Question

A.How are power and focal length of lens related ?
B.You are provided with two lenses of focal length 20cm and 40 cm respectively.....

Answer 1

How are power and focal length of a lens related? You are provided with two lenses of focal length 20 cm and 40 cm respectively. ... More the powerof a lens more is its converging power. Thus, the lens of shorter focal lengthwill have more
converging power
The power of a lens is related to its focal length as
P = 1/f (in m).
More the power of a lens more is its converging power. Thus, the lens of shorter focal length will have more converging power

Answer 2

Answer:

A. The relation between power and focal length of lens is inversely proportional. That means Power ∝ $\frac{1}{focal length}$

B. The power of two lenses will be 5D and 2.5D

Explanation:

• Focal length: The distance between the lens and its focus point is called focal length. (focus means the point at which the light rays meet   or from which the light rays emerge after refraction.)
• Focal length is expressed be the symbol 'f'.
• Power: Power indicates the degree of convergence or divergence of light rays after refraction.
• Symbol of power is 'P' and its SI unit is 'dioptre'(D).
• The relation between Power (P) and focal length (f) is: P ∝ $\frac{1}{f}$
• Lens-1:  f1 = 20cm = 0.2 m

                     Power = 1/0.2 = 5D

• Lens-2:  f2 = 40cm = 0.4 m

                     Power = 1/0.4 = 2.5D