Advertisements
Advertisements
प्रश्न
An eye has a near point distance of 0.75 m. What sort of lens in spectacles would be needed to reduce the near point distance to 0.25 m? Also calculate the power of lens required. Is this eye long-sighted or short-sighted?
Advertisements
उत्तर
A person suffering from hypermetropia can correct the defect by wearing spectacles containing convex lenses. In order to find the power of the convex lens required, we have to first calculate its focal length.
Given that the near point of the hypermetropic person is 0.75 m from the eye (the person can see objects kept at the normal near point of 0.25 m in front of the eye if the images of the objects are formed at the person's own near point of 0.75 m from the eye).u=-0.25m(the distance of the object at the normal near point)v=-0.75m (the near point of the defective eye in front of the lens)
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/-0.75-1/-0.25`
`1/f=((-1+3)/0.75)`
∴f =2.67 m
Now that we know the focal length of the convex lens, its power can be calculated.
Poer `P=1/(f "in meters")`
Hence, the power of the convex lens required to rectify the defect is +0.37 D.
This eye is long-sighted.
APPEARS IN
संबंधित प्रश्न
In a Std. X class out of 40 students 10 students use spectacles, 2 students have positive power and 8 students have negative power of lenses in their spectacles.
Answer the following questions:
(1) What does the negative power indicate?
(2) What does the positive power indicate?
(3) Generally which type of spectacles do most of the students use?
(4) What defect of eyesight do most of the students suffer from?
(5) Give two possible reasons for the above defect.
Which defect of vision can be rectified:
by using a concave lens?
What is the other name of old age hypermetropia?
A man can read the number of a distant bus clearly but he finds difficulty in reading a book.
- From which defect of the eye is he suffering?
- What type of spectacle lens should he use to correct the defect?
Name the defect of vision which can be corrected by a converging lens. Show clearly by a ray diagram how the lens corrects the defect.
The defect of vision which cannot be corrected by using spectacles is:
(a) myopia
(b) presbyopia
(c) cataract
(d) hypermetropia
To read a book held at a distance of 25 cm, will she need converging or diverging spectacle lenses?
By closing the eyes and gently pressing them by your palms, you may see some specks of brilliant light. How do you get this sensation while there is no light entering your eyes?
State the main functions of the following:
Tears
The diagram alongside represents a section of a mammalian eye.
(i) Label the parts 1 to 5 of the diagram.
(ii) State the function of the parts labelled 4 and 5.
(iii) With the help of a diagram show the short 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.
|
The near point of the eye of a person is 50 cm. Find the nature and power of the corrective lens required by the person to enable him to see clearly the objects placed at 25 cm from the eye?
Differentiate between:
Myopia and Hypermetropia.
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.
Due to elongation of _______ and increase in curvature of the eye lens, a person cannot see distant objects clearly.
Nearsightedness : concave lens : : farsightedness : _______
What are the causes of ‘Myopia’?
Assertion: Myopia is the defect of vision in which a person cannot see distant objects clearly.
Reason: This due to eye-ball being too short.
Observe the figure whether it is correct or not and explain the phenomenon.
