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## Friday, February 19, 2016

### class8 light and vision (Refraction and eyes)

Class 8 light and vision (reflection and eye) leanwell science/Basic science

Q. Why are multiple images are formed when two mirror are placed at an angle to each other ?

Ans: This happen because image formed by first mirror act as an object for 2nd mirror.

No of image = (360/q) -1[0 degree or Parallel to each other --> Infinite number]

Q. What do you mean by persistence of vision?

Ans : Image of an object retain on retina for 1/16 th of a sec even after removed is called persistence of vision. This form the basis of the principle of cinematography.

Q. How persistence of vision form basis of the principle of cinematography?

Ans: in moving cinema, we take still pictures. The sequence of still pictures are projected on screen at rate of 24 images per second (persistence of vision is 1/16th of sec.)The successive images merge smoothly and give us feeling of movie.

Q. A person whose retina is damaged cannot see why?

Ans: This happens because retina acts as seen on which image is formed by eye lens.

Q. Does diffused reflection means failure of the law of reflection?

Ans: NO. Law of reflection apply to all reflecting surfaces whether plane or curved.

Q. Why are you not able to see complete rainbow at all places?

Ans: this is because rainbow is formed due to dispersion of light through water droplets in atmosphere.

Q. Why are red signals used in traffic signals to stop vehicles?

Ans: This is because red light scattered least through atmosphere due to longer wave length and seen from distance.

Q. When two prisms are placed in succession we can get white light back. Why?

Ans: This happen because the dispersed colour of first prism when pass through the inverted prism converge back into white.

Q. An object is placed in front of a plane mirror. If the mirror is moved away from the object through a distance x, by how much distance will the image move?

Ans: 2x meter

Q. A concave mirror is made up by cutting a portion of a hollow glass sphere of radius 30 cm. Calculate the focal length of the mirror.

Ans: f = r/2 = 15 cm

Q. An object 0.5 m tall is in front of a plane mirror at a distance of 0.2 m. what is the size of the image formed

And: Hi = Ho = 0.5m

Q. Write uses of concave mirror?

Ans: Concave mirrors are used in torches, search lights and head lights of vehicles to get parallel beams of light. They are used as shaving mirrors to see larger image of the face. They are used by dentists to see larger images of the teeth. Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces

Q.Why does convex mirror used as rear-view mirrors in vehicles?

Ans: Convex mirrors give erect diminished images of objects. They also have a wider field of view than plane mirrors.

Q. Why concave mirrors are used In torches, search lights and headlights of vehicles?

Ans: In torches, search lights and headlights of vehicles the bulb is placed very near to the focus of the reflector to get parallel beams of light.

Q. What is the angle of incidence, when a ray of light falls on the spherical mirror from its centre of curvature?

Ans: The angle of incidence is zero, when a ray of light falls on the spherical mirror
from its centre of curvature.

Q. What is Astigmatism?

Ans: Astigmatism is a condition in which the surface of the cornea is not spherical,
it causes a blurred or slightly twisted image to be received at the retina.
It is quite common to have only one eye affected, whilst the other is normal.

Q. Choose correct one

1) A stick in water appears broken due to

(a) Reflection of light
(b) Refraction of light
(c) total internal reflection
(d) none of these

Ans: (b) Refraction of light

2) Refractive index of diamond with respect to vacuum is 2.5, then
the velocity of light in diamond is.

(a) 25×10^8 m/s
(b)2.5×10^8m/s
(c)1.2×10^8m/s
(d)2.1×10^8m/s

Ans: (c)1.2×10^8m/s

3) Lateral displacement is directly proportional to:

(a) Angle of incidence
(b) angle of refraction
(c) Thickness of the slab
(d) velocity of light

Ans: (a) Angle of incidence  and (c) Thickness of the slab

4) A drop of water appears like pearl due to

(a) refraction
(b) total internal reflection
(c) reflection
(d) none

Ans: (b) total internal reflection

5) Myopia can be corrected using

(a) concave lens
(b) convex lens
(c) cylindrical lens
(d) Plano convex lens

Ans: (a) concave lens

CBSE NCERT Formative Assessment : MCQ ( Reflection and refraction)

8th Physics: Light and Our Eyes links

Class 8 light and vision Solved CBSE Test paper - 01 Download File

Class 8 light and vision Solved CBSE Test paper - 02 Download File

Class 8 light and vision Solved CBSE Test paper -03 Download File

Class 8 light and vision Solved CBSE Test paper - 04 Download File

## Tuesday, February 2, 2016

### 10th Our Environment CBSE Board solved Questions

Class 10 our environment board solved Questions 2016

1. What is the function of ozone in upper atmosphere?

Ans: It shields the surface of the earth from ultraviolet rays from the Sun.

