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Gist of Lesson for Quick Revision (By JSUNIL) 1. An eye is the most important organ of the human body. It is associated with vision and acts as a vital optical instrument. 2. An eye is spherical in shape of about  2.3 cm diameter  and is covered externally by a hard thick and opaque layer called Sclerotica. The outermost layer of the eye is sclerotic that prtects and holds the eye. 3. Cornea is transparent part bulging  out of eye and allows the light to enter in the eye. Cornea is also known as the window or aperture of the eye. 4. Iris is a  circular  diaphragm having a hole in its centre .this hole is called pupil 5. Iris has muscles and coloured pigments that provide different colour y eyes 6. Iris controls the amount of light entering the eye through the pupil.  7. The pupil becomes small when bright light falls on the eye. However, it becomes wide when there is dim light. Read more key points Visit Links:   Human eyes and Colorful world...

CBSE ADDA: X Chemistry: Periodic Classification of elements 1. Periodic table: The table which classifies all the known elements on the basis of their properties in such a way that elements with similar properties are grouped together. The first classification of elements was as metals and non-metals. This served only limited purpose mainly because of two reasons: (i). All the elements were grouped in to these two classes only. The groupcontaining metals was very big. (ii) Some elements showed properties of both-metals and non-metals and they could not be placed in any of the two classes. 2. All earlier attempts on the classification of the elements were based on their atomic weights. 3. The first attempt towards the classification of elements was made by Johann Dobereiner, a German chemist in 1829. He made sets of three elements which showed similar chemical properties and he called it triads. 4. The Triad was approximately the mean of the atomic weights of the other two members and t...

QUESTIONS BANK CBSE BOARD PAPER  Questions:   If K and L shells of an atom are full, then what would be the total number of electrons in the atom ? What is the valency of this element ? Questions:   List three main features of Rutherford�s nuclear model of an atom. Questions:   On the basis of the number of protons, neutrons and electrons in the samples given below identify    (a) the cation     (b) the pair of isobars and            (c) the pair of isotopes. Sample Protons Neutrons Electrons A 17 18 16 B 18 19 18 C 17 20 17 D 17 17 17 Questions:   Composition of the nuclei of two atomic species A and B are given as under : Particala A B Protons 17 17 Neutrons 18 20 (i) What are the mass numbers of A and B ?    (ii) How are they related to each other ? Questions:   (i) Draw a sketch of Bohr�s model of an atom with three shells...

CHAPTER-10 OUR ENVIRONMENT:  CBSE SOLVED GUESS PAPER 1 Marks Questions: [5] 1. Which of the following organism is likely to have maximum concentration of DDT : Algae, fish, water flea, frog, bird 2. What is the role of bacteria and fungi in an ecosystem? 3. What are trophic levels? 4. How much energy is transferred to the next trophic level in a food chain? 5. Which trophic level has the highest concentration of toxic substances in a food chain? Two marks questions [10] 6. Mention some sources of CFC�s. 7. Distinguish between a food chain & a food web 8. What would happen if there are no decomposers on earth? 9. What is a food chain? Write a five step food chain found in grass land with frog as one of the members.  What will happen to organisms at different trophic levels if all the frogs are removed? 10. The number of malaria patients increased tremendously when a large number of frogs were exported  from the village .What could be the cause for it? Explain with the ...