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The bird appears to be inside the cage. This happens due to persistence of vision.

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.

(1) In nature we field objects of various colours. Perception of colour means to be able to respond to colour.

(2) We can distinguish between various colours due to perception of colour.

(3) The cone-shaped cells on the retina of the eye respond to the various colours when light is bright and communicate to the brain about the colours of the image formed on the retina. This gives us the proper idea about the colours of the object.

(4) If, in the retina of a person, the cone-shaped cells responding to certain specific colours are absent, the person is unable to distinguish between the colours. As a result, he lacks perception of colour.

• A convex lens can be regarded as made of a very large number of portions of triangular prisms. The bases of these prisms are towards the central thicker portion of the lens.
• A ray of light passing through a prism bends towards its base. Hence, a convex lens converges the rays falling on it.

A circle of red light is seen.

1. A concave lens is used to correct myopia (nearsightedness).

2. In some optical instruments, a combination of a concave lens and a convex lens is used.

A convex lens is used 

(1) to read words in small print 

(2) to correct hypermetropia (Far-sightedness)

A lens having both spherical surfaces curved inwards is called a concave lens or double concave lens or biconcave lens. It is thicker at the edges than in the middle.

[Note: A concave lens is also called a diverging lens.]

• A concave lens can be regarded as made of a very large number of portions of triangular prisms. The bases of these prisms are towards the edges of the less, i.e, away from the central thinner portion of the lens.
• A ray of light passing through a prism bends towards its base. Hence, a concave lens diverges the rays of light falling on it.

(1) The retina of the human eye contains a large number of light sensitive cells. 

These cells are of two shapes : (i) rods and (ii) cones.

(2) The cone-shaped cells respond to various colours of light when light is bright.

(3) Thus, the perception of colour is due to the presence of the cone-shaped cells in the retina.

(4) In the retina of some persons, cone-shaped cells responding to certain specific colours are absent.

Hence, these persons are unable to distinguish between certain colours, i.e., they are colour-blind. This defect is known as colour blindness.

The impression of the image on the retina lasts for about \(\frac{1}{16}\) th of a second after the removal of the object. If a burning stick of incense is moved at a rate of more than sixteen revolutions per second, we see a circle of red light due to persistence of vision.

In the retina of some persons, cone-shaped cells responding to certain specific colours are absent. 

Hence, these persons are unable to distinguish between certain colours, i.e., they are colour-blind.

Uses of a compound microscope:

1. It is used to observe blood corpuscles, plant and animals cells, microorganisms like bacteria, etc.

2. It is used in a pathological laboratory to observe blood, urine, etc.

3. It is a part of a travelling microscope used for measurement of very small distance.

(1) The retina in the eye is made of many light sensitive cells. The rod-shaped cells respond to the intensity of light while the cone-shaped cells j respond to various colours.

(2) The cone-shaped cells do not respond to faint light. They function only in bright light. Hence, one can sense colours only in bright, light.

(1) A colour-blind person cannot distinguish between different colours. For example, he cannot distinguish between red and green colours. Also he cannot distinguish between blue and green colours. Red and green, both appear grey. Since a colour blind person cannot distinguish between red and green colours, it is difficult for him to cross a road. There is a possibility of an accident while crossing a road.

(2) A colour-blind person cannot distinguish between two objects of different colours, which are otherwise identical, e.g., clothes.

(3) A colour-blind person may have an inferiority complex and hence may find it difficult to mix with other persons.

Real image:

1. A real image is formed when the light rays starting from an object meet after reflection or refraction.

2. It can be projected on a screen.

3. It is inverted with respect to the object.

Virtual image:

1. A virtual image is formed when the light rays starting from an object (when extended backward) appear to meet after reflection or refraction. 

2. It cannot be projected on a screen. 

3. It is erect with respect to the object.

1. Concave lenses are used for proper working of medical equipment, scanner, CD player – the instruments that employ laser rays.

2. One or more concave lenses are used in a small safety device, fitted in the peep hole in a door, due to which we can see a large area outside the door.

