Chapter : 5. Human Eye & Colourful World
Short sightedness or myopi
1. Short sightedness or myopia
Symptoms : Eye cannot see clearly beyond a certain distance. It means that the far point of the defective eye has shifted from infinity to a finite distance ahead.
Reasons : It is so because the image of distant objects is formed in front of the retina. It is shown in fig.
Myopic eye vision.
Causes :
(i) The lens may be thicker (more converging) that the normal eye lens.
(ii) The eye ball may be elongated,
Fig. Elongated eye.
Correction : The extra converging power of eye lens is compensated by using a concave (diverging) lens of proper power (focal length) as shown in fig.
Fig. Myopia corrected by a concave lens
Calculation : Let distance of far point F’ from eye = x. Then for lens to be used, u = ∞, v = –x, f = ?
From lens formula,
या f = –x
प्रThe lens used must have focal length equal to the distance of the far point from the eye (–ve sign means concave lens).
Symptoms : Eye cannot see clearly beyond a certain distance. It means that the far point of the defective eye has shifted from infinity to a finite distance ahead.
Reasons : It is so because the image of distant objects is formed in front of the retina. It is shown in fig.
Myopic eye vision.
Causes :
(i) The lens may be thicker (more converging) that the normal eye lens.
(ii) The eye ball may be elongated,
Fig. Elongated eye.
Correction : The extra converging power of eye lens is compensated by using a concave (diverging) lens of proper power (focal length) as shown in fig.
Fig. Myopia corrected by a concave lens
Calculation : Let distance of far point F’ from eye = x. Then for lens to be used, u = ∞, v = –x, f = ?
From lens formula,
या f = –x
प्रThe lens used must have focal length equal to the distance of the far point from the eye (–ve sign means concave lens).
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