) None of these
Answer: (a)
Explanation
When the oil is mixed in water, it becomes spherical. It is because of the surface tension between the molecules of the oil drop.
Muscular Force
To lift a table from the ground to a certain height, we need to apply a force; or if you want to throw a cricket ball, also you have to apply a force. These forces are exerted by the muscles of our arm or body. The force applied by the muscles of our body is known as the muscular force. The muscular force depends upon the strength of the body or the mass of the object. Human use the muscular force of the animals to do various work. For example, ploughing of the field by an ox, or carrying of loads from one place to another by camels or donkey.
A weight lifter lifts the weight with the help of his hand, and has to apply large force to do this. The force, which the weight lifter has to apply to lift the weight is called _______.
(a) Cellular force
(b) Nuclear force
(c) Muscular force
(d) more... Current Affairs CategoriesArchive
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