1© Manhattan Press (H.K.) Ltd. The human eye Visual defects and their corrections Visual defects and their corrections 12.3 Properties of vision Visual

© Manhattan Press (H.K.) Ltd. 1

• The human eyeThe human eye• • Visual defects and their Visual defects and their

correctionscorrections

12.3 Properties of 12.3 Properties of visionvision

• • Visual angleVisual angle


12.3 Properties of vision (SB p. 220)

The human eye tough layer that covers eyeball

transparent layer that covers front

portion of eyeincident light is refracted

muscles that controls aperture of pupil (amount of light )

muscle that controls thickness of lens

Look at near objects, lens is thicker

Look at distant objects, lens is thinner

lens focus light on retina



The human eye

Accommodation- process whereby shape of lens changes in order for eye to see objects at different distances

The far point is the point furthest from the eye where an object can be seen clearly by the eye without straining it.

At infinity


The distance of the near point from the normal eye is known as the least distance of distinct vision.


The human eye

The point of the least distance from the eye such that an object can be seen clearly without strain is called the near point.

At 25 cm from eye

Go to More to Know 9More to Know 9

Go to More to Know 10More to Know 10



Visual defects and their corrections

1. Short-sightedness (or myopia)

A short-sighted person cannot see distant objects clearly.




1. Short-sightedness (or myopia)

focal length of lens = far-point

distance

Go to

Example 8Example 8




2. Long-sightedness (or hypermetropia)

A long-sighted person can only see distant objects clearly.




2. Long-sightedness (or hypermetropia)

Go to

Example 9Example 9




3. Presbyopia

Go to

Example 10Example 10

Presbyopia is the loss of accommodative power of the eye due to age.

Eye is unable to focus both distant and near objects




4. Astigmatism

Astigmatism is the defect of the eye arising from its inability to see clearly in one particular plane.

due to different focal lengths (F1 & F2) in different planes



Visual angle

The angle α subtended at the eye by the object is known as the visual angle.

Distant object:smaller smaller in size

Near object:larger larger in size


End


Visual Acuity (VA)

VA is the eye’s ability to recognize small, fine details, and is measured in terms of the angular size of the smallest character that can be recognized. Acuity is frequently expressed as the ratio between the distance to the target (usually 6.5 m) and the distance at which the target element would subtend 1 minute. The figure shows the construction of a Snellen eye-chart letter H to measure visual acuity.

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Spectral sensitivity of eye

The eye is a poor photometer; it is very inaccurate at judging the absolute level of brightness. Under normal conditions of illumination, the eye is most sensitive to yellow-green light at a wavelength of 550 nm, and its sensitivity drops off on either side of this peak. The figure shows the relative sensitivity of the eye to different wavelengths for normal levels of illumination (photopic vision) and under conditions of dark adaptation (scotopic vision). Return to

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Q: Q: The far point and the near point of a short-sighted person are 200 cm and 15 cm respectively from the eye.(a) Which type of lens placed 2.0 cm from the eye would enable him to see an object at infinity clearly?(b) Where would the near point be when he uses spectacles of the focal length calculated?

Solution



Solution:Solution:


(a) The lens must form a virtual image at a distance of 200 cm from the eye or (200 – 2.0) cm = 198 cm from the lens for him to see an object at infinity clearly. We have u = ∞ and v = –198 cm (the image is virtual). Thus,

Therefore, the lens required is a diverging lens of focal length 198 cm or of power 1/-1.98 = -0.505 D.(b) The original near point is 15 cm from the eye or (15 – 2) cm = 13 cm from the lens. The new position of the near point is such that an object placed at that position produces a virtual image 13 cm from the lens. We have v = –13 cm and f = –198 cm. Thus,

Therefore, the distance of the new near point from the eye= 13.9 + 2.0 = 15.9 cm

cm 198 19811111

f,

vuf

cm 913 131

1981111 .u,

vfu

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Q: Q: The near point of a man is 75 cm from the eyes. Which type of lenses should he use in order to be able to read a book held 25 cm from his eyes?

Solution



Solution:Solution:

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For an object placed at a distance of 25 cm from the eyes, the virtual image must be 75 cm away from the man to be able to see clearly. We have u = 25 cm and v = –75 cm (the image is virtual). Thus,

Therefore, a pair of converging lenses of focal length 37.5 cm is required.

cm 537 751

251111 .f,

vuf


Q: Q: A man’s near point is 50 cm from his eye, and his far point 3 m away. Find the focal lengths of the two sets of spectacle lenses he needs to enable him to see distant objects, and to read print 25 cm away.

Solution



Solution:Solution:

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To correct for his short-sightedness, we have u = ∞ and v = –3 m.

A set of diverging lens of focal length 3 m is required.

To correct for his long-sightedness, we have u = 25 cm and v = –50 cm.

A set of converging lens of focal length 50 cm is required.

m 3 3

11111

f,vuf

cm 50 501

251111

f,

vuf

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1© Manhattan Press (H.K.) Ltd. The human eye Visual defects and their corrections Visual defects and their corrections 12.3 Properties of vision Visual