Advertisements
Advertisements
Question
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.
Solution
(a) The focal length of lens X, ie., f1 = 15 cm = 0.15 m
Power of lens X, `p_-1=1/f_1=1/0.15=6.67` D
Let the power and the focal length of the lens Y be P2 and f2, respectively.
The resultant power of the combination of two lenses is the algebraic sum of the powers of the individual lenses.
∴ Power of combination of lenses P = P1 + P2
⇒5 = 6.67 + P2
⇒P2 = 5 - 6.67 = - 1.67 D
The power of the lens is related to the focal length of the lens as:
`p_2=1/f_2`
`-1.67=1/f_2`
`f_2=1/-1.67=0.60`m=-60cm
`f_2=1/-1.67=-0.60m=-60cm`
The focal length of the lens Y is -60 cm.
(b) The focal length of the lens Y is negative. Therefore, the lens is concave and diverging in nature.
APPEARS IN
RELATED QUESTIONS
Define 1 dioptre of power of a lens.
A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.
The lens mentioned in 6(b) above is of focal length 25cm. Calculate the power of the lens.
What can you see in a completely dark room? If you switch on an electric bulb in this dark room as a light source, explain how you could now see:
(a) the electric bulb
(b) a piece of white paper
Give the usual name for the following:
A point inside a lens through which the light passes undeviated.
Which has more power : a thick convex lens or a thin convex lens, made of the same glass? Give reason for your choice.
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.
A screen is placed a distance 40 cm away from an illuminated object. A converging lens is placed between the source and the screen and its is attempted to form the image of the source on the screen. If no position could be found, the focal length of the lens
Which lens has more power a thick lens or a thin lens?
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 |
