Thursday, 28 February 2013



Defects of vision

Myopia(or) Short sightedness --Human eye can see clearly the objects lying at short distances from it. but not the far off objects
Causes of Myopia
1. Increase in the length of the eye ball as if distance of the retina from the eye has increased.
2. Decrease in focal length of eye lens when the eye is fully relaxed.
Remedy-- To correct a myopic eye, the person has to wear spectacle with a concave lens of suitable focal length. 

Hypermetropia (or) Long sightedness-- It is that defect of a human eye in which a person  can see clearly the objects lying at large distances from it but the nearby objects cannot be seen clearly.
Causes of Hypermetropia
(1) Decrease in length of eye ball as if distance of retina from the eye lens has decreased 
(2) Increase in the focal length of the eye lens when the eye is fully relaxed.
Remedy-- To correct a hypermetropic eye, the person has to wear spectacle with a convex lens of suitable focal length. 

Presbyopia-- In this defect old person cannot read and write comfortably.
Cause of Presbiopia--
(1)Gradual weakening of ciliary muscles 
(2)Diminishing flexibility of eye lens.
Remedy--An old person has to use spectacles with a convex lens of suitable focal length
When a person suffers from both myopia and hypermetropia his spectacles have bi-focal lenses (i.e) both concave and convex lenses. 

AstigmatismThe defect by which the person is notable to differentiate horizontal and vertical position, is called astigmatism.Remedy----It can be rectified by using cylindrical lenses.  
Figure 11.2-(a), (b) The myopic eye, and (c) correction for myopia with a concave lens

Figure 11.3-(a), (b) The hypermetropic eye, and (c) correction for hypermetropia


                                    IMPORTANT POINTS

 Eye lens  --- double convex lens 
The retina --- contains numerous light sensitive cells which  generate electrical signals which are sent to brain via optic nerve
 Cornea---The transparent spherical membrane covering the front of the eye.
 Iris--The coloured diaphragm between the cornea and lens.
Pupil ---The small hole in the iris.
 Eye lens---  It is a transparent lens made of jelly like material.
 Ciliary muscles--These muscles hold the lens in position.
 Retina--The back surface of the eye.
Blind spot--The point at which the optic nerve leaves the eye. An image formed at this point is not sent to the brain.
 Aqueous humour---A clear liquid region between the cornea and the lens.
 Vitreous humour--The space between eye lens and retina is is filled with another liquid called Vitreous humour.
Persistence of vision--The image of an object seen persists on the retina for 1/16 second even after the removal of the object. This continuance of sensation of eye for some timed is called persistence of vision.
Colour blindness-- It is said to occur when a person cannot distinguish between colours
Accomodation--The ability of the eye to focus both near and distant objects, by adjusting the focal length, is called the accommodation of the eye.
Far Point of the Eye-- It is the farthest point at which the object can be seen clearly. For a normal eye, the far point lies at infinity.
Near point of the Eye-- It is the closest point at which an object can be seen clearly. For normal eye, the near point lies at 25 cm from the eye(least distance of distinct vision).


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The structure of human eyes part1

Wednesday, 27 February 2013

Human Eye and colourful world-HOTS Questions


HOTS Questions

1. What is the least distance of distinct vision of a normal human eye ?

2. Name the muscle responsible for bringing change in the focal length of the eye lens ?

3. Name one defect of vision which cannot be corrected by any type of spectacle lenses ?

4. State one effect produced by the scattering of light by the atmosphere ?

5. What is the nature of image formed on the retina of the eye ?

6. What type of lens is used for correcting hypermetropia ?

7. Who was the first person to obtain the spectrum of sunlight ?

8. What is the function of optic nerve in human eye ?

9. What is range of vision ?

10. Why do different colours deviate through different angles on passing through a prism?

11. As light rays pass from air into glass prism, are they refracted towards or away from the normal ?

12. Which color has largest wavelength ?

13. Which defect of vision can be rectified using a concave lens ?

14. What phenomenon causes twinkling of star on a clear night ?

15. What is meant by scattering of light ?

16. Why does the sky appear black instead of blue to an astronaut?

17. What is the basic cause of atmospheric refraction?

18. Why does clear sky look blue?

19. Can visible light be scattered by atoms/molecules in earth̢۪s atmosphere?

20. What is a spectrum?

21. Name the defect of vision in person

a. Whose near point is more than 25cm away?.

b. Whose far point is less than infinity.

More Questions for Practice

1. A person can see only objects beyond 1m. From his eyes. Name the defect of the eye.

2. Out of light of blue and red colours which one is scattered most?

3. What is the function of crystalline lens in the human eye?

4. Which phenomenon is responsible for increasing the apparent length of the day by 4 minute?

5. What is the far point of a person suffering from Myopia.

6. What name is given to front transparent part of human eye?

7. Where do we see  1) Concave and (2) Convex lens in bifocal lenses.

8. What is the nature of image formed by our eye?

9. Name the liquid which is present between eye lens and cornea.

10. Where does most of the refraction of light in an eye occurs?

11. Which kind of lens is an eye lens?

12. What is the cause of dispersion of light?

13. Under very dim light, we are able to see the objects but can not distinguish between colours why ?

14. What is the range of vision for normal human eye ?

15. How is the amount of light entering the eye controlled ?

16. What is the colour of danger signal? Why?

17. What is rainbow? How is rainbow formed?

18. State two causes of myopic vision.

19. How an uncorrected myopic eye sees far off objects.

20. What is presbyopia? Name the type of lens which can be used to correct presbyopia.

21. Explain why planet do not twinkle at night?

22. Explain about the colour of the sun at sunrise and sunset.

23. Define the term (1) Near point (2) Far point

24. Why is a normal eye not able to see clearly the object closer than 25 cm.

25. Draw a ray diagram to show the refraction of light through a glass prism on the diagram mark.

(a) Incident ray (b) Emergent ray and (c) Angle of deviation

26. How is the dispersed white light recomposed ?

27 The near point of a hypermetropic eye is at 75 cm from the eye. What is the power of the lens required to enable him to read clearly a book held at 25 cm from the eye.

