Question

the radius of curvature of a convex lens are 15cm. and 10 cm respectively. if refractive index of lens is 1.5,then find its focal length

Answer 1

Given :

➳ Radius of curvature of a convex lens are 15cm and 10cm.

➳ RI of lens = 1.5

To Find :

➠ Focal length of convex lens.

Solution :

⇒ Lens maker's formula :

$\bigstar\:\underline{\boxed{\bf{\dfrac{1}{f}=(n-1){\huge(}\dfrac{1}{R_1}-\dfrac{1}{R_2}{\huge)}}}}$

፨ where,

✏ f denotes focal length

✏ n denotes RI of lens

✏ R¹ and R² denotes radii of curvatures

We know that,

➛ Radius of curvature of convex mirror is taken positive.

➛ Radius of curvature of concave mirror is taken negative.

$\longrightarrow\tt\:\dfrac{1}{f}=(n-1){\huge(}\dfrac{1}{R_1}-\dfrac{1}{R_2}{\huge)}\\ \\ \longrightarrow\tt\:\dfrac{1}{f}=(1.5-1){\huge(}\dfrac{1}{15}-\dfrac{1}{(-10)}{\huge)}\\ \\ \longrightarrow\tt\:\dfrac{1}{f}=0.5{\huge(}\dfrac{2+3}{30}{\huge)}\\ \\ \longrightarrow\tt\:\dfrac{1}{f}=0.5\times \dfrac{5}{30}\\ \\ \longrightarrow\tt\:f=\dfrac{60}{5}\\ \\ \longrightarrow\underline{\boxed{\bf{f=+12\:cm}}}$

Answer 2

Explanation:

1÷f= o.5(1÷15-1÷-10)

1÷f= 0.5 ×5÷30

f= 6÷0.5= 12