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The optical prescription for a pair of s...

The optical prescription for a pair of spectacles is:
Right eye: `-3.50 D` Left eye: `-4.00` D
a) Are these lenses thinner at the middle or at the edges?
b) Which lens has a greater focal length?
c) What is the weaker eye?

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a) Thinner at the middle b) Lens of lower power: `=-3.50 D` c) Left eye
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Shashank went to the optician to get his eyes checked. He observed that the doctor combined two lenses and put these in front of his eyes so that he could see clearly. The powers of the two lenses used in the combination were -3 D and 4 D. Which of these will be the focal length of the combination of lenses Shashank has to wear?

The ciliary muscles of eye control the curvature of the lens in the eye and hence can alter the effective focal length of the system. When the muscles are fully relaxed, the focal length is maximum. When the muscles are strained, the curvature of lens increases. That means radius of curvature decreases and focal length decreases. For a clear vision, the image must be on the retina. The image distance is therefore fixed for clear vision and it equals the distance of retina from eye lens. It is about 2.5cm for a grown up person. A perosn can theoretically have clear vision of an object situated at any large distance from the eye. The smallest distance at which a person can clearly see is related to minimum possible focal length. The ciliary muscles are most strained in this position. For an average grown up person, minimum distance of the object should be around 25cm. A person suffering from eye defects uses spectacles (eye glass). The function of lens of spectacles is to form the image of the objects within the range in which the person can see clearly. The image o the spectacle lens becomes object for the eye lens and whose image is formed on the retina. The number of spectacle lens used for th eremedy of eye defect is decided by the power fo the lens required and the number of spectacle lens is equal to the numerical value of the power of lens with sign. For example, if power of the lens required is +3D (converging lens of focal length 100//3cm ), then number of lens will be +3 . For all the calculations required, you can use the lens formula and lensmaker's formula. Assume that the eye lens is equiconvex lens. Neglect the distance between the eye lens and the spectacle lens. Q. A far sighted man connot see objects clearly unless they are at least 100cm from his eyes. The number of the spectacle lenses that will make his range of clear vision equal to an average grown up person will be

The ciliary muscles of eye control the curvature of the lens in the eye and hence can alter the effective focal length of the system. When the muscles are fully relaxed, the focal length is maximum. When the muscles are strained, the curvature of lens increases. That means radius of curvature decreases and focal length decreases. For a clear vision, the image must be on the retina. The image distance is therefore fixed for clear vision and it equals the distance of retina from eye lens. It is about 2.5cm for a grown up person. A perosn can theoretically have clear vision of an object situated at any large distance from the eye. The smallest distance at which a person can clearly see is related to minimum possible focal length. The ciliary muscles are most strained in this position. For an average grown up person, minimum distance of the object should be around 25cm. A person suffering from eye defects uses spectacles (eye glass). The function of lens of spectacles is to form the image of the objects within the range in which the person can see clearly. The image o the spectacle lens becomes object for the eye lens and whose image is formed on the retina. The number of spectacle lens used for th eremedy of eye defect is decided by the power fo the lens required and the number of spectacle lens is equal to the numerical value of the power of lens with sign. For example, if power of the lens required is +3D (converging lens of focal length 100//3cm ), then number of lens will be +3 . For all the calculations required, you can use the lens formula and lensmaker's formula. Assume that the eye lens is equiconvex lens. Neglect the distance between the eye lens and the spectacle lens. Q. Minimum focal length of eye lens of a normal peson is

The ciliary muscles of eye control the curvature of the lens in the eye and hence can alter the effective focal length of the system. When the muscles are fully relaxed, the focal length is maximum. When the muscles are strained, the curvature of lens increases. That means radius of curvature decreases and focal length decreases. For a clear vision, the image must be on the retina. The image distance is therefore fixed for clear vision and it equals the distance of retina from eye lens. It is about 2.5cm for a grown up person. A perosn can theoretically have clear vision of an object situated at any large distance from the eye. The smallest distance at which a person can clearly see is related to minimum possible focal length. The ciliary muscles are most strained in this position. For an average grown up person, minimum distance of the object should be around 25cm. A person suffering from eye defects uses spectacles (eye glass). The function of lens of spectacles is to form the image of the objects within the range in which the person can see clearly. The image o the spectacle lens becomes object for the eye lens and whose image is formed on the retina. The number of spectacle lens used for th eremedy of eye defect is decided by the power fo the lens required and the number of spectacle lens is equal to the numerical value of the power of lens with sign. For example, if power of the lens required is +3D (converging lens of focal length 100//3cm ), then number of lens will be +3 . For all the calculations required, you can use the lens formula and lensmaker's formula. Assume that the eye lens is equiconvex lens. Neglect the distance between the eye lens and the spectacle lens. Q. Maximum focal length of a eye lens of a normal person is

LAKHMIR SINGH & MANJIT KAUR-REFRACTION OF LIGHT-Exercise
  1. The focal length of a lens is +150mm. What kind of lens is it and what...

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  2. Fill in the following blanks with suitable words: a) The reciprocal ...

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  3. An object of height 4 cm is placed at a distance of 15 cm in front of ...

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  4. An object of height 4.25 mm is placed at a distance 10 cm from a conve...

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  5. A convex lens of power 5 D and a concave lens of power 7.5 D are place...

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  6. A convex lens of focal length 25 cm and a concave lens of focal length...

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  7. The power of a combination of two lenses X and Y is 5 D. If the focal ...

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  8. Two lenses A and B have focal length of +20 cm and, -10 cm, respective...

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  9. a) What do you understand by the power of a lens? Name one factor on w...

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  10. a) Two lenses A and B have power of i) +2D and ii) -4 D respectively. ...

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  11. The focal lengths of four convex lens P,Q, R and S are 20cm, 15 cm, 5 ...

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  12. A converging lens has a focal length of 50cm. The power of this lens i...

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  13. A diverging lens has a focal length of 0.10m. The power of this lens w...

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  14. The power of a lens is +2.0D. Its focal length should be:

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  15. If a spherical lens has a power of, -0.25 D, the focal length of this ...

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  16. The power of a concave lens is 10 D and that of a convex lens is 6 D. ...

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  17. The power of a converging lens is 4.5 D and that of a diverging lens i...

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  18. A convex lens of focal length 10 cm is placed in contact with a concav...

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  19. The optical prescription for a pair of spectacles is: Right eye: -3....

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  20. A person got his eyes tested by an option. The prescription for the sp...

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