Basic Physics

# Contact lens, farsightedness, nearsightedness

Nearsightedness eyes can be normalized using eyeglass lenses and contact lenses. Eyeglass lenses are affixed to the eyeglass while contact lenses are attached to the eyeball. Eyeglass has a certain distance from the eye while contact lenses stick to the eye so that the distance between the contact lens and the eye can be ignored.

An example of problems experienced by a person with nearsightedness and farsightedness is explained, what types of lenses are used to normalize the eyes, what is the focal length and power of the lens. This focuses on the use of contact lenses in normalizing nearsightedness and farsightedness

Farsightedness eye and contact lens

The farthest distance that can be seen clearly by a person with nearsightedness is 50 cm. For nearsightedness eyes to see infinite objects clearly, determine (a) the type of lens used, (b) the focal length of the lens and the type of lens, (c) the power of the lens.

Contact lenses are very close to the eyes so ignore the distance between the lens and the eye. The distance between objects and image with the contact lenses is considered the same as the distance between objects and image with the eyes.

Solution:

(a) The type of lens used

On the topic of nearsightedness, it has been explained that the lens used to normalize nearsighted eyes is a concave lens or diverging lens or negative lens.

(b) The focal length of the lens

Objects that are observed nearsighted eyes through contact lenses are at infinite distant, so the lens must form an image at a distance of 50 cm in front of the contact lens. The image is in front of the lens, so the image is virtual (compare the explanation on the topic of the image formation by the concave lens)

Known:

The object distance (do) = infinite

The image distance (di) = -50 cm (negative means the image is virtual)

Wanted: The focal length (f)

Solution:

1/f = 1/do + 1/di

1/f = 1/~ + (- 1/50)

1/f = 0 – 1/50

1/f = – 1/50

f = – 50/1 cm = -50 cm = -0.5 meters.

The focal length is negative, means that the lens used is a concave lens or a diverging lens.

(c) The power of the lens

P = 1/f = 1/-0.5 m = -2 Diopters

The power of the lens is -2 D. Negative sign means that the lens used is a concave lens or divergent lens.

Read :  Equation of loupe (magnifying glass)

Nearsightedness eye and contact lens

The closest distance that can be seen clearly by a person with nearsightedness is 50 cm. For the person to be able to read at a distance of 25 cm like the average normal eye, determine (a) the type of lens used, (b) the focal length of the lens, (c) the power of the lens.

Contact lenses are very close to the eyes so ignore the distance between the lens and the eye. The distance between objects and image with contact lenses is considered the same as the distance between object and image with the eyes.

Solution

(a) The type of lens used

The lens used to normalize nearsighted eyes is a convex lens or convergent lens or positive lens.

(b) The focal length of the lens

For the observed object to be 25 cm in front of the eye, the lens must form an image at a distance of 50 cm in the front of the eye. The image is in front of the lens, so the image is upright and virtual.

Known:

The object distance (do) = 25 cm

The image distance (di) = -50 cm (negative means the image is virtual)

Wanted: The focal length of the contact lens

Solution:

1/f = 1/do + 1/di

1/f = 1/25 + 1/-50

1/f = 2/50 – 1/50

1/f = 1/50

f = 50/1 = 50 cm = 0.5 meters

The focal length is positive means that the lens used is a convex lens or convergent lens or positive lens.

(c) Power of lens

P = 1/f = 1/0.5 = +2 Diopters

The power of the lens is +2 D. Positive sign means that the lens used is a convex lens or convergent lens.