Ans: So that the time and energy required in segregation may be saved and waste may be disposed off quickly

3. The following organisms form a food chain. Which of these will have the highest concentration of non-biodegradable chemicals ? Name the phenomenon associated with it. Insects, Hawk, Grass, Snake, Frog.

Ans: Hawk ; Biomagnifications

4.. Write the full name of the group of compounds mainly responsible for the depletion of ozone layer.

Ans: Chloroflurocarbons

5. List two examples of natural ecosystem.

Ans: Forests ,Ponds,Lakes

6. Differentiate between biodegradable and non-biodegradable substances with the help of one example each.

List two changes in habit that people must adopt to dispose non-biodegradable waste, for saving the environment.

Ans: Biodegradable substances – Any substance that can be broken down into simpler substances by biological process is called biodegradable. Ex. – Human Excreta/ Vegetable peels, etc.

Non-biodegradable substances – Any substance that can can’t be broken down into simpler substances by nature or decomposers. Ex. – Plastic/ glass (or any other) (any one)

Habits that people must adopt to dispose non-biodegradable waste, for saving the environment:

- Reuse of things such as poly-bags, etc.,

- Recycle of waste

- Use of cotton /jute bags for carrying vegetables

7. What will be the amount of energy available to the organisms of the 2nd trophic level of a food chain, if the energy available at the first tropic level is 10,000 joules?

Ans: 10% of 10,000 joules = 1000 J

8. ‘‘Energy flow in food chains is always unidirectional.’’ Justify this statement. Explain how the pesticides enter a food chain and subsequently get into our body.

Ans: The flow of energy is unidirectional. The energy that is captured by the autotrophs does not revert back to the solar input and the energy which passes to the herbivores does not come back to autotrophs.

Therefore, in food chain the energy moves progressively through the various trophic levels it is no longer available to the previous level. Thus, The flow of energy in food chain is unidirectional.

The pesticides enter a food chain and subsequently get into our body in following way:

(i) Pesticides used for crop protection when washed away into the soil or water bodies absorbed by plants.

(ii) On consumption they enter our food chain and being non – biodegradable these chemicals get accumulated progressively and enter our body.

9. What is ozone ? How and where is it formed in the atmosphere? Explain how does it affect an ecosystem.

Ans: Ozone (O3) is a molecule formed by three atoms of oxygen.

Ozone is formed at at the higher levels of the atmosphere by action of UV radiation on oxygen (O2) molecule.

The higher energy UV radiations split apart some moleculer oxygen (O2) into free oxygen (O) atoms. These atoms then combine with the molecular oxygen to form ozone as shown—

O2 ⎯⎯UV⎯→ O + O then, O + O2 ⎯⎯⎯→ O3 (Ozone)

Ozone shields the surface of the earth from ultraviolet (UV) radiation from the Sun. This radiation is highly damaging to organisms, for example, it is known to cause skin cancer in human beings.

10. Construct an aquatic food chain showing four trophic levels.

Ans: Hydrilla →Scorpio→ Fish → Crane

11. To protect the food plants from insects, an insecticide was sprayed in small amounts but it was detected in high concentration in human beings. How did it happen?

Ans: As insecticides are not degradable, these get accumulated progressively at each tropic level. As human beings occupy the top level in any food chain, the high concentration of insecticide gets accumulated in their bodies. This phenomenon is known as biological magnification

12. Why do most food chains have 3- 4 steps only?

Ans: The loss of energy at each step is so great that very little usable energy remains after four trophic levels. Therefore, food chains generally consist of only three or four steps.

13. What will be impact on ecosystem if bacteria and fungi are removed from the Environment?

Ans: The decomposes bacteria and fungi secrete enzymes that breakdown organic remains into simpler soluble substances that are absorbed by saprophytes. They clean the earth from organic matter hence Environmental pollutioncreated and food chain disturbed if bacteria and fungi are removed from the Environment.

14. Why is a pond self sustaining unit while an aquarium may not be?

Ans: A pond contains microorganisms that break-down the dead remains and waste products of organisms into simple inorganic substances that are used by other organism. Since, pond is a natural ecosystem while an aquarium is created by man and is artificial, thus a pond self sustaining while an aquarium may not be.

15. Which level shows the maximum biological magnification? Why?

Ans: The top tropic level in any food chain has the maximum concentration of chemicals. From the soil, these are absorbed by the plants along with water and minerals, and from the water bodies these are taken up by plants and animals.

10th Science – Chapter: Our Environment links

10th Science - Our Environment: Summative assignments

10th Chapter: Our Environment Questions Paper for FA-III