7th Class

        Light   Light is the important part of our life. Some objects produce light and are called luminous object, while some do not produce light and therefore are called non luminous object. The big source of light is the Sun. The other sources of light are, burning candles, torches, electric bulbs, etc. Light travels through vacuum and also through some mediums, for example, through air, glass and other transparent objects. The best example of traveling of light through vacuum is traveling of sun light from the Sun to the Earth. The objects that allow the passing of light through them are called transparent object, i.e. glass, water, air, etc. The objects that allow passing only of some portion of light through them, are called translucent objects.   Look at the following picture of translucent object more...

       Formation of Image by Plane Mirror   The picture of our face in the mirror is called image and our face is called object. The formation of our image in the mirror is possible due to the reflection of light. In the absence of light, the formation of image is not possible. If we put a mirror in front of our face in the dark (light is not available) we cannot see the image of the face. Light is therefore an important factor for the formation of image.   Images formed are of two types:

• Virtual image
• Real image

The image, which cannot be obtained on the screen is called virtual image. The image formation of our face in the plane mirror is virtual image.       The image, which is formed by the mirror by the actual intersection of more...

       Spherical Mirrors   The name of the spherical mirror itself indicates that it is the part of the sphere. There are two types of spherical mirrors, convex and concave mirror. The reflective side of the concave mirror bulges away from the observer and in the convex mirror it bulges towards the observer. For the better understanding of the concave and convex mirror, a tablespoon may be taken as an example. The inner surface of the table spoon is the shape of concave and the outer surface is the shape of convex.   Look at the following pictures of convex mirror                        Fig.(1) Virtual image (In convex mirror)                                       Fig.(2) more...

       Lenses and Formation of Image   There are two types of lens:

• Convex
• Concave

The middle portion of the convex lens is thicker than its edges and the middle portion of the concave mirror is thinner than its edges. The images formed by convex lenses are: (a) real and inverted; (b) virtual and erect. The formation of image by a convex lens is very similar to the formation of image by concave mirror.   Look at the following pictures of lenses                                                                          Fig. (1) Side view of convex lens more...

       Water   Water is a chemical substance with the chemical formula H2O. A molecule of water contains one oxygen and two hydrogen atoms connected by covalent bonds. Water exists in nature in all the three states. Water exists as vapour in air, as liquid in rivers, sea and under the ground and as solid in the polar regions and as snow on the mountains. Water covers about three-fourth of the earth's surface. Only 2.5% of the water available on the earth is fresh water and suitable for drinking, rest is in the form of oceans as salty water.

         Water Sources   The various sources of water are: Surface water: Water is available on the earth's surface in the form of oceans, rivers, lakes, etc. Note: About 70.9% of earth's surface is covered by water. Ground water: The rain water gets collected deep inside the layers of the earth as groundwater. Traditionally, wells were used for utilizing the ground water. Now a days groundwater is pumped with the help of power operated tube wells. Rainwater: Rainfall is the primary source of water. It replenishes the surface water and groundwater..

       Scarcity of Water   The reasons for the scarcity of water are: Pattern of Rainfall: The rainfall occurs mainly in rainy season. Therefore, there is scarcity of water in the other seasons. Loss of Water: All the rain water that falls on the ground cannot be utilized and is lost due to evaporation and runs off the land into streams. Overuse: Overuse of water is another reason for its scarcity. Pollution: Pollution of water bodies is one of the main causes of scarcity of water. Deforestation: Deforestation of hill slopes and other areas leads to excessive runoff of water. In addition to this, negligence and non-proper maintenance of old water tanks, ponds and wells leads to water shortage                What is the chemical more...

       Time   Time is used to measure the duration in which things occur, and interval between two occurrences. Time give us the idea of the duration of the occurrence of a certain event. For example, repetition of the day, night and seasons, phases of the Moon, appearance of certain star in the sky etc. In ancient time, people observed stars and constellations for such estimations. Appearance of the certain star in the sky was the indicator of new season. But this way of estimation was very inconvenient as stars and constellations appear only at night and could not be seen at cloudy nights. Therefore, they started to develop new systems to measure the day and night durations.      Sand Clock                 Sand clock is one of the instruments, which was used earlier to more...

       Distance and Displacement   Distance and displacement are two different physical quantities. Let us study them in detail                  Distance When an object moves from one place to other, it covers some distance. Distance is the total length of the path on which the object move. Distance does not shows the direction. It just represents the length, which an object has traveled. Therefore, distance is a scalar quantity.       Displacement is the distance between initial point from where object started to move and the final point where the object stopped. Displacement shows finally the object in which direction and at how far from the initial place sifted. Therefore displacement shows direction as well as distance. And it is a vector more...

       Speed   When a body moves, it covers some distance. How much distance does it cover in a given time or how long will it take to cover a particular distance? Such questions arise in our day to day life. To solve such queries a term "speed" is used. Speed is related to time and distance. With the help of speed we can find the distance or the required time to cover the particular distance. The standard unit of the time is second. The distance covered by an object in a unit time is called speed. Speed is a scalar quantity as it does not tell about the direction. \[\text{Speed}\,\,\text{=}\,\,\frac{\text{Distance}}{\text{Time}}\]                   Uniform Speed If an object does not change its speed while moving, the object is said to be in more...