Question

Aniket from std X uses spectacles. The power of lenses in his spectacles is-.
Answer the following questions:
(A) State the type of lenses used in his spectacles.
(B) Name the defect of vision Aniket is suffering from.
(C) Find the focal length of the lenses used in his spectacles.

Answer 1

Hint: Check the sign of the power. If it’s positive then we have a convex lens. If it's negative then we have a concave lens. Then depending on the lens used look for the defect.

Complete step by step solution
(A) The power of the lens is given as -0.5D Since, the power is negative, so the type of lens used is Concave. Concave lens is a Diverging lens. So diverging lenses have negative focal lengths and so do they have negative power.
(B) The defect of vision Aniket is suffering from is Myopic or Nearsightedness. Myopic type is the one in which close objects appear clearly. But for ones that don't. Glasses for correction of Myopia are Concave lenses (curved inwards), which moves the focus of the light to help you see clearly.
(C) $\text{Power of lens = }\dfrac{1}{F}$
       $-0.5D=\dfrac{1}{F}$
       $F=\dfrac{-1}{F}$ $=-2metre$
So the focal length of the lenses used comes out to be -2m which is negative because the lens used is concave.

Additional Information
 Myopia: It occurs when the eyeball is too long, relative to the focusing power of the cornea and lens of the eye. This causes light rays to focus at a point in the front retina, rather than directly on its surface. Placing concave lenses in front of a myopic eye reduces the refraction of light and lengthens the focal length so that the image is formed on the retina. These are called “minus power lenses” because they reduce the focusing power of the eye. By doing so, minus lenses move the focus of light from a point in front of the retina backward so it falls directly onto the surface of the retina. This shift corrects the blurry distance vision caused by myopia, restoring clear vision.
The power of the lenses is measured in units called dioptres (D). The high power number of the lenses used for their correction.


Note Study the concepts of Optics, Type of lenses and their properties, the defects of the eye and the lenses used for their correction. The formulas regarding calculating Focal lengths and Power should be known to one.