Lens Combination Numerical Class-12 Nootan ISC Physics Solution Ch-16 Refraction of Light at Spherical Surfaces : Lenses. Step by step solutions of Kumar and Mittal Physics of Nageen Prakashan as council latest prescribe guideline for upcoming exam. Visit official Website CISCE for detail information about ISC Board Class-12 Physics.
Lens Combination Numerical Class-12 Nootan ISC Physics Solution Ch-16 Refraction of Light at Spherical Surfaces : Lenses
| Board | ISC |
| Class | 12 |
| Subject | Physics |
| Book | Nootan |
| Chapter-16 | Refraction of Light at Spherical Surfaces : Lenses |
| Topics | Numericals on Lens Combination |
| Academic Session | 2025-2026 |
Numericals on Lens Combination (Lenses Placed in Contact)
Refraction of Light at Spherical Surfaces : Lenses Numerical-VI Class-12 Nootan ISC Physics Solution
Que-45: A convex lens of 20 cm focal length is placed in contact with a concave lens of 25 cm focal length. Find the position and nature of the image of an object placed at a distance of 2 m from this combination.
Ans- 1/f = 1/20 – 1/25
=> 5-4/100 = 1/100
f = 100
m = f/f+u
=> 100/100-200 = -1
same size real and inverted and same distance of object
Que-46: A convex lens forms the image of the same size of an object placed at a distance of 20 cm. If another lens is placed in contact with the first, the image is formed at a distance of 5 cm from the combination on the other side. What are the focal lengths of the lenses used?
Ans- If first case object is placed at 2f
i.e. 2f = 20 cm
=> f = 10 cm
again 1/f = 1/5 – 1/-20
=> 1/f = 4+1/20
=> 1/f = 1/4
=> 1/f = 1/f1 + 1/f2
=> 1/4 = 1/10 + 1/f2
=> f2 = 5-2/20 = 6.66 cm
Que-47: Rays coming from an object situated at infinity, fall on a convex lens and an image is formed at a distance of 16 cm from the lens. When a concave lens is kept in contact with the convex lens, the image is formed at a distance of 20 cm from the lens combination. Calculate the focal length of the concave lens.
Ans- 1/f = 1/f1 + 1/f2
=> 1/20 = 1/16 + 1/f2
=> f2 = -80 cm
Que-48: Two thin lenses, one convex and the other concave of focal lengths 15 cm and 20 cm respectively, are put in contact. Find the focal length of the doublet lens.(ISC 2000)
Ans- Power of lens 1
=> P1 = 1/f1 = 1/0.2 = 5 D
Power of lens 2
=> P2 = 1/f2 = 1/-0.15 = -6.67 D
P = P1+P2
=> 5 – 6.67 = -1.67 D
Focal length f = 1/ P
=> 1/-1.67 = -0.598 m
=> -60 cm
Que-49: A thin convex lens of focal length 20 cm is kept in coaxial contact with another thin lens so that the combination becomes a converging lens of focal length 100 cm. What is the focal length of the second lens?
Ans- 1/f = 1/f1 + 1/f2
=> 1/100 = 1/20 + 1/f2
After solving,
=> 1/f2 = -1/100
=> f2 = -25 cm (concave)
–: End of Lens Combination Numerical Class-12 Nootan ISC Physics Solution Ch-16 :–
Return to : – Nootan Solutions for ISC Class-12 Physics
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