Lens
Lens is a special transparent material designed in such way that it bends parallel rays of light. In terms of its shape, a lens can be a segment of cone, plane or sphere. The idea behind the lens design is to bend rays of light in order to cross them and form and image, which can be larger or smaller than the actual object under observation. This whole process of light bending is called refraction, an according to historians, the early Greeks were the first to introduce this concept. Nowadays, lenses find application in all types of optical instruments such as binoculars, telescopes, cameras, movie projectors, microscopes.
The process of refraction occurs because as light travels from one type of material through another, it changes its velocity, since glass has much higher density than air. The amount of light that bends depends on the angle at which the light rays make contact with the glass.
We can generally divide lenses into two types - convex and concave. The convex lenses are thicker in the middle than at the edges, while the concave ones are thinner in the middle. Convex lenses make the light rays to come together at a point after they exit the lens, while the concave lens cause the rays of light to separate.
The point at which the light rays come together after interacting with convex lens is called focal point, while the distance between the lens center and the focal point is called focal length. Since light can travel in both directions passing through the lens, it is considered that every lens has to focal points on either side. In the case of concave lens, since they don't focus the light in a single point, their focal point has different meaning. Concave lenses produce an image that appears to have originated from a point on the opposite side of the lens. But the focal length has the same definition as in convex lenses. Every lens has a specific magnification power (M), which can be calculated by dividing the height of the image produced (h1) by the height of the real image (h2), or M=h1/h2. When the magnification power is less than one, then the image is smaller than the object (M<1), when M equals one, then we have the same size. A negative value would mean that the image is inverted and real, while positive value means the image is upright and virtual.
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