Advertisements
Advertisements
Question
How does the power of a lens change if its focal length is doubled?
Advertisements
Solution
We know that
`"D"_1 = (100 " cm")/("F.L")` ...(i)
When focal length is doubled
`1"D"_2 = (100 " cm")/("2 F.L") = 50/"FL"` ...(ii)
`"D"_2/"D"_1 = 100/(2 "F.L") xx "F.L."/100 = 1/2`
`therefore "D"_2 = "D"_1/2`
∴ Power is Halved.
If the focal length of a lens is doubled, then its power gets halved.
APPEARS IN
RELATED QUESTIONS
Kavita from 10th is using spectacles. The power of the lenses in her spectacles is –2.5 dioptre. Answer the following questions:
1) Which lenses are used in her spectacles?
2) State the defect of vision Kavita is suffering from.
3) Find the focal length of the lenses used in her spectacles
Name the physical quantity whose unit is dioptre.
Which of the two has a greater power: a lens of short focal length or a lens of large focal length?
A converging lens has focal length of 50 mm. What is the power of the lens?
A diverging lens has focal length of 3 cm. Calculate the power.
What is the unit of power of a lens? Define the unit of power of a lens.
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?
The following diagram shows the object O and the image I formed by a lens. Copy the diagram and on it mark the positions of the lens LL’ and focus (F). Name the lens.
Find the power of a convex lens of focal length of + 25 cm.
The same size images are formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is ______ cm.
