Advertisements
Advertisements
Question
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.
Advertisements
Solution
Since the image is real and same size. The position of image should be at 2F.
It is given that the image of the needle is formed at a distance of 50 cm from the convex lens. Hence, the needle is placed in front of the lens at a distance of 50 cm.
Object distance, u = – 50 cm
Image distance, v = 50 cm
Focal length = f
According to the lens formula,
`1/v-1/u = 1/f`
`1/f = 1/50 - 1/(-50)`
`=1/50 + 1/50 = 1/25`
f = 25 cm
Power of lens, `p = 100/f ("cm")D`
= `100/25D`
= 4D
APPEARS IN
RELATED QUESTIONS
Calculate the focal length of a corrective lens having power +2D.
What does sign of power (+ve or –ve) indicate?
How unit of power related to focal length of a lens?
Find the focal length of a lens of power −2.0 D. What type of lens is this?
Which type of lens has (a) a positive power, and (b) a negative power?
Which has more power : a thick convex lens or a thin convex lens, made of the same glass? Give reason for your choice.
The optician's prescription for a spectacle lens is marked +0.5 D. What is the:
(a) nature of spectacle lens?
(b) focal length of spectacle lens?
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.
How is the power of a lens related to its focal length?
How is the sign (+ or -) of power of a lens related to its divergent or convergent action?
A symmetric double convex lens in cut in two equal parts by a plane perpendicular to the principal axis. If the power of the original lens was 4 D, the power a cut-lens will be
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.
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?
A convex lens is of focal length 20 cm. Find its power.
Power of a lens is – 4D, then its focal length is
Assertion and reasoning type
- Assertion: Myopia is due to the increase in the converging power of eye lens.
- Reason: Myopia can be corrected with the help of concave lens.
The above lens has a focal length of 10 cm. The object of height 2 mm is placed at a distance of 5 cm from the pole. Find the height of the image.
When monochromatic red light is used instead of blue light in a convex lens, its focal length will ______.
