Advertisements
Advertisements
Question
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).
Solution
A person suffering from hypermetropia can correct the defect by wearing spectacles with 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 eye is 50 cm in front of the eye (the person can see an object kept at the normal near point of 25 cm from the eye if the image of the object is formed at the person's own near point of 50 cm from the eye).
u = -25 cm (the distance of the object at the normal near point)
v = -50 cm (the location of the near point of the defective eye)
f = ? (focal length)
The focal length can be calculated using the lens formula `1/v-1/u=1/f`
Substituting the values in the formula, we get `1/50-1/-25=1/f`
`1/-50+1/25=1/f`
`((-1+2))/50=1/50=1/f`
∴f=50cm=0.5cm
Now that we know the focal length of the convex lens, the power can be calculated.
Power P=`1/f(termertrs)`
∴` P=1/0.5=+diopters`
Hence, the power of the convex lens required to rectify this defect is +2 dioptres.
APPEARS IN
RELATED QUESTIONS
Explain with the help of labelled ray-diagram, the defect of vision called hypermetropia, and hot it is corrected by a lens.
A person suffering from the eye-defect myopia (short-sightedness) can see clearly only up to a distance of 2 metres. What is the nature and power of lens required to rectify this defect?
Though a woman can see the distant object clearly, she cannot see the nearby objects clearly. She is suffering from the defect of vision called:
(a) long-sight
(b) short-sight
(c) hind-sight
(d) mid-sight
Name the following:
The photosensitive pigment present in the rods of the retina.
A person cannot read newspaper placed nearer than 50 cm from his eyes. Name the defect of vision he is suffering from. Draw a ray diagram to illustrate this defect. List its two possible causes. Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length.
What eye defect is hypermetropia? Describe with a ray diagram how this defect of vision can be corrected by using an appropriate lens.
List two causes of presbyopia. Draw labelled diagram of a lens used for the correction of this defect of vision.
Due to elongation of _______ and increase in curvature of the eye lens, a person cannot see distant objects clearly.
Correlate the given sequence:
Hypermetropia : Convex lens : ______ : Concave lens
