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प्रश्न
An eye has a far point of 2 m. What type of lens in spectacles would be needed to increase the far point to infinity? Also calculate the power of lens required. Is this eye long-sighted or short-sighted?
उत्तर
The far point of an eye can be increased by wearing spectacles with concave lens. In order to find the power of the concave lens required, we have to first calculate its focal length.
Given that the far point of the short-sighted person is 2 m in front of the eye (the person can see objects kept at infinity if the images of the objects are formed at the person's own far point of 2 m from the eye).
`u=oo("the distance of the object")`
`v=-2m("the far point of the defective using the formula ")1/f=1/v-1/u`
f=? (focal length)
The focal length can be calculated using the formula `1/f=1/v-1/u`
Substituting the values in the formula, we get `1/f=1/-2-1/oo=(-1)/2 ("Sinc e, 1/oo=0")`
∴ f=-2 m
Now that we know the focal length of the concave lens required, its power can be calculated.
Power `P=1/f` (for f in meters)
∴` P=1/-2=-0.5 D`
Hence, the power of the concave lens required to rectify the defect is -0.5 D.
This eye is short-sighted.
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संबंधित प्रश्न
Name the defect of vision in a person:
whose far point is less than infinity
The near-point of a person suffering from hypermetropia is at 50 cm from his eye. What is the nature and power of the lens needed to correct this defect? (Assume that the near-point of the normal eye is 25 cm).
What is Hypermetropia (far sightedness)?
Rewrite the following table so as to match second and third column with first column.
|
Column I
|
Column II
|
Column III
|
|
(i) Myopia
|
Old age problem
|
Bifocal lens
|
|
(ii) Presbyopia
|
Nearsightedness
|
Concave lens.
|
A person is unable to see objects distinctly placed within 50 cm from his eyes.
(a) Name the defect of vision the person is suffering from and list its two possible causes.
(b) Draw a ray diagram to show the defect in the above case.
(c) Mention the type of lens used by him for the correction of the defect and calculate its power. Assume that the near point for the normal eye is 25 cm.
(d) Draw a labeled diagram for the correction of the defect in the above case.
With respect to human eye explain:
(i) How is the image formed on the retina?
(ii) How is the amount of light entering the eye-controlled?
(iii) What type of lens is used for the correction of ‘Long sight’ defect?
(iv) With the help of a ray, diagram show the defect of the eye and then its correction after the use of a lens.
Choose the Odd One Out:
Which of the following statement is correct?
Observe the figure and answer the following questions:
- Name the defect of vision represented in the above figure.
- State the reasons for this defect.
- How is it corrected?
- Draw the diagram to show the correction of this defect.
Complete the following table by observing the given figures:
| Figure → |  |
 |
| Points ↓ | ||
| (a) Name of the defect | ______ | ______ |
| (b) Position of the image | ______ | ______ |
| (c) Lens used to correct the defect | ______ | ______ |
