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Lens

It is a transparent medium that refracts the light and is limited with two spherical surfaces

and is usually made of glass or plastic

Uses of lenses:

1 – in medical eye glasses ((for reading or walking))

2 – in fixing watches.

3 – In war they use binoculars to follow the battle.

4 – used in manufacture of many things like

Projector, cameras and magnifying lenses.

The types of lenses:

| |1 Convex lens (converging) |Concave lens (diverging) |

|Its structure |It is thick at the centre and less thickness at the tips. |It is thin at its centre and thicker at the tips. |

|Its function |Collect the light rays falling on it |diverge the light rays falling on it |

| |so it is called converging lens |so it is called diverging lens |

| | [pic] [pic] |[pic] [pic] |

Study the previous figure and identify the following concepts:

|Concept |Definition |Figure |

|1-The centre of curvature of the lens|Is the centre of the sphere where this | |

|face (c): |face is a part of it . | |

|2-The optical centre of the lens (p) |Is a point inside the | |

| |lens placed on the principal axis in | |

| |the mid distance between its faces . | |

|3 -The radius |Is half the radius of | |

|of curvature of the face of the lens |the sphere where the face is a part of | |

|(p): | | |

|4-The principal axis |Is the line between the optical centre | |

| |of the lens passing by the optical | |

| |centre of the lens | |

|The secondary axis |It is any line passes by the optical | |

| |center of the lens except the principal| |

| |axis. | |

|The focus of the lens (F) (principal |It is the point of collection of the | |

|focus) |refracted light rays | |

| |it is produced when a beam of parallel | |

| |rays falls parallel t the principal | |

| |axis of a lens. | |

| |It is real in case of the convex lens. | |

| |And virtual in case of concave lens. | |

|The focal length of the lens (f) |It is the distance between the focus | |

| |and optical center of the lens. | |

| | | |

| |F = 1/2 r | |

Note

Each lens has only one principal axis and more than one secondary axes.

The lens has two centers of curvature?

Or … the lens has two foci. ?

■ because it has two circular surfaces.

A: The convex lens

The focus of the convex lens (converging):

If the sun ray or any light from any distant source fall on the lens we notice that ray passing through the lens is collected in one point called “the focus of the lens”.

Activity >>>Determine the focal length of the convex lens

Materials:

A convex lens - screen - lens holder

- distance source of light (can use the sun ray)

The steps

|Steps |Observation |

|1 -Place the lens on a holder where the distance light source is |Parallel rays fall on the lens |

|facing one of its faces. | |

|2 move the screen closer and farther from the lens until you get the|Rays collect in one lit point which call focus of the lens. |

|lit point which is the «focus of the lens». | |

|3-Measure the distance between this point and the optical centre of |This distance = 1/2 radius of curvature of the face of the lens and|

|the lens which is the focal length (f) of the convex lens. |called focal length. |

Conclusion

1 – convex lens is a converging les collect refracted rays.

2 – th point of collection rays is called " the focus of the lens (F)"

3 – the distance between the focus of he lens and its optical center is called " focal length (f)

4 – (f) = 1/2 ( r ) radius of curvature.

The image formed by the convex lens can be received on the screen?

Because it is a real image

A piece of paper can be burned by a convex lens?

Because convex lens can converge and direct sunlight in a point which is burned.

The path of rays falling on the concave lenses

|The path of the incident ray |The path of the emergent ray |Explanation Figure |

|The incident light ray passing through the |pass inside the lens and passes without | |

|optical centre of the lens continues |refraction | |

|The incident light ray parallel to the principal |exits the lens passing through the focus | |

|axis, | | |

|The incident light ray passing through the focus,|exits the lens parallel to the principal | |

| |axis | |

** the position of the images formed and their properties can be determined by using only two rays of the previous three rays.

The cases of the formation of the images by the convex lens (collective):

To determine the position and characteristics of the formed image by the convex lens, follow the following steps:

1-Use the protractor and draw the convex lens

2-Draw the principal axis of the lens (it is a straight line passing by the focus and the optical centre of the lens).

