If you can see objects nearby with no problem, but reading road signs or making out the writing on the board at school is more difficult, then you probably are nearsighted. Myopia simply occurs when the shape of the eyeball is too long or when the cornea/lens is too steep. The magnitude of this dictates the power of the lens for corrective eyewear.
Myopia occurs when the eyeball is either too long, or more rounded shape. Another less frequent cause of myopia is that the cornea, or lens is too curved. There is also some evidence that myopia may also be caused by too much close work.
The sense of vision starts off when light enters through the cornea and into the pupil. At this point it penetrates through the lens and focuses on the point of the retina called the macula. The shape of the cornea, lens help bend (refract) light rays in a manner that allows light to be focused on the retina/macula. If you are nearsighted, the light rays from a distant point are focused at a place in front of the retina. The light will only be focused in that one place, and by the time it reaches the retina it will have defocused again, forming a blurry image.
Since the eyes continue to grow during childhood, nearsightedness almost always occurs before the age of 20. The degree of myopia increases as the body grows rapidly, and will eventually level off in adulthood. During the years of rapid growth, frequent changes in prescription eyewear may be needed to maintain clear vision.
Myopia is often suspected when a teacher notices a child squinting to see a blackboard or a child performs poorly during a routine eye screening. A comprehensive eye health examination will detect myopia. Periodic examinations should follow after myopia has been discovered to determine whether the condition is changing, and whether a change in prescriptive eyewear is needed. ( this paragraph was copied and pasted)
Corrective lenses are prescribed to help focus light precisely onto the retina, where a clear image can be formed. Depending on the degree of myopia, glasses or contact lenses may be needed at all times for clear vision. If the patient has myopia due to a long eyeball the eyeglasses would be optimal for them, but if it is due to the steepness of the cornea or lens then contact lens would be the desired choice.