Defect Of Vision 1.

water conservation

Laundry, and Dishes
  • 1994 was the year that federally mandated low-flow showerheads, faucets, and toilets started to appear on the scene in significant numbers.
  • On average, 10 gallons per day of your water footprint (or 14% of your indoor use) is lost to leaks. Short of installing new water-efficient fixtures, one of the easiest, most effective ways to cut your footprint is by repairing leaky faucets and toilets.
  • If you use a low-flow showerhead, you can save 15 gallons of water during a 10-minute shower.
  • Every time you shave minutes off your use of hot water, you also save energy and keep dollars in your pocket.
  • It takes about 70 gallons of water to fill a bathtub, so showers are generally the more water-efficient way to bathe.
  • All of those flushes can add up to nearly 20 gallons a day down the toilet. If you still have a standard toilet, which uses close to 3.5 gallons a flush, you can save by retrofitting or filling your tank with something that will displace some of that water, such as a brick.
  • Most front-loading machines are energy- and water-efficient, using just over 20 gallons a load, while most top-loading machines, unless they are energy-efficient, use 40 gallons per load.
  • Nearly 22% of indoor home water use comes from doing laundry. Save water by making sure to adjust the settings on your machine to the proper load size.
  • Dishwashing is a relatively small part of your water footprint—less than 2% of indoor use—but there are always ways to conserve. Using a machine is actually more water efficient than hand washing, especially if you run full loads.
  • Energy Star dishwashers use about 4 gallons of water per load, and even standard machines use only about 6 gallons. Hand washing generally uses about 20 gallons of water each time.

Yards and Pools
  • Nearly 60% of a person's household water footprint can go toward lawn and garden maintenance.
  • Climate counts—where you live plays a role in how much water you use, especially when it comes to tending to a yard.
  • The average pool takes 22,000 gallons of water to fill, and if you don't cover it, hundreds of gallons of water per month can be lost due to evaporation.

  • The water it takes to produce the average American diet alone—approximately 1,000 gallons per person per day—is more than the global average water footprint of 900 gallons per person per day for diet, household use, transportation, energy, and the consumption of material goods.
  • That quarter pounder is worth more than 30 average American showers. One of the easiest ways to slim your water footprint is to eat less meat and dairy. Another way is to choose grass-fed, rather than grain-fed, since it can take a lot of water to grow corn and other feed crops.
  • A serving of poultry costs about 90 gallons of water to produce. There are also water costs embedded in the transportation of food (gasoline costs water to make). So, consider how far your food has to travel, and buy local to cut your water footprint.
  • Pork costs water to produce, and traditional pork production—to make your sausage, bacon, and chops—has also been the cause of some water pollution, as pig waste runs into local water sources.
  • On average, a vegan, a person who doesn't eat meat or dairy, indirectly consumes nearly 600 gallons of water per day less than a person who eats the average American diet.
  • A cup of coffee takes 55 gallons of water to make, with most of that H2O used to grow the coffee beans.

Electricity, Fuel Economy, and Airline Travel
  • The water footprint of your per-day electricity use is based on state averages. If you use alternative energies such as wind and solar, your footprint could be less. (The use of biofuels, however, if they are heavily irrigated, could be another story.) You would also get points, or a footprint reduction, for using energy-star appliances and taking other energy-efficiency measures.
  • Washing a car uses about 150 gallons of water, so by washing less frequently you can cut back your water use.
  • A gallon of gasoline takes nearly 13 gallons of water to produce. Combine your errands, car pool to work, or take public transportation to reduce both your energy and water use.
  • Flying from Los Angeles to San Francisco, about 700 miles round-trip, could cost you more than 9,000 gallons of water, or enough for almost 2,000 average dishwasher loads.
  • A cross-country airplane trip (about 6,000 miles) could be worth more than 1,700 standard toilet flushes.
  • Traveling from Chicago to Istanbul is just about 10,000 miles round trip, costing enough water to run electricity in the average American home for one person for more than five years.

Industry—Apparel, Home Furnishings, Electronics, and Paper
  • According to recent reports, nearly 5% of all U.S. water withdrawals are used to fuel industry and the production of many of the material goods we stock up on weekly, monthly, and yearly.
  • It takes about 100 gallons of water to grow and process a single pound of cotton, and the average American goes through about 35 pounds of new cotton material each year. Do you really need that additional T-shirt?
  • One of the best ways to conserve water is to buy recycled goods, and to recycle your stuff when you’re done with it. Or, stick to buying only what you really need.
  • The water required to create your laptop could wash nearly 70 loads of laundry in a standard machine.
  • Recycling a pound of paper, less than the weight of your average newspaper, saves about 3.5 gallons of water. Buying recycled paper products saves water too, as it takes about six gallons of water to produce a dollar worth of paper.