Power of Lenses. The Human Eye Revision Notes

In these revision notes for Power of Lenses. The Human Eye, we cover the following key points:

• What is power of lens and where it is used?
• How can we calculate power of a lens?
• How can we calculate the power of s system of lenses?
• What is the structure of human eye?
• How does the human eye work?
• What are some defects of vision?
• What can we do to correct the sight when needed?
• How can we know what kind of optical glasses are needed in various situation?
• What is the range of normal vision?

Power of Lenses. The Human Eye Revision Notes

There is a quantity used to determine how able is a lens to collect rays at points near it. This quantity is the reciprocal of focal length and is known as (optical) Power of a lens, P. The only condition is that focal length must be written in metres. Its formula therefore is:

Power of lens is measured in Dioptre [D].

When two lenses are in contact, the focal length of this system is calculated by the formula

Therefore, the optical power of such a system of lenses, is

When two lenses are not in contact, we must also consider the distance between them in order to calculate the optical power of this system. The formula of optical power in this case becomes

where d is the distance (in metres) between the two lenses and P₁ and P₂ are the individual optical powers.

The eye has a spherical shape called eyeball, most of which has a white colour when healthy. The cornea is the clear front surface of the eye. It lies directly in front of the iris and pupil, and it allows light to enter the eye.

Attached to the pupil there is a converging lens, which refracts the light rays entering the eye, forming an inverted image on the retina, at the back of the eyeball.

The optic nerves attached to the retina carry these signals from the eye to the brain. The brain then converts the signals into upright image.

Ciliary muscle is a circular muscle that enables the lens to change shape for focusing.

The inside of the eyeball is filled with a watery liquid.

Light intensity entering the eye is controlled by the iris. If there is too much light, the iris enlarges, making the pupil smaller. In this way, the light intensity entering the eye decreases. This occurs during the day, in places exposed to sunlight. During the night the reverse process does occur, i.e. the iris shrinks, making the pupil larger. As a result, more light enters the eye and therefore, the light intensity increases.

The eye lens is flexible, i.e. it becomes thicker or thinner as needed. This is made possible by the action of ciliary nerves as stated earlier. When the eye lens changes thickness, its focal length also changes. Thus, the eye can make focusing adjustments for objects at various distances. This property of eye is known as eye accommodation.

Practically, a person with normal vision has an infinite distance of eye accommodation because he/she may see objects at very high distances without having any trouble. Therefore, the far point of normal people is at infinity. On the other hand, the nearest point in which the eye can accommodate effortlessly is known as the near point. For normal people the near point is 25 cm. This means the eye can be damaged when we try to see objects closer than 25 cm, because this causes tension in the eyes.

Many people have trouble in seeing clearly the objects around them. Their vision is blurred when it should not be so. This phenomenon occurs due to many factors, both inherited and acquired, and it is known as eye defect.

Basically, there are six types of eye defects:

1. Long-sightedness (Hyperopia). This occurs when light rays converge behind the retina. Converging lenses are used to correct this pathology.
2. Near-sightedness (Myopia). It occurs when light rays converge before the retina. Diverging lenses are used to correct this pathology.
3. Colour-blindness. This is another anomaly of the eye, in which the patient is not able to distinguish the colours of objects.
4. Astigmatism. This pathology is a result of an irregularly shaped cornea or eye-lens. If the eye-lens or cornea are not perfectly spherical, the eye can form a correct image only in some directions, but not in others. To correct this defect, cylindrical lenses are typically used.
5. Cataract. Cataract occurs when the vision becomes cloudy. Such a pathology can be corrected through a surgical intervention.
6. Blindness. It represents a pathology involving a lack of vision. It may also refer to a loss of vision that cannot be corrected with glasses or contact lenses. Partial blindness means the patient has very limited vision. Complete blindness means the patient cannot see anything and is not able to see any light.