Hilmadani's Blog

The speed of light in the air and in others me

dium is different. It is caused by the difference of the

particle density which builds it. Because of the diffe-

rence of the density, the light undergoes a deflection-

of direction and a change of speed. So, when light pas

ses through a boundary plane of two mediums with different density, the light is transmitted with direction that has been changed.

Light refraction is the phenomenon of ben-

ding of light that spreads from one medium to another medium which density are different.

Based on the some phenomenons can be con

cluded that :

1. Incident ray from spacious medium to dense medi- um is refracted approaching normal line.

2. Incident ray from dense medium to spacious medium is refracted moving away the normal line.

1. A normal line is a perpendicular

at the boundary plane where that

ray falls.

2. Incident angle of ray is an angle

formed by the beam incident ray

with a normal line.

3. Refracted angle of ray is an angle

formed by the beam of refracted

ray with a normal line.

Pay-attention the explanation below :

incident

ray i air

Boundary Plane

glass

r reflected

ray

According to Cristian Huygens refractive in- dex (absolute refractive index) of matter is defined as follows :

Refractive index (η) of a matter is ratio between light velocity in vacuum to light velocity in that matter.

η =

Refractive index is stated with :

in which : η = refractive index of medium

C = light velocity in vacuum is 3x10⁸ m/s

C_n= light velocity in medium

Example :

A stream of sunlight spreads from the air to the housing window pane (refractive index of glass = 3/2). What is the light velocity in the glass ?

In the discussion before we know that light –

spreads in the form of electromagnetic wave. In wave there is relationship between velocity,wavelength and frequency, namely C = λ f. From this equation if rela- ted with η =

will obtained relationship

be- cause light frequency is constant when passing through one medium to another medium, then f = f_n , so

η =

In which :

λ = light wavelength in the air

λ_n= light wavelength in the medium

Example :

Red light spreads from the air to the water. If refractive index of water = 4/3 and wavelength of red light in air = 6.300 Ǻ. What is the wavelength of the redlight in the water ?

( Ǻ is read angstrom, the value of 1 Ǻ = 10^{-10 m ).}

The value of refractive index of a medium shows den- sity of the medium. The larger refractive index of me- dium then the larger also medium density. On the con trary, the smaller refractive index of medium,then the smaller also medium density.

Examples of the light refraction in daily life :

1. A straight straw if immers its half into water in the glass, it will be seen the straw seems bending in the water.

2. A fish in pool appears by our eyes is not in its real position.

3. A pool that contains clear and silent water is seen shallower than the real. Etc.

Several optical things that can make light refraction are :

planparallel glass, prism, and lens.

Willebrod Snell, a mathematic expert, through his series of experiments found out the relation between the incident ray and the refracted ray, which later known as Snellius Law, i.e. :

1. The incident ray, the normal line and the refracted ray lie on one flat plane.

2. The proportion of the projection of the incident ray and the refracted ray at a boundary plane between two mediums is a constant number. That constant number is defined as a relative refraction index.

Explantion pay-attention the statements below :

A’

Boundary Plane

B’

Medium

Based on Snellius Law, the refraction index of a medium can be found from the proportion of the pro- jection of incident ray and refracted ray at the bounda ry plane of both medium, as seen at figure above.

The cutting point of the incident ray and refracted ray at the boundary plane of the medium becomes the cir- cle center point (O). If the cutting point of the circle side of incident ray (A) and refracted ray (B) are projected to the boundary plane, then A’O is the projection of in cident ray (AO) and B’O is the projection of refracted ray (BO). Thus, the refraction index of its medium(η) can be formulated as follow :

η =

- 2 -

Can you prove from the figure that

Example :

1. Draw light refraction from the air to glass medium, if given glass refractive index is

2. Draw light refraction from the air to another medi um which refractive index is

The value of medium refraction index shows the den- sity of the medium. To make us more understand about the direction of the refracted ray when it passes through the boundary plane between two mediums, we use the properties of the refracted ray, i.e. as follows :

1. The incident ray from less dense medium to denser medium will be refracted close to the normal line.Thus

the refracted angle (r) < the incident angle (i)(Figure1)

2. At figure 2, the incident ray, which is perpendicu- lar with the boundary plane, undergoes no directi- on changes ( it is not refracted, but only transmitted).

3. The incident ray comes from a denser medium to a less dense medium will be refracted away from the normal line. Thus, the refracted angle (r) will be larger than the incident angle (i) ( Figure 3 ).

Pay-attention the figure below !

Figure 1 : Figure 2 : Figure 3 :

N N N

i less dense medium r

denser medium

r i

The light refraction come from a denser medi-

um to a less dense medium produces a larger angle of refracted ray (r) than the angle of incident ray (i).

Thus if the angle of incident ray increases, then the angle-

of refracted ray will be larger too, or getting closer to the boundary plane as shown in the figure about critical angle and a total internal reflaction below:

N N N N

Less dense

i > θ

i = θ

medium

Denser

medium

light source

A critical angle(θ) is the incident angle(i) when the reflected angle(r) formed is 90⁰,or when the reflec ted ray parallels to the boundary plane.

If the incident angle is larger than the critical angle, then a total internal reflection will occur.The e- xamples of total reflection phenomenon in daily life are fatamorgana, the glowing of diamond and fiber optics.

The penetrated dark object, which can transmit almost all the light that hit it, is called optical object.

Several optical objects that can make light refrac- tion are plan-parallel, glass, prism, and lens.

1. Plan-parallel Glass.

Plan-parallel glass is a thick glass which surface is flat. The incident ray passes through a plan-parallel galss will undergo two refractions. The first refraction is when the incident ray heading for the plan-parallel glass and the second refraction is when the ray leaving the plan-parallel glass. Pay-attention the figure below !

air i

glass

i’

r’

The incident ray come from air to glass is refracted close to normal line inside the glass. Then,the ray that propagates from inside the glass into the air is refrac- ted away from normal line. The direction of incident ray heading toward the plan-parallel glass and the direction of the exit ray from the plan-paralellel glass are parallel.

Thus, based on the figure above it can be concluded that :

i = r’ and r = i