How Do Glasses Work
Glasses are basically optical devices, which fall under the category of Lenses. Mirrors also fall under the category of optical devices. The basic difference between mirrors and lenses is the underlying principle. Mirrors work on the principle of reflection of light whereas lenses refract the light.
Refraction: Refraction is a phenomenon by which light bends when it travels from one medium to another. The Snell's Law gives a relationship between the angle of incidence i and angle of refraction r as follows:
i=Angle of Incidence
r=Angle of Refraction
This law is much simpler to apply for a plane glass slab, but when the medium gets complex, as in case of lenses, the physics gets more complex too.
Real Image: A real image is the one that is formed when the rays of light converge after reflection/refraction.
Virtual Image: A virtual image is the one where the rays of light do not actually meet after reflection/refraction but appear to meet at a point when drawn backwards.
For all discussions, left side of the lens will be considered negative and right side as positive.
Lenses: Lenses are carefully crafted or molded pieces of glass such that they refract light to form real or imaginary images.
Principle Axis: The Horizontal axis about which the lens is symmetrical.
Pole: The central point of any lens is known as its pole.
Focal Point: The point at which a beam parallel rays of light converges(in case of convex lens) or seems to converge(in case on concave lens) after refraction is called as the focal point.
Lenses can be categorized broadly into two categories:
1) Concave: These are also known as divergent lenses since they diverge the rays of light from an object. The focus of the convex lens lies on the -ve axis. Since the rays diverge after Refraction, they do not form a real image, and the image is always on the negative axis.
2)Convex: These are known as convergent lenses since they converge the rays of light at one point(except in one case where the object is between Focus and the Pole), forming a real image of the objects.
Eye is a lens that refracts light that falls on it and forms an inverted image(since it is a convex lens) on the retina. This image is in turn converted into electro-chemical impulses in the neuron. These impulses are conveyed to the brain via the optical nerve.
Common Eye Defects:
Myopia: Myopia or near sightedness is the ability to see clearly an object when it is near the eyes but inability to focus on objects that are far. The degree of myopia determines the maximum distance up to which one can perceive objects clearly. This is caused when the image on the objects are formed not on the retina, but before it.
Hypermetropia: Hypermetropia or far sightedness is the ability to see objects which are far but inability to focus on objects that are near. The degree of Hypermetropia determines the least distance up to which the objects can be seen clearly. This is caused when the image, instead of forming on the retina, forms behind it.
Presbyopia: Presbyopia is also an inability to see near objects, but the causes of presbyopia are different than that of Hypermetropia.
How Glasses Work
After a basic understanding of refraction, lenses, eye and its defects, it is not difficult to guess how glasses might work. The simple principle of the glasses is to correct the defect of the eye by rectifying the image and making it fall on the retina.
In case of Myopia, since the image forms in front of the retina, concave lens, being divergent lens, can diverge the rays before they fall on the cornea of the eye. Since, the rays are slightly diverged now, the image will now form farther away than it did previously. Depending upon the degree of Myopia, lenses of appropriate focal length are used.
In case of Hypermetropia, the image falls behind the retina. Hence, a convergent type of lens, i.e. convex lens, would converge the light rays before they enter the eye. Since, the rays are slightly converged now, the image will form closer than it did previously. Depending on the degree of Hypermetropia, appropriate lenses are used.