3. Concave lenses are used in spectacles to correct nearsightedness (myopia).

4. A concave lens is used to spread light emitted by the small bulb in a torch over a wide area.

5. A concave lens is used in front of the eyepiece or inside the eyepiece fitted in a camera, telescope and microscope – the instruments employing convex lenses.

Convex lenses are used in a simple microscope, compound microscope, refracting telescope, camera, projector, spectroscope, spectacles for correcting farsightedness (hypermetropia) and binoculars.

To observe the object with a compound microscope, the distance between the object and objective lens is adjusted.

The working of a lens is similar to that of a triangular prism. When a ray of light passes through a lens, it is refracted twice: When entering the lens and when emerging from the lens. There is a change 5 in its direction of propagation every time and as both the changes occur in the same sense, the direction of propagation of the emergent ray is different from that of the incident ray.

(1) The cone-shaped cells in the retina of a person respond to colours. This makes the perception of colours possible.

(2) In the retina of colour-blind persons, coneshaped cells responding to certain specific colours are absent. 

Hence, they are unable to distinguish between different colours.

A colour-blind person cannot distinguish between different colours. If a driver is colour-blind, he will not be able to distinguish between the colours of the signal and the colours on different sign boards. This will lead to an accident. Hence, it is risky to issue a driving license to a person suffering from colour-blindness.

(1) Some people, in old age, suffer from myopia (nearsightedness) as well as hypermetropia (farsightedness).

(2) Myopia is corrected using a concave lens of appropriate power. Hypermetropia is corrected using a convex lens of appropriate power. Therefore, they need a bifocal lens.

In old age, people usually suffer from both myopia and hypermetropia. Therefore, they need spectacles having bifocal lenses.

The upper part of a bifocal lens is a concave lens to correct myopia. The lower part of a bifocal lens is a convex lens to correct hypermetropia.

The tendency of the pupil in the human eye to adjust the opening for light, depending on the intensity of incident light, to control and regulate the amount of light entering the eye is called adaptation.

Simple microscope:

1. In a simple microscope, only one convex lens is used. 

2. In this case, the object is placed within the focal length of the convex lens. 

3. In this case, the image is erect. 

4. It is used to observe minute parts of a watch, to read words in small print, etc.

Astronomical refracting telescope:

1. In an astronomical refracting telescope, two convex lenses, objective lens and eyepiece are used. 

2. In this case, the object is far away from the objective lens. 

3. In this case, the image is inverted. 

4. It is used to observe satellites, planets, stars, etc.

Compound microscope:

1. In a compound microscope, the focal length and cross section of the objective lens are respectively smaller than the focal length and cross section of the eyepiece. 

2. In this case, to observe the object, the distance between the object and the objective lens is adjusted. 

3. It forms a magnified image of a small object. 

4. It is used to observe blood corpuscles, plant and animal cells, etc.

Astronomical refracting telescope:

1. In an astronomical refracting telescope, the focal length and cross section of the objective lens are respectively greater than the focal length and cross section of the eyepiece.

2. In this case, to observe the object, the distance between the objective lens and eyepiece is adjusted.

3. It forms a near image of a distant object.

4. It is used to observe sateulites, planets, stars, etc.

A transparent biconvex crystalline lens is located just behind the pupil in the human eye.

The pupil in the human eye is useful for controlling and regulating the amount of light entering the eye. The pupil contracts in the presence of too much light and dilates when light is insufficient, thus changing the amount of light entering the eye.

The pupil is a small circular opening of changing diameter at the centre of the iris in the human eye.

(1) Magnification: \(\frac{h_2}{h_1}\)

(2) Power of a lens: \(\frac{1}{f}\)

(3) Focal length: f

(4) Distance of an object from a lens: u.

(1) The less the distance between the screen in a cinema hall and the person watching the movie, the more is the intensity of light falling on the eye.

(2) This results in great contraction of the pupil of the eye causing a strain. Hence, you cannot enjoy watching a movie from a very short distance from the screen in a cinema hall.

(i) Convex lenses are used in the spectacles used by Surabhi.

(ii) Surabhi is suffering from hypermetropia (farsightedness).

