The correct answer is 1.08 m.
Concept:
- Lens: The transparent curved surface which is used to refract the light and make an image of any object placed in front of it is called a lens.
- The lens whose refracting surface is upside is called a convex lens and the lens whose refracting surface is inside is called a concave lens or diverging lens.
- The convex lens is also called a converging lens.
When two or more lenses are in contact with each other then, the equivalent focal length is given by:
\(\frac{1}{F} = \frac{1}{{{f_1}}} + \frac{1}{{{f_2}}}\)
Where f1 and f2 are focal lengths of the two lenses
- Power is the ability to bend light and it is equal to the reciprocal of the focal length of the lens/mirror in meters.
Power (P) = 1/f
Where f is the focal length of the lens/mirror.
- The unit of power of a lens is Dioptre when the focal length of the lens is taken in meter (m).
- The focal length of the concave lens is negative and hence its power is also negative.
- The focal length of the convex lens is positive and hence its power is also positive.
Lens formula: \(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\) where f is focal length, v is image distance and u is object distance.
Power of lens: It is the reciprocal of the focal length of the lens.
\(P=\frac{1}{f}\) where f is the focal length in meters.
P = \(\frac{100}{f}\) where f is the focal length in centimeters.
Explanation:
For convex lens
Power = \(\frac{1}{f}\)
f = \(\frac{1}{p}\) = \(\frac{100}{5}\) = 20 cm
Given: Object distance, u = -30 cm, Focal length, f = 20 cm
v=?
\(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\)
\(\frac{1}{20} = \frac{1}{v} - \frac{1}{-30}\)
\(\frac{1}{20} = \frac{1}{v} + \frac{1}{30}\)
\(\frac{1}{20} - \frac{1}{30} = \frac{1}{v} \)
\(\frac{1}{v} = \frac{3-2}{60}\)
\(\frac{1}{v} = \frac{1}{60}\)
v = 60 cm
For concave lens
f = \(\frac{1}{p}\) = \(\frac{100}{-5}\) = -20 cm
Object distance, u = -90-(-60)= -90 + 60 = -30cm, focal length, f = -20cm
v = ?
\(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\)
\(\frac{1}{-20} = \frac{1}{v} - \frac{1}{-30}\)
\(\frac{1}{-20} = \frac{1}{v} + \frac{1}{30}\)
\(\frac{1}{-20} + \frac{1}{-30} = \frac{1}{v} \)
\(\frac{1}{-20} - \frac{1}{30} = \frac{1}{v} \)
\(\frac{1}{v} = \frac{-3-2}{60} = \frac{-5}{60} = \frac{-1}{12}\)
v= -12 cm
Distance between original object and final image = 30 + (90-12) = 30 + 78 = 108 cm = 1.08 m.