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A professor reads a greeting card receiv...

A professor reads a greeting card received on his 50th birthday with + 2.5 D glasses keeping the card 25 cm away. Ten years later, he reads his farewell letter with the same glasses but he has to keep the letter 50 cm away. What power of lens should he now use?

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The correct Answer is:
D
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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

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

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

Amit is a domestic help (or maid) working at Mr Dave's house. One day Amit complained to Mr Dave that he had difficulty in reading the letter which he had received from his parents. Mr Dave, realising that Amit had an eye defect, took him to an eye specialist doctor. The doctor tested his eyes carefully and told Amit to wear spectacles containing certain type of lenses having specified power. Mr Dave bought the required spectacles for Amit. By wearing this spectacle, Amit could read and write easily. He was very happy and thanked Mr Dave. What could be the eye defect Amit was suffering from?

Amit is a domestic help (or maid) working at Mr Dave's house. One day Amit complained to Mr Dave that he had difficulty in reading the letter which he had received from his parents. Mr Dave, realising that Amit had an eye defect, took him to an eye specialist doctor. The doctor tested his eyes carefully and told Amit to wear spectacles containing certain type of lenses having specified power. Mr Dave bought the required spectacles for Amit. By wearing this spectacle, Amit could read and write easily. He was very happy and thanked Mr Dave. What values are displayed by Mr Dave in this episode?

Amit is a domestic help (or maid) working at Mr Dave's house. One day Amit complained to Mr Dave that he had difficulty in reading the letter which he had received from his parents. Mr Dave, realising that Amit had an eye defect, took him to an eye specialist doctor. The doctor tested his eyes carefully and told Amit to wear spectacles containing certain type of lenses having specified power. Mr Dave bought the required spectacles for Amit. By wearing this spectacle, Amit could read and write easily. He was very happy and thanked Mr Dave. What could be the two possible reasons responsible for his eye defect? What type of lenses do you think the doctor recommended for Amit's spectacles ?

A far sighted person has a near point of 60"cm" . What power lens should be used for eye glasses such that the person can be read this book at a distance of 25 cm.

A man with normal near point (25 cm) reads a book with small print using a magnifying glass : a thin convex lens of focal length 5 cm . (a) What are the closest and the farthest distances at which he can read the book when viewing through the magnifying glass ? (b) What is the maximum and the minimum angular magnifications (magnifying powers) possible using the above simple microscope ?

A man walks on a straight road from his home to a market 2.5 km away with a speed of 5 km h^(-1). Finding the market closed, he instantly turns and walks back home with a speed of 7.5 km h^(-1). What is the (a) magnitude of average velocity, and (b) average speed of the man over the interval of time (i) 0 to 30 min, (ii) 0 to 50 min, (iii) 0 to 40 min ? [Note: You will appreciate from this exercise why it is better to define average speed as total path length divided by time, and not as magnitude of average velocity. You would not like to tell the tired man on his return home that his average speed was zero !]

HC VERMA-OPTICAL INSTRUMENTS-Exercises
  1. A simple microscope is rated 5 X for a normal relaxed eye. What Nvill ...

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  2. Find the maximum magnifying power of a compound microscope having a 25...

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  3. The separation between the objective and the eyepiece of a compound mi...

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  4. An eye can distinguish between two points of an object if they are sep...

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  5. A compound microscope has a magnifying power of 100 when the image is ...

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  6. A compound microscope consists of an objective of focal length 1 cm an...

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  7. An optical instrument used for angular magnification has a 25 D object...

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  8. An astronomical telescope is to be designed to have a magnifying power...

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  9. The eyepiece of an astronomical telescope has a focal length of 10 cm....

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  10. A Galilean telescope is 27 cm long when focussed to form an image at i...

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  11. A farsighted person cannot see objects placed closer to 50 cm. Find th...

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  12. A nearsighted person cannot clearly see beyond 200 cm. Find the power ...

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  13. A person wears glasses of power - 2.5 D. Is the person short sighted o...

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  14. A professor reads a greeting card received on his 50th birthday with +...

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  15. A normal eye has retina 2 cm behind the eye-lens. What is the power of...

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  16. The near point and the far point of a child are at 10 cm and 100 cm. I...

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  17. A nearsighted person cannot see beyond 25 cm. Assuming that the separa...

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  18. A person has near point at 100 cm. What power of lens is needed to rea...

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  19. A lady uses + 1.5 D glasses to have normal vision from 25 cm onwards. ...

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  20. A lady cannot see objects closer than 40cm from the left eye and close...

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