Question

Doctor has prescribed a lens having power $+1\cdot 5$ what will be the focal length of lens? What is the type of lens and what must be the defect of vision?

Answer 1

A lens is a transparent optical device which consists of two spherical surfaces or one spherical surface and another is a plane surface. A lens can be of two types:
-Convex lens (convergent lens)
-Concave lens (divergent lens)
Power of lens: it is reciprocal of the focal length. From the power of the lens, we can easily measure the amount of deviated light ray produced by the lens. Mathematically, it is given as:
$\text{P}=\dfrac{1}{\text{F}}$
F= focal length of the lens.

Complete step by step solution
In our question $\text{P}=+1\cdot 5\text{ D}$
Using $\text{P}=\dfrac{1}{\text{F}}$ , we will find out the focal length of the lens.
So, it will become
$1\cdot 5=\dfrac{1}{F}$
$\Rightarrow F=\dfrac{1}{1\cdot 5}$
$\Rightarrow 0\cdot 6m$
Now, we can see that the focal length is positive and we also know that a convex lens has positive focal length and a concave lens has negative focal length.
$\therefore$ It is a convex lens. It is also known as a converging lens.
Now, Hypermetropia is a common vision disorder. It results from the overall power of the eye not being strong enough, or the length of eye being too short. Surgical correction is a treatment for laser eye surgery and clear lens exchange.
It is a form of refractive error in which parallel rays of light coming from infinity are focused behind the light sensitive layer of the retina, when the eye is at rest. This may be corrected with a lens. Convex lens is used to correct farsightedness.

Note
Remember that a concave lens has negative focal length and a convex lens has positive focal length. Concave lenses are used to correct nearsightedness and convex lenses are used to correct farsighted. Do not forget that concave lenses are diverging lenses and convex lenses are converging lenses.