Advertisements
Advertisements
प्रश्न
What is the focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm? Is the system a converging or a diverging lens? Ignore thickness of the lenses.
Advertisements
उत्तर
Focal length of the convex lens, f1 = 30 cm
Focal length of the concave lens, f2 = −20 cm
Focal length of the system of lenses = f
The equivalent focal length of a system of two lenses in contact is given as:
`1/"f" = 1/"f"_1 + 1/"f"_2`
`1/"f" = 1/30 - 1/20`
`1/"f" = (2 - 3)/60`
`1/"f" = -1/60`
∴ f = −60 cm
Hence, the focal length of the combination of lenses is 60 cm. The negative sign indicates that the system of lenses acts as a diverging lens.
संबंधित प्रश्न
Calculate the focal length of a corrective lens having power +2.5 D.
What does sign of power (+ve or –ve) indicate?
Define 1 dioptre of power of a lens.
A student uses a lens of focal length 40 cm and another of –20 cm. Write the nature and power of each lens
Consider two statements A and B given below:
A: real image is always inverted
B: virtual image is always erect
Out of these two statements:
The focal length of a convex lens is 25 cm. What is its power?
A converging lens has focal length of 50 mm. What is the power of the lens?
An object of height 4.25 mm is placed at a distance of 10 cm from a convex lens of power +5D. Find (i) the focal length of the lens, and (ii) the size of the image.
The power of a combination of two lenses X and Y is 5 D. If the focal length of lens X be 15 cm :
(a) calculate the focal length of lens Y.
(b) state the nature of lens Y.
A converging lens has a focal length of 50 cm. The power of this lens is:
Mohan obtained a sharp inverted image of a distant tree on the screen placed behind the lens. He then moved the screen and tried to look through the lens in the direction of the object. He would see:
(1) a blurred image on the wall of the laboratory.
(2) an erect image of the tree on the lens.
(3) no image as the screen has been removed
(4) an inverted image of the tree at the focus of the lens.
A double convex lens has two surfaces of equal radii R and refractive index \[m = 1 \cdot 5\]
A double convex lens has focal length 25 cm. The radius of curvature of one of the surfaces is double of the other. Find the radii, if the refractive index of the material of the lens is 1.5.
Surabhi from std. X uses spectacle. The power of the lenses in her spectacle is 0.5 D.
Answer the following questions from the given information:
- Identify the type of lenses used in her spectacle.
- Identify the defect of vision Surabhi is suffering from.
- Find the focal length of the lenses used in her spectacle.
A lens forms an upright and diminished image of an object, irrespective of its position. What kind of lens is this? Draw an outline ray diagram to show the formation of the image. State the position and one more characteristic of the image.
Match the following:
| Column - I | Column - II | ||
| 1 | Retina | a | Path way of light |
| 2 | Pupil | b | Far point comes closer |
| 3 | Ciliary muscles | c | near point moves away |
| 4 | Myopia | d | Screen of the eye |
| 5 | Hypermetropia | e | Power of accommodation |
The focal length of a concave lens is 20 cm. The focal length of a convex lens is 25 cm. These two are placed in contact with each other. What is the power of the combination? Is it diverging, converging or undeviating in nature?
In what unit is the power of a lens expressed?
