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State power of a lens S.I. unit.

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#### Solution

The S.I. unit of power is diopter (D).

#### RELATED QUESTIONS

Calculate the focal length of a corrective lens having power +2D.

Calculate the focal length of a corrective lens having power +2.5 D.

Define the power of a lens.

How unit of power related to focal length of a lens?

Define 1 dioptre of power of a lens.

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

What is the power of a convex lens of focal length 0.5 m?

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?

Fill in the following blank with suitable words :

The reciprocal of the focal length in metres gives you the _________ of the lens, which is measured in ___________.

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For converging lenses, the power is __________ while for diverging lenses, the power is ___________.

A convex lens of power 5 D and a concave lens of power 7.5 D are placed in contact with each other. What is the :

(a) power of this combination of lenses?

(b) focal length of this combination of lenses?

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*.

What do you mean by focal plane of a lens?

State the condition for the following a lens has both its focal lengths equal .

State the condition for the following a ray passes undeviated through the lens .

**Complete the following sentence:**

If half part of a convex lens is covered, the focal length ……………… change, but the intensity of image ……………….

How is the power of a lens related to its focal length?

The power of a lens is + 1.0 D is :

Name the type of lens whose power is positive.

The image of an object formed by a lens is real, inverted and of the same size as the object. If the image is at a distance of 40 cm from the lens, what is the nature and power of the lens ? Draw ray diagram to justify your answer.

How does focal length of a lens change when red light incident on it is replaced by violet light? Give reason for your answer.

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 normal eye is not able to see objects closer than 25 cm because

A thin converging lens is formed with one surface convex and the other plane. Does the position of image depend on whether the convex surface or the plane surface faces the object?

A double convex lens has two surfaces of equal radii *R* and refractive index \[m = 1 \cdot 5\]

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 symmetric double convex lens is cut in two equal parts by a plane containing the principal axis. If the power of the original lens was 4 D, the power of a divided lens will be

Consider three converging lenses L_{1}, L_{2} and L_{3} having identical geometrical construction. The index of refraction of L_{1} and L_{2} are \[\mu_1 \text{ and } \mu_2\] respectively. The upper half of the lens L_{3} has a refractive index \[\mu_1\] and the lower half has \[\mu_2\] following figure . A point object O is imaged at O_{1} by the lens L_{1} and at O_{2} by the lens L_{2}placed in same position. If L_{3} is placed at the same place,

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(b) there will be an image at O_{2}.

(c) the only image will form somewhere between O_{1} and O_{2}

(d) the only image will form away from O_{2}.

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

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 pin of length 2.00 cm is placed perpendicular to the principal axis of a converging lens. An inverted image of size 1.00 cm is formed at a distance of 40.0 cm from the pin. Find the focal length of the lens and its distance from the pin.

A convex lens produces a double size real image when an object is placed at a distance of 18 cm from it. Where should the object be placed to produce a triple size real image?

A 5.0 diopter lens forms a virtual image which is 4 times the object placed perpendicularly on the principal axis of the lens. Find the distance of the object from the lens.

Consider the situation described in the previous problem. Where should a point source be placed on the principal axis so that the two images form at the same place?

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.

How is accommodation produced?

Which lens has more power: a thick lens or a thin lens?

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.

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.

Focal length : metre : : power of lens : _______

The power of the magnifying glass depends on the distance of the magnifying glass from object.

Find the power of a convex lens of focal length of + 25 cm.

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

According to Rayleigh’s scattering law, the amount of scattering of light is inversely proportional to the fourth power of its ______.

The power of lens depends on the focal length of the lens

Increase in the converging power of eye lens cause ‘hypermetropia'

**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 |

**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.

What is power of accommodation of eye?

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?

A convex lens with radii of curvature R_{1} = R_{2} is immersed in water. Assuming that the refractive indices of glass and water are 3/2 and 4/3 respectively, its focal length f_{1} in comparison to that in air, f, is ______.

An object is placed in front of a converging lens. Obtain the conditions under which the magnification produced by the lens is

- negative and
- positive.

A point object is placed at O in front of a glass sphere as shown in figure.

Show the formation of the image by the sphere.

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.

When monochromatic red light is used instead of blue light in a convex lens, its focal length will ______.

The power of convex lens of focal length 20 cm is ______.