A depression in the center of the macula of the retina, the area of the most acute vision, where only cones are present and where blood vessels are lacking. Also called central fovea, central pit. fovea is responsible for sharp central vision (also called foveal vision), which is necessary in humans for reading, driving, and any activity where visual detail is of primary importance. Fovea (or Fovea centralis) is the depression in the inner retinal surface, about 1.5 mm wide, the photoreceptor layer of which is entirely cones and which is specialized for maximum visual acuity.
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An owl can see very well in night but not during the day because a. It has more no. of cones and only a few rods b. It has more no. of rods and only a few cones c. Same no. of cones and rods Ans: (b) CHAPTER 16 : LIGHT WORKSHEET –SA_II (2013-2014) STD. VIII SCIENCE I. Answer the following question : 1. Name the angle between the normal and the incident ray. 2. Name the angle between the normal and the reflected ray. 3. In an image formed by a mirror the left of the object appears on the right and vice-versa. This is known as _______________ 4. Splitting of light into its colours is known as___________ 5. Our eyes sees everything around us due to _________ light. 6. Name the small opening present in Iris. 7. Visually challenged persons can read and write using __________ system. II. Choose the correct Answer : 1. When we say that a person has black, blue or green eyes, we refer actually to the colour of a. Lens b. Cornea c. Iris d. Pupil 2. The lens focuses light on the back of the eye, on a layer called a. cornea b. retina c. lens d. pupil 3. Among the following which cells are sensitive to bright light a. rods b. cones c. Both d. None 4. Among the following which cells are sensitive to dim light. a. rods b. cones c. Both d. None of them 5. An owl can see very well in night but not during the day because a. It has more no. of cones and only a few rods b. It has more no. of rods and only a few cones c. Same no. of cones and rods Question: Why does Ethanoic acid called glacial acetic acid? (Imp.) Ans. On cooling, pure ethanoic acid is frozen to form ice like flakes. They look like glaciers, so it is called glacial acetic acid. Question: Why is the conversion of ethanol to ethanoic acid an oxidation reaction? (Imp.) Ans. When ethanol is oxidised, hydrogen is removed from its molecule and oxygen is added so as to form ethanoic acid. Thus, conversion of ethanol into acetic acid is an oxidation reaction. Question: A mixture of ethyne and oxygen is burnt for welding. Can you tell why a mixture of ethyne and air is not used? (Imp.) Ans. Ethyne has only two hydrogen atoms and two carbon atoms in its molecule. It burns in air with a sooty flame, because the oxygen from air is insufficient to burn the carbon completely. Thus, pure oxygen is used which completely burns carbon and hydrogen and produces a very hot flame. Question: Why is the conversion of ethanol to ethanoic acid considered an oxidation reaction? Ans. Conversion of ethanol to ethanoic acid is considered an oxidation reaction since it involves: (i) addition of oxygen to the ethanol molecule (ii) removal of hydrogen from the ethanol molecule. A sigma bond is a type of covalent bond formed by the axial overlapping of two half filled atomic orbitals. The atomic orbitals undergoing sigma bond formation overlap along the internuclear axis and involve head on head overlap. The electron cloud is thus cylindrically symmetrical about the internuclear axis.
Three types of these axial overlaps are possible. These are between the s and the pz orbitals and are as under. s-s overlap: It involves the overlap of two half filled s-orbitals approaching towards each other to form a bond. The bond so formed is called as s-s sigma bond. s-p overlap: It involves the mutual overlap of half filled s-orbital of one atom with half filled pz -orbital of another atom. The bond so formed is called as s-p sigma bond. p-p overlap: It involves the mutual overlap of half filled pz -orbital of one atom with half filled pz -orbital of another atom. The bond so formed is called as p-p sigma bond. A pi (π) bond is formed by sideways overlapping as the axes of the orbitals remain parallel to each other, and perpendicular to the inter-nuclear axis. A px- px and a py- py bond are pi (π) bond. Question fro5 Board paper SA-1 -2013
Solve these Questions: Q. If ab + bc + ca= 0 find the value of 1/(a2-bc) + 1/(b2-ca) + 1/(c2- ab) Q . if x2-bx+c = (x+p)(x-q) , then factorize x2 –bxy+cy2 Q. If a + b + c = 0 ; show that ( a2 / bc ) + ( b2/ ca ) + ( c2 / ab ) = 3 |