Advertisements
Advertisements
Question
A combination of lenses for a camera contains two converging lenses of focal lengths 20 cm and 40 cm and a diverging lens of focal length 50 cm. Find the power and focal length of the combination.
Advertisements
Solution
Focal length (f1) of the 1st converging lens = 20 cm = 0.2 m
Focal length (f2) of the 2nd converging lens = 40 cm = 0.4 m
Focal length (f3) of the diverging lens = −50 cm = −0.5 m (negative sign due to sign convention)
Power = `1/"Focal length of lens (in metre)"`
Power (P1) of 1st lens = `1/0.2` = 5 D
Power (P2) of 2nd lens = `1/0.4` = 2.5 D
Power (P3) of diverging lens = `1/0.5` = −2 D
Net power of the combination = P1 + P2 + P3
= 5 + 2.5 + (−2)
= 7.5 − 2
= 5.5 D
Net focal length of the combination = `1/"Net power"`
= `1/5.5`
= 0.1818 m
= 18.18 cm
RELATED QUESTIONS
Calculate the focal length of a corrective lens having power +2D.
Find the power of a concave lens of focal length 2m.
The lens mentioned in 6(b) above is of focal length 25cm. Calculate the power of the lens.
Give the usual name for the following:
A point inside a lens through which the light passes undeviated.
The focal length of a convex lens is 25 cm. What is its power?
What is the power of a convex lens of focal length 0.5 m?
The power of a lens is, −2 D. What is its focal length?
What is the nature of a lens having a power of + 0.5 D?
What is the nature of a lens whose power is, −4 D?
A doctor has prescribed a corrective lens of power, −1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?
An object of height 4 cm is placed at a distance of 15 cm in front of a concave lens of power, −10 dioptres. Find the size of the image.
What do you understand by the power of a lens?
A diverging lens has a focal length of 0.10 m. The power of this lens will be:
A normal eye is not able to see objects closer than 25 cm because
A double convex lens has two surfaces of equal radii R and refractive index \[m = 1 \cdot 5\]
A diverging lens of focal length 20 cm and a converging lens of focal length 30 cm are placed 15 cm apart with their principal axes coinciding. Where should an object be placed on the principal axis so that its image is formed at infinity?
How is accommodation produced?
If focal length of a convex lens is 20 cm at what is the power of the lens?
Power of a lens is – 4D, then its focal length is
When monochromatic red light is used instead of blue light in a convex lens, its focal length will ______.
The lens of power + 1·0 D is ______.
A convex lens is a converging lens whose focal length and power are both:
A greater numerical value of the power of a lens means:
For a lens immersed in a medium of refractive index \[n_1\], the power is related to the focal length by:
The power of a lens of focal length \[f = 0.5\,m\] is:
A lens has a power of \[+2\,D\]. Its focal length is:
For a lens with \[f = +12\,cm\], \[R_1 = +10\,cm\] and \[R_2 = -15\,cm\], the lens maker's formula \[\frac{1}{f}=(n-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)\] gives the refractive index of the glass as:
