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The image is real, inverted and smaller than the object.

Correct answer is: 24 cm

The image is real, inverted and of the same size as that or the object.

In this case, the focal length (f) of the lens is 20 cm. 

∴ 2f = 40 cm.

When the object distance is 60 cm (which is greater than 2f), the image will be smaller than the object. When the object distance becomes 40 cm (which is equal to 2f), the image will be of the same size as that of the object.

Correct answer is: -5 D

We have seen a lens in a microscope and a telescope.

Correct answer is: 2.5 D

Correct answer is: 8 D

A lens is a transparent material bound by two surfaces, out of which at least one surface is spherical.

[Note: A lens is normally made of glass or plastic.]

Data: Convex lens, P = +2.5 D, f = ?

P = \(\frac{1}{f}\)

∴ 2.5 D = \(\frac{1}{f}\)

∴ f = \(\frac{1}{2.5D}\) = 0.4 cm = 40 cm

The focal length of the lens = 40 cm.

The combination of the lenses will behave as a concave lens.

The effective power of the combination of the lenses will be + 5 D.

(i) f = -30 cm

(ii) The combination will behave as a concave lens.

f = 20 cm, P = 5 D

i. The focal length of a lens with power 2.5 D is 40 cm (0.4 m).

ii. The power of a lens with focal length 20 cm is 5D.

iii. The minimum distance of distinct vision for a normal human eye is 25 cm.

iv. If two lenses with focal lengths 10 cm and 20 cm respectively are kept in contact with each other, the effective power of the combination is 15 D.

v. A convex lens is used as a simple microscope.

i. Inside water, an air bubble behaves like a concave lens.

ii. (b) \(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\) represents the lens formula.

iii. The power of a convex lens of focal length 25 cm is +4.0 D

iv. A lens does not produce any deviation of a ray of light passing through its optical centre.

v. The image formed by a concave lens is always virtual and erect.

The unit of power of a lens is the dioptre (D). One dioptre is the power of a lens whose focal length is one metre.

1 dioptre (D) = \(\frac{1}{1\,metre\,(m)}\)

[Note: The dioptre, the SI unit of power of a lens, is denoted by D.]

The power of a convex lens is positive while that of a concave lens is negative.

Power of accommodation of the eye: The eye lens is held in its position by the ciliary muscles. When we look at a nearby object, the ciliary muscles compress the eye lens so that it becomes rounded. Hence, the focal length of the eye lens decreases. Therefore, the image is formed on the retina of the eye and hence the nearby object is seen clearly.

When we look at a distant object, the ciliary muscles relax so that the eye lens becomes flat. Hence, the focal length of the eye lens increases. Therefore, the image is formed on the retina of the eye and hence the distant object is seen clearly. This ability of the eye lens to adjust its focal length is called the power of accommodation of the eye.

Problems of vision are related to 

(i) weakening of ciliary muscles 

(ii) change in the size of the eyeball 

(iii) irregularities on the surface of cornea 

(iv) formation of a membrane over the eye lens.

The muscles which hold the eye lens in its position, and bring about changes in the shape (curvature) of the eye lens, and hence of focal length are known as ciliary muscles.

Minimum distance of distinct vision; Though the focal length of the eye lens is adjustable, it cannot be decreased below a certain limit. Hence, if an object is very close to the eye, it cannot be seen clearly. For a normal human eye, the minimum distance from the eye at which an object is clearly visible without stress on the eye, is called the minimum distance of distinct vision. For the normal human eye, it is 25 cm.

The minimum distance from the normal eye, at which an object is clearly visible without stress on the eye is called the minimum distance of distinct vision.

The focal length of the eye lens of a normal eye in relaxed position of eye muscles is about 2 cm.

The process of focusing the eye on objects at different distances is called accommodation. It is brought about by changing the curvature of the f elastic eye lens making it thinner or thicker.

The second focal point of the eye lens of a normal eye in relaxed position of eye muscles lies on the retina.

A telescope is used to observe a distant object such as mountain, moon, planet, star in the magnified form.

Yes. 

The eye is the natural optical device.

Persistence of vision: We see an object when its image is formed on the retina. The image disappears when the object is removed from our sight. But this is not instantaneous and the image remains imprinted on the retina for about \(\frac{1}{16}\) th of a second after the removal of the object.

The sensation on the retina persists for a while. This effect is known as the persistence of vision. It is due to persistence of vision that we continue to see the object in its position for about \(\frac{1}{16}\) th of a second after it is removed.

Example: When a burning stick of incense is moved fast in a circle, a circle of red light is seen.

Microscope, telescope, binoculars, camera, projector.

The working of a television set and motion picture is based on the phenomenon of persistence of vision. 

[Note: These are the examples of persistence of vision in daily life.