(iii) Focal length of the lenses used in her spectacles = \(\frac{1}{power\,of\,the\,lens}=\frac{1}{0.5\,D}\) = 2 m.

i. Concave lenses are used in the spectacles used by Aniket.

ii. Aniket is suffering from myopia (nearsightedness).

iii. Focal length of the lenses used in his spectacles = \(\frac{1}{power\,of\,the\,lens}=\frac{1}{-0.5\,D}\)

= -2 m (Concave lens ∴ Minus sign)

(1) Convex lens – Presbyopia 

(2) Astronomical telescope – To observe astronomical objects such as stars, planets, etc. 

(3) Compound microscope – To observe minute objects 

(4) Simple microscope – To see small objects clearly.

(1) Conical cells – Colour of an image

(2) Rod like cells – Intensity of light

(3) Pupil – Aperture 

(4) Cornea – Transparent.

(1) The retina of the human eye contains a large number of light sensitive cells. 

These cells are of two shapes : (i) rods and (ii) cones.

(2) The rod-like cells respond to the intensity of light.

(3) The conical cells respond to various colours of light. They respond differently to red, green and blue colours. They do not respond to faint light.

(1) A person suffering from hypermetropia can see distant objects clearly as the image of a distant object is formed on the retina, but cannot see nearby objects clearly as the image of a nearby object would be formed behind the retina instead of on the retina.

(2) A convex lens converges the rays of light passing through it. When spectacles of convex lenses of appropriate power are used, the rays of light coming from a nearby object are converged to proper extent before they are incident on the eye lens. Therefore after the converging action of the j eye lens, the image of a nearby object is formed on j the retina of the eye and hence the nearby object | can be seen clearly.

(1) Nearsightedness – Image in front of the retina

(2) Farsightedness – Image behind the retina

(3) Presbyopia – Ciliary muscles become weak

Presbyopia is the defect of vision in which aged people find it difficult to see nearby objects comfortably and clearly without spectacles

Reason of presbyopia: The power of accommodation of eye usually decreases with ageing. The muscles near the eye lens lose their ability to change the focal length of the lens. Therefore, the near point of the eye lens shifts farther from the eye.

This defect is corrected using a convex lens of appropriate power. The lens converges light rays before they fall on the eye lens such that the action of the eye lens forms the image on the retina.

i. False. 

(A convex lens is used as a magnifying glass.)

ii. True.

iii. True.

iv. True.

i. Flattened eyeball

ii. Diverging

iii. Image on the opposite side at infinity

iv. Inverted image

v. Negative power of the lens.

vi. Magnification.

vii. Dioptre.

viii. Eye lens

ix. Convex lens

x. Image behind the retina

xi. Simple microscope

xii. Refraction, dispersion and internal reflection of light.

i. False. 

(A convex lens forms a real image or a virtual image depending on the object distance.)

ii. True.

iii. True.

iv. False. 

(The magnification produced by a concave lens is always positive.)

v. True.

Farsightedness:

1. In hypermetropia, a human eye can see distant distinctly but is unable to see nearby objects clearly.

2. In this case, the image of a nearby object would be formed behind the retina.

3. This defect can be corrected using a convex lens of appropriate power.

Nearsightedness:

1. In myopia, a human eye can see near objects distinctly, but is unable to see distant objects clearly.

2. In this case, the image of a distant object is formed in front of the retina.

3. This defect can be corrected using a concave lens of appropriate power.

i. Myopia. It is a defect of vision; others are instruments.

ii. Spectrometer. It is an instrument; others are defects of vision.

iii. Retina. It is a part of the eye; others are defects of vision.

iv. Kaleidoscope. Others are optical instruments.

v. Colour-blindness. Others are examples of persistence of vision.

vi. Rainbow. Others are celestial bodies.

i. True.

ii. True.

iii. False. 

(A concave lens is a diverging lens.)

iv. False. 

(A convex lens is a converging lens.)

v. True.

A concave lens is used to correct myopia.

A convex lens can be used to burn paper in sunlight at noon.

Correct answer is: u = -10 cm

The image is virtual, erect and larger than the object.