3-Determine on it the position of the focus (f) and twice the focal length (c) on the principal axis from both sides of the lens.

4-Draw a ray coming from the highest point of the object so it falls parallel to the principal axis thus refracts and leaves the lens and passes through the focus

5-Draw a ray from the same point passing by the visual centre of the lens thus leaves with no refraction.

6-To determine the position when two respected of the two penetrating rays determines the image of the lit point.

7-Determine the position and characteristics of the images formed in the six cases shown in the following table and compare the results you obtain to those present in the table.

The Properties of the image formed by the lenses

|Position of the body |Position of the images |Characteristics of the images |cases of image formation |

| | | | |

|More than |Between the focus and twice |Real, inverted, and smaller the | |

|twice the focal length |the focal length |object | |

|At twice the |At twice the focal |Real, inverted and equal to the | |

|focal length |length |object | |

|Between the |At a distance large than |Real, inverted and enlarge | |

|focus and twice |twice the focal length | | |

|the focal length | | | |

|At the focus |No image is |No image is formed | |

| |formed | | |

|At a distance |Forms in front of |Virtual, upright and | |

|smaller than |the lens at the |enlarged | |

|the focal length |object side | | |

Science, technology and Society

The opposite figure shows how to make an activity to identify properties of the image formed by a convex lens by changing the focal length.

B: Concave lens

We can replace convex lens by a concave one in the last activity, the formed image can not be received on the screen, because the incoming rays diverge after refraction:

The properties of the formed image by the concave lens:

|Position of the object |Position of the image |Properties of the image formed. |Image formation |

|In front of the concave lens. |Before the object in the |The image formed by the concave | |

| |same side. |lens is always virtual, smaller | |

| | |and erect. | |

The concave lens is known as the diverging lens?

Because it diverges the rays after they pass through it.

The image formed by the concave lens is a virtual image?

Because it can not be received on a screen.

Applications on lenses:

Lenses are used in many things as :

1 – optical devices as

- Telescopes : use to enlarge images for the heavenly bodies.

- Microscope : use to magnify images of tiny bodies.

2 – use in making medical glasses to treat the vision defects.

The eye works in such way:

- the light rays reflect from the body to fall on the eye.

- The light rays refract when passing through the cornea then the lens (convex lens).

- The refracted rays are collected on the retina forming a small, real inverted image.

Note

The optic nerve transmits the image to the brain which re-corrects the image to become erect and in its natural size.

• the diameter of the eye ball is distance between the optical center of the lens and the retina.

• To see clearly, the image must be formed on the retina.

• The normal person can see clearly near objects at distance not less than 25cm, and for objects at a distance up to 6 meter.

The use of lenses to treat the vision defects

Short-sightedness:

It is a vision defect through which near objects can be seen clearly but far objects seem distorted.

Long-sightedness:

It is a vision defect through which far objects only can be seen clearly but close objects are not seen clearly.

Vision defects occur?

Because the eye cornea is not always convex or the eye is not always spherical.

The convex lens is used to correct the long-sightedness??

To collect the rays coming from the near object, so the image is formed on the retina.

The use of lenses to treat the vision defects

| |1 Short-sightedness |Long – sightedness |

|The images of near objects |Can be seen clearly |Cannot be seen clearly |

|The images of far objects |Cannot be seen clearly |Can be seen clearly |

|The position of the images concerning|In front of the retina |Behind the retina |

|the retina | | |

|4- causes |The diameter of the eyeball is too large |The diameter of the eyeball is too short |

| |Or |Or |

| |The lens is too convex |The lens is thin |

|The correction |By using a concave lens |By using a convex lens |

|short- Long sightedness |[pic] |[pic] |

|Correcting short- Long sightedness |[pic] |[pic] |

Contact lenses:

The contact lenses are used instead of the glasses. It is very thin lenses made of plastic, and can stick to the eye cornea by the eye fluid

Science, Technology, and Society

Cataract ( Science integration )

Cataract is one of the most dangerous diseases that injure the eye as a result of:

- old age,

- illness,

- side effects of drugs

- genetic readiness.

