Understanding Lenses

• A lens is a piece of transparent material (usually glass) that changes the direction of light that passes through it.

• There are two main types of lenses: convex and concave lenses.

• A convex lens, or converging lens, bends light rays towards each other, focusing them at a particular point known as the focal point. The image produced can be real or virtual, depending on the position of the object.

• A concave lens, or diverging lens, spreads out light rays so that they diverge from a certain point. The image produced by a concave lens is always virtual and reduced in size.

The Eye and its Features

• The human eye functions much like a camera, with various parts responsible for focusing light and producing images.

• The cornea is a clear front surface of the eye, which starts to bend (or refract) incoming light.

• The iris is the coloured part of the eye, which adjusts to control the amount of light that enters the eye.

• The pupil is the dark centre in the iris, which the light passes through.

• The lens of the eye further refracts light and focuses it onto the retina, which is a layer at the back of the eye containing cells sensitive to light.

Principles of the Human Eye and Lenses

• When we look at objects at different distances, the lens of the human eye changes shape to ensure that the light is focused on the retina. This is known as accommodation.

• The near point is the closest point at which an object can be seen clearly without straining the eyes. For a typical adult, the near point is about 25cm.

• The far point is the furthest point an object can be while still being seen clearly without straining the eye. For a normal eye, the far point is at infinity.

• Short sightedness (myopia) and long sightedness (hypermetropia) are common vision defects that are corrected using lenses. In short sightedness, light is focused in front of the retina, and a concave lens is used to correct it. In long sightedness, light is focused behind the retina, and a convex lens is used for correction.

• Presbyopia is a condition that typically emerges as a person ages, whereby the eye's lens loses its flexibility and thus its ability to adjust focus, making it difficult to see objects up close. This condition may also be remedied with the use of a convex lens.

• Astigmatism is another common eye condition where the eye does not focus light evenly onto the retina, resulting in a distorted or blurred image. This condition can be caused by an irregularly shaped cornea or lens, and may be corrected with specifically designed glasses or contact lenses.

Principal Concepts of Lenses

• The principle of reversibility of light states that if the path of a ray of light through a system is reversed, it will retrace its original path.

• The focal length of a lens is the distance between the lens and the focal point, where parallel rays of light converge after passing through the lens.

• The power of a lens, measuring in dioptres (D), is the reciprocal of the focal length measured in meters (1/f). A lens with a longer focal length has less refracting power and is weaker.