Waves and Light: Lenses

Key Concepts in Lenses

Lenses are optical devices which transmit and refract light, converging or diverging the beam. A lens can focus light to form an image.
The principle of refraction governs the manner in which lenses bend light. As light passes from one medium to another (for example, from air to glass), its speed changes, causing it to change direction.
Lenses are characterised by their focal length, which is the distance from the lens to the point where light rays coming from a point at infinity converge.
There are two primary types of lenses: converging (or convex) lenses and diverging (or concave) lenses. Converging lenses bring light together at a point, forming a real image, while diverging lenses spread light out, forming a virtual image.

A converging lens is thicker at the middle than at the edges. Parallel rays of light that enter the lens converge to a point on the axis, called the focal point. The distance from the centre of the lens to the focal point is the focal length.
Real images can be formed by converging lenses. A real image is formed when light rays physically meet, and can be projected onto a screen. The image may be either smaller or larger than the object, depending on the object’s distance from the lens.

A diverging lens is thinner at the middle and thicker at the edges. Parallel light rays that hit the lens are spread out, or diverged, such that they appear to come from a point on the axis, creating a virtual image.
Virtual images are always formed by diverging lenses. A virtual image is the apparent source of light rays and cannot be projected onto a screen. Virtual images formed by diverging lenses are always upright and smaller than the object.

The lens equation relates the object distance (u), image distance (v), and the focal length (f) of the lens: 1/f = 1/v - 1/u.
This equation is applicable to both types of lenses, but sign conventions must be followed in the calculations depending on the type of lens in question.
The size of an image compared to the size of the original object is called the magnification. It’s calculated by the ratio of image distance to object distance (m = v/u).
The magnification can also be calculated by comparing the height of the image to the height of the object (m = height of image/height of object).

Lenses are commonly used in devices that manipulate light, such as cameras, telescopes and microscopes, to produce clear images.
In the human eye, the cornea and the lens work together to focus light onto the retina.
Corrective eyewear like spectacles and contact lenses use the principles of refraction to correct vision by adjusting the focus of light onto the retina.
Projectors use lenses to produce large images from small transparencies, while magnifying glasses can produce a magnified image of a small object.