Science 2nd prep 2nd term unit 2 lesson 4 part 2

Barrel changes its path as in figure when you push it from asphalt to sand then to asphalt.

Similarly, as light travels from transparent medium like air to another medium like (glass) , its path changes due the difference of light velocity through different transparent media. And this phenomenon is known as ((Light refraction)).

Light refraction: It is the change of light path when it travels from a transparent medium to another transparent medium of different optical density.

**The ability of the transparent medium to refract the light is called the ((optical density of the medium)) .

Optical density of the medium It is the ability of the transparent medium to refract the light .

Note : **each medium has its own optical density .

**as optical density increases the speed of the light through it decreases and vice versa.

Activity >>> to show the light refraction .

Materaials and tools : - thick rectangular glass block .- white paper sheet - pencil - ruler - protractor - laser pen .

Steps :

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Light refraction

Science 2nd prep 2nd term unit 2 lesson 4 part 2 1 – draw rectangular glass block on the white paper sheet and mark around th1

block using pencil .

2 – direct a ray from laser pen to point of incidence A on the side of rectangular glass and draw its path to represent he incident ray .

3 – draw the path of the emergent ray from point B on opposite side of glass .

4 – remove rectangular glass and join the two points A and B with straight line which represent refracted ray .

5 – draw at A and B dotted vertical lines , where each line represents the normal at the point of incidence on the interface.

Observation : -when the light ray travels form air into glass or vice versa , it refracts . -the angle of incidence (60) is not equal to the angle of refraction (34.5)

-the angle of incidence (60) is equal to the angle of emergence (60) -the incident light ray is parallel to the emergent light ray .

Conclusion : The light refraction phenomenon occurs when the light ray travels from a

transparent medium to another transparent medium of different optical density .

Angle of incidence It is the angle between the incident light ray and the normal at the point of incidence on the interface.

Angle of refraction : It is the angle between the refracted light ray and the normal at the point of incidence on the interface.

Angle of of emergence : It is the angle between the emergent light ray and the normal at the point of emergence on the interface.

What is meant by ?The angle of emergence in a prism is 50

This means that the angle between the emergent ray and the line perpendicular to the interface at the point of emergence is 50º

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Concepts related to light refraction

Laws of light refraction

Science 2nd prep 2nd term unit 2 lesson 4 part 2

Facts about light refraction :

1 –when light ray travels from transparent medium of lower optical density to another of higher optical density , it refracts near the normal.

***The angle of incidence is greater than the angle of

refraction.

2 –when light ray travels from a transparent medium of higher optical density to another of lower optical density , it refracts far from normal .

***the angle of incidence is less than the angle of refraction .

3 –when a light rays falls perpendicular to the interface between two different transparent media it passes to the other medium without refraction .

***angle of incidence = zero.

***we can deduce that the light refraction depends on optical density of the medium .

**there is a relation between velocity of light through air and any other transparent medium which is known as ((Absolute refractive index)).

Absolute refractive index of a medium : It is the ratio between velocity of light through air to the velocity of light

through another transparent medium .Absolute refractive index = velocity of light through air / velocity of light through the medium

Note : if the refractive index of a medium is high , this means that its optical density is high and vice versa .

The absolute refractive index of any transparent medium is always greater than one ?

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Science 2nd prep 2nd term unit 2 lesson 4 part 2 Because the velocity of light through air is always greater than that through any other transparent medium.

What is meant by: the absolute refractive index of glass is 1.5 ?This means that the ratio of velocity of light through air to that through

glass is 1.5.

Enrichment information : The ratio between the absolute refractive index of medium to the absolute refractive index of another medium is known as the relative refractive index.

Problems : If the velocity of light through water is 2.25 ×108 m/s , calculate the absolute refractive index of water, knowing that the velocity of light through air is 3 ×108

m/s.

Solution :

Absolute refraction of water = velocity of light in air / velocity of light in water .

=3 ×108 / 2.25 ×108 = 1.33

Activity >>> to identify the concepts of the critcal angle and total internal reflection .

Materails and tools : A source of light

A trough filled with water .

Steps ; -fix the source of light near the base of the trough (which filled with water) at

one of its sides to be able to control the angle of incidence of light ray.

-what do you observe when the angle of incidence of light ray is:

- -less than 48.6 - -equal to 48.6

- -more than 48.6

Observation :

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Critical angle and total internal reflection:

Science 2nd prep 2nd term unit 2 lesson 4 part 2 By increasing the angle of incidence gradually, the angle of refraction increases

too .

Explanation : -when angle of incidence is less than 48.6 , the light ray is refracted away from

the normal .

-when the angle of incidence becomes 48.6 , the light ray refracts tangent to the separating surface and the angle of incidence is known as ((critical angle)) .

-when the angle of incidence is more than 48.6 (exceeds the ciritical angle) , the light ray reflects instead of being refracted .

Note >>> Critical angle of water is 48.6 º

Conclusion : -when the angle of refraction in the least optical dense medium (air) is 90 , the

angle of incidence in the most optical dense medium (water ) is called ((Critical angle)) of this medium.

-when the light ray is incident in more optical dense medium with an angle greater than critical angle of that medium , it reflects in the same medium which is known as ((total internal reflection)),

Critical angle : It is the angle of incidence of light ray which travels from high optical dense medium to lower one which results in it being refracted at 90º to the normal.

Total internal reflection : It is the return of a light ray when it incident in a medium of larger optical dense by an angle larger than the critical angle of this medium.

Note >>> optical fibers transmit light by total internal reflection.

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Science 2nd prep 2nd term unit 2 lesson 4 part 2

We can summarize the previous activity in the following figure :

A pencil which is partially immersed in water appears as being broken !!

Due to the refraction of light rays coming from immersed part in water .

Submerged object in water is seen in an apparent position slightly above its real position.Due to refraction of light rays coming from the object (away from the normal) where, the eye sees the extensions of these refracted rays.

Examples : - Fish that is put in a glass basin seems at a position higher than its real position .

-Bottom of swimming pool filled with water seems higher than its true position.

To pick up a coin which has fallen in deep beaker, we must look at it vertically ?

Because incident light ray perpendicular to interface between air and water passes without refraction, so we can see it in its real position.

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Natural phenomena related to reflection and refraction of light

1 – stcejbo fo sepahs tnerappA:

2 –stcejbo fo snoitisop tnerappA :

3 - egariM

Science 2nd prep 2nd term unit 2 lesson 4 part 2

It is a natural phenomenon that takes places on the desert roads at noon especially in summer times. Where objects on road sides seems as if they had inverted images on a wet area.

Reasons for the occurrence of mirage phenomenon :

-the air above the desert road is heated at noon which leads to formation of different air layers of different temperature by convection currents .

-when an object (as tree) exists on the road side, the rays coming from it refract several times going form higher cold layers to lower hot layers .

-at hottest air layer (which above the earth surface directly) , a total internal reflection

happens to the rays coming from object .SO, this layer appears as watery area and a virtual inverted image is formed below the object.

Enrichment information:A group of refraction and reflections of sun rays occurs when the sun ray is incident on water drops or snow crystals suspended in air leading to analyzing the light into a bow of spectrum colors known as rainbow.

Real life application: project of soap bubbles toy :

- you can make soap bubbles toy which can form colors of spectrum when light is incident on it.- this happens by adding an amount of liquid soap to water and drops of glycerin to increase the duration of bubbles existence in air.

Questions on lesson

Complete the following statements : 1 – the optical density of a medium differs from one medium to another due to

the change in in the …………………………………….. through such medium .

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Science 2nd prep 2nd term unit 2 lesson 4 part 2 2 – light …………………………….. is the change of light path when it travels from a

transparent medium to another transparent medium of different .……………………………

3 – the angle of …………………..is the angle between the refracted light ray and the ………………….. at the point of incidence n the separating surface .

4 – if angle of incidence is more than the angle of refraction, this means that the light ray travels from a medium of …………………………. Optical density to another of …………………….. optical density.

5 – when a light ray travels form water or glass to air , the angle of ……………………….. is greater than the angle of …………………………………

6 -when light ray falls perpendicular to the interface between two transparent media different in optical densities, it ……………………….. to the other medium

without……………………………

7 – the absolute refractive index of the medium is the ratio between ………………. To..………………

8 – from the natural phenomena that are related to the reflection and refraction of light are ……………………………….., ……………………………………. and ..………………………………………

9 – when we look at a fish in trough filled with water , its ……………………….position appears t be lower than the ………………………… position .

10 …………………………………… - is a natural phenomenon that takes place on desert roads at noon in summer days due to light refraction.

Choose the correct answer : 1 – the ability of the transparent medium to refract the light is called the

………………………. Of the mediuma. refractive index b. density c. optical density d. viscosity .

2 – the angle between refracted light ray and the normal at the point of incidence on the separating surface is…………………………………

a. angle of reflection b. angle of refraction .c. angle of incidence d. no correct answer.

3 – angle between emergent light ray and the normal at the point of emergence on the interface is called the angle of .……………………………

a. incidence b. reflection c. refraction d. emergence .

4 – light refraction is due to the difference in ………………….. through different media .

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Science 2nd prep 2nd term unit 2 lesson 4 part 2 a. sound intensity b. nature of surfaces

c. light velocity d. all the previous answers .

5 – the angle of incidence is greater than the angle of refraction when a light ray travels from .…………………………………

a. air to water b. air to glass .c. water to air d. a and b are correct .

6– when a light ray travels from air to water , it ..………………………………… a. refracts near the normal b. refracts far from the normal c. passes without refraction. d. reflects .

7 – the absolute refractive index of any material is always ……………………………

a. more than one b. less than one c. equal one .

8 – a pencil seems broken when it is placed in a glass cup of water due to the ……………………. Of light .

a. reflection b. refraction c. total internal reflection d. no correct answer.

The critical angle lies in the medium of the ………………………… optical dense. 9a. more b. less c. same d. no correct answer.

10 – from the natural phenomena that is resulted from total internal reflection of light is .………………………………

a. echo b. mirage c. seeing objects higher than its normal position. d. no correct answer.

Write scientific term : 1 – Changing the path of light when travels from a transparent medium to

another transparent medium of different optical density ..……………………………………………

2 – the ratio between velocity of light though air to the velocity of light through another transparent medium .

……………………………………………………

3 – the incident angle of light ray which travels from high optical dense medium to lower one when angle of refraction equals 90º

………………………………………………………

4 – a phenomenon that appears in the desert as a result of total internal reflection.

..…………………………………………………………

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Science 2nd prep 2nd term unit 2 lesson 4 part 2 Give reasons for :

1 – the pencil which is partially immersed in water appears broken ?…………………………………………………………………………………………………………………………………………………

2 – the floor of swimming pool appears higher than its real position ?……………………………………………………………………………………………….………………………………………………

3 – sometimes, when a light rays incident in a transparent medium, it refracts tangent to the separating surface ?

………………………………………………………………………………………………………………………………………………

4 – occurrence of mirage phenomenon in desert regions at noon?………………………………………………………………………………………………………………………………………………

Show by drawing : 1 – The path of light ray falling perpendicular to the interface between two

transparent media of different optical densities ?

2 – The path of a light ray that is incident from water to air with an angle greater than the critical angle of water ?

3 – The path of rays by which Soha sees the image of coin which is put a basin containing water (from one side) ?

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