Question

A convex lens of focal length 25cm and a concave lens of focal length 10cm are placed in close contact with each other. Calculate the focal length of this combination. is it a converging or diverging lens?

Answer 1

Answer:Given

f1 = 25cm (Focal length of convex lens is positive)

⇒ f1 = 0.25m (1m = 100cm)

P (in dioptre) =  

⇒ Power of convex lens,(F=25 cm) P1 =  

⇒ P1 =  

∴ P1 = 4D

f2 = -10cm (Focal length of concave lens is negative)

⇒ f2 = -0.1m (1m = 100cm)

⇒ Power of the concave lens(F=10 cm), P2 =  

⇒ P2 =  

∴ P2 = -10D.

Power of combination = P1 + P2.

P = 4D – 10D = -6D.

Lens power of the combination is -6D. Since it is negative therefore the new lens will behave like concave lens having a focal length of 16.6cm.

Answer 2

Answer:

→ FOCAL LENGTH OF COMBINATION ( F ) = [ - 50 / 3 ] cm ←

♒ Nature Of Combination Of Lens = DIVERGING LENS ≈ [ Concave Lens ]

Explanation:

★ RAY OPTICS ★

★ ★ COMBINATION OF LENSES ★ ★

★ GIVEN ;

» Focal length of Convex Lens ( F¹ ) = 25 cm

» Focal length of Concave Lens ( F² ) =-10 cm

★ Focal length of Combination ( F )= ???

______ [ BY USING FORMULA ] _________

[ $\frac{1}{f} \: = \: \frac{1}{ {f}^{1} } \: + \: \frac{1}{ {f}^{2} }$ ]

★ 1 / F = 1 / 25 + 1 / -10

» 1 / F = [ ( -10 )+ 25 ] / 25 × ( -10 )

» F = -250 / 15

⏩ » F = [ - 50 / 3 cm ] « ⏪

_______________________________________

ANSWER :- FOCAL LENGTH OF COMBINATION OF LENSES IS ;

[ ★ F = - 50 / 3 cm ★ ]

♣ HENCE ; NATURE OF COMBINATION OF LENS IS "" DIVERGING "" .

» BECAUSE FOCAL LENGTH IS [ NEGATIVE ] .