Defects Of eye
Common defects of vision include myopia (nearsightedness), hypermetropia (farsightedness), and Presbyopia.
(1) Myopia
Myopia is commonly known as nearsightedness, is a refractive error of the eye that affects the ability to see distant objects clearly.
Myopia is a condition where the eye can see nearby objects clearly but has difficulty focusing on objects that are far away. In a myopic eye, distant objects appear blurred.
A person with this defect has the far point nearer than infinity. Such a person may see clearly upto a distance of a few metres.
In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself
Myopia often occurs when the eyeball is elongated, or the cornea is too curved.
Correction of Myopia
Wearing glasses or contact lenses with concave lenses helps diverge the incoming light rays, allowing them to focus on the retina.
(2) Hypermetropia
Hypermetropia, also known as farsightedness or long-sightedness, is a common refractive error of the eye where distant objects can be seen more clearly than close ones
It is a condition where the eye can see distant objects clearly but has difficulty focusing on nearby objects. In a hypermetropic eye, close objects appear blurred.
The near point, for the person, is farther away from the normal near point (25 cm).
In a Hypermetropia eye,image of a closeby object are focussed at a point behind the retina
Hypermetropia often occurs when the eyeball is shorter than normal, or the cornea is too flat.
Correction of Hypermetropia Wearing glasses or contact lenses with convex lenses helps converge the incoming light rays, allowing them to focus on the retina.
REFRACTION OF LIGHT THROUGH A PRISM
A prism is a transparent optical object with flat, polished surfaces that refract light. The most common type of prism is a triangular prism, which has a triangular base and rectangular sides. When light passes through a prism, it undergoes refraction, bending, and dispersion.
Process of Refraction in a Prism
(i)Incidence of Light: A beam of white light is incident on one of the prism's surfaces at an angle to the normal.
(ii)Refraction at the First Surface: As the light enters the prism, it slows down and bends towards the normal due to the change in medium from air to glass (or another transparent material). This is the first refraction.
(iii)Traveling Through the Prism: The light travels through the prism, and its path may be affected by the angles and shape of the prism.
(iv)Refraction at the Second Surface: As the light exits the prism, it speeds up and bends away from the normal, moving from the glass into the air. This is the second refraction.
(v) This angle between incident ray and emergent ray is called the angle of deviation
Dispersion of White Light by Glass Prism
When light falls on the prism it splits the incident light into band of colours. The sequence of colours observed are VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange and Red). This band of colour is known as Spectrum. So, this splitting of incident light into different colours is known as Dispersion. This splitting is due to bending of light rays at different angles. Violet light bends most whereas red light bends least. The phenomenon of rainbow is also due to dispersion of light.
The band of these seven colours formed because of dispersion of light is known as Spectrum.