The result of Cataract:

The difficulty of vision as a result of the darkness of the eye lens.

Treatment of cataract:

Exchange the eye lens with plastic lens transplanted permanently in the eye by surgery.

Land measurement:

Land surveyors and topographical scientists use a mirror provided with later ray to determine heights and distances and to make very accurate measurements to calculate the time that a light beam bounced from a distant point and returns to its source.

Questions on lesson:

Complete the following statements:

1 – the convex lens ……………………………. The rays fallen on it.

2 – the focus of the lens is the point of collection of the ………………………….. light rays after ……………….. from the lens.

3 – the ………………… lens is called converging lens, while the ……………………. Lens is called diverging lens.

4 – the focal length of the convex lens equals the distance between ………………………. And …………………..

5 – the radius of the convex lens = ……………………….. of its focal length.

6 – the incident ray that passes through the …………………………. Of the convex lens, it passes in a straight line without ……………………………

7 – if an object is put in front of a convex lens at a distance of 30 cm. and its image is formed behind the lens at a distance of 30 cm, so the focal length of the lens is ………………………….

8 – no image is formed fro the object, when it is at the …………………………. Of the convex lens.

9 – when an object is placed at the focus of a convex lens, the rays penetrate as …………………….

10 – the image formed by the concave lens is always …………………………., …………………. And …………………..

11 – the normal person can see clearly the near objects at a distance not less than ………….. ….and far objects at a distance up to ……………………….

12 – the short-sighted person needs a medical eye glasses with …………………………… lenses.

13 – in short-sightedness, the images of the far objects are formed …………………………. Retina. While in long-sightedness, the images of the near objects are formed ……………………………the retina.

14 – the contact lenses are very ……………………. Lenses made of ………………………. And can stick to the eye …………………………. By the eye fluid.

15 - ……………………….. and ……………………. Are from reasons of cataract disease.

Chose the correct answer:

1 – lenses used in the …………………..

a. cameras b. medical glasses. C. binoculars d. all the pervious things

2 – Concave lens is called …………………. Lens.

a. diverging b. converging c. collecting d. no correct answer.

3 – the imaginary point in the center of the lens which lies on the principal axis is called the ……………

a. real focus, b. secondary axis c. optical center d. center of curvature of lens.

4 – the distance between the focus of the lens and its optical center is called……………………….

a. principal axis, b. secondary axis c. focal length of lens d. radius of curvature.

5 – if a light ray falls on a convex lens and passing through its focus, so it ………………….

a. passes in a straight line without deviation

b. refracts parallel to the principal axis.

c. passes from the optical center in a straight line

d. no correct answer.

6 – when the object is at the center of curvature of the lens ( c ) , the image is formed at a distance …………………………….. the radius of curvature.

a. equals b. greater than c. smaller than d. no correct answer.

7 – the optical piece that forms an equal , inverted image of the body is the …………………..

a. spherical mirror b. plane mirror c. convex lens d. concave lens.

8 – an object is placed at the focus of a convex lens, the image will be ……………………….

a. between the focus and the center of curvature.

b. at the center of curvature.

c. not formed.

d. no correct answer.

9 – pupil's eye contains ………………………..

a. convex lens b. concave lens c. plane mirror d. convex mirror

10 – the images are formed on …………………….. in the eye.

a. retina b. lens c. cornea d. eye pupil

11 – the images that are formed on retina are ………………………

a. erect b. real c. enlarged d. magnified.

12 – vision defects occur because …………………………….

a. the eye cornea is not always convex b. the eye is always spherical

c. the eye is not always spherical d. a and c

13 – a …………………….. is used to treat the short-sightedness.

a. convex lens b. concave lens c. concave mirror d. convex mirror

14 –contact lenses can stick to the eye …………………….. by the eye fluid.

a. retina b. cornea c. lens d. no correct answer.

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1 – Short-sightedness.

The vision

Lenses

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