Human Eye Vision Defects

Human Eye Vision Defects

Human Eye

The human eye basically consists of a convex lens. This lens forms a real and inverted image of an object on the retina. There are two kinds of vision cells in the retina. They are called rods and cones due to their peculiar shape. Rods decide the intensity of light, whereas cones distinguish the color of light.

The least distance of distinct vision is 25 cm.

Defects of the Human Eye and Their Remedies

1. Myopia (Short-sightedness)

• A person suffering from myopia can see near objects clearly, but far objects are not clear.

• Causes:

  • Elongation of the eyeball along the axis.

  • Shortening of the focal length of the eye lens.

  • Overstretching of ciliary muscles beyond the elastic limit.

• Remedy:

  • A diverging (concave) lens is used.

2. Hyperopia (Hypermetropia or Long-sightedness)

• A person suffering from hypermetropia can see distant objects clearly but cannot see near objects clearly.

• Causes:

  • Shortening of the eyeball along the axis.

  • Increase in the focal length of the eye lens.

  • Stiffening of ciliary muscles.

• Remedy:

  • A converging (convex) lens is used.

3. Presbyopia

• This defect is generally found in elderly persons. Due to the stiffening of ciliary muscles, the eye loses much of its accommodating power. As a result, distinct as well as nearby objects cannot be seen.

• Remedy:

  • Two separate lenses or a bifocal lens are used.

4. Astigmatism

• This defect arises due to differences in the radii of curvature of the cornea in different planes. As a result, a point in one plane of an object appears in focus, while those in another plane are out of focus.

• Remedy:

  • A cylindrical lens is used.

Simple Microscope

A simple microscope is simply a convex lens with a small focal length. The object to be enlarged is placed within the focus of the lens.

The magnifying power of a simple microscope is given as:

M=DfM = \frac{D}{f} M=fD​

Where:

• D=25 cm D = 25 \, \text{cm} D=25cm (least distance of distinct vision)

• f = f = f = focal length of the lens.

Compound Microscope

A compound microscope consists of two convex lenses fitted coaxially in a hollow tube. The lens facing the object is called the objective, and the lens towards the eye is called the eyepiece.

• The aperture of the objective is smaller than that of the eyepiece.

• Both lenses have smaller focal lengths, which increases the magnifying power of the instrument.

Electron Microscope

An electron microscope uses a beam of accelerated electrons as a source of illumination. Since the wavelength of electrons is shorter than that of visible light, the electron microscope has a higher resolving power.

Telescope

Telescopes are used to view distant objects that are not visible to the naked eye. Telescopes can be divided into:

• Astronomical Telescope

• Terrestrial Telescope

• Galilean Telescope

Astronomical Telescope

An astronomical telescope consists of two convex lenses placed coaxially in a hollow tube. The lens facing the object is called the objective, and the lens towards the eye is called the eyepiece. The final image formed is inverted.

• The objective has a large aperture to easily collect rays from the object.

• The focal length of the objective is larger than that of the eyepiece.

Terrestrial Telescope

• A terrestrial telescope is similar to the astronomical telescope but with one additional convex lens placed between the objective and the eyepiece.

• This lens inverts the image formed by the objective, so the final image is erect.

Galilean Telescope

• A Galilean telescope consists of two lenses. The objective is convex, and the eyepiece is concave. The final image formed is erect.

History of the Optical Telescope

• The optical telescope was invented by Hans Lippershey, but Galileo Galilei was the first to use it systematically to study the night sky.

Movie (Film) or Slide Projector

A movie (film) or slide projector projects a magnified image of an object on a screen.

• The object (inverted) is placed between FFF and 2F2F2F of a projection lens (a convex lens) so that its real, inverted, magnified image is formed between 2F2F2F and infinity on the other side, i.e., the image is erect.

Final Thoughts

The human eye works like a camera, forming a real, inverted image on the retina. It has two types of cells: rods for light intensity and cones for color detection. Common defects include myopia (short-sightedness), hyperopia (long-sightedness), and astigmatism, which can be corrected with concave or convex lenses. Presbyopia, common in older people, requires bifocal lenses.

Instruments like microscopes and telescopes are based on similar optical principles. Simple microscopes use convex lenses for magnification, while compound microscopes offer greater power with two lenses.

Telescopes, like astronomical and terrestrial ones, help us view distant objects, and the electron microscope uses electron beams for much higher resolution.