Question

A doctor prescribes a corrective lens of power -0.5D to a person. The focal
length of lens and the type is
A. -2m and concave lens
B. +2m and convex lens
C.+2m and concave lens

D. -2m and convex lens
​

Answer 1

Answer:

a. -2m and concave lens

Explanation:

We know that,

$\mathtt{Power = \frac{1}{focal \: length} }$

$\mathtt{ - 0.5 = \frac{1}{focal \: length} }$

$\mathtt{focal \: length = \frac{1}{ - 0.5} }$

$\color{purple} \boxed{ \color{blue}\boxed{ \color{cyan}\boxed{\mathtt{ \colorbox{red}{f ocal \: length = - 2 \: m}}}}}$

Answer 2

Answer:

The focal length of the lens is -2m and it is a type of concave lens.

Explanation:

What is the power of a lens?

• The power (P) of a lens is defined as the reciprocal of its focal length (f), i.e.,
  $P=\frac{1}{f}$

• It gives us a measure of the converging or diverging ability of a lens.
• It is measured in Dioptres (D).

Significance of positive (+) and negative (-) signs used for denoting the power of a lens:

• In a convex lens, the light beams get converged after passing through the lens. The converging power is denoted by a +ve sign.
• In a concave lens, the light beams diverge after passing through the lens. The diverging power is denoted by a -ve sign.

Calculation:

Here, it is given that the power of the concerned lens is -0.5D.
The negative sign denotes that this lens must be a concave lens.

∴ The focal length of the lens

⇒ $P=\frac{1}{f}$

⇒ $-0.5=\frac{1}{f}$

⇒$f=\frac{1}{0.5} = -2m$

Conclusion:

Thus, the focal length of the lens comes out to be -2m and the lens is a concave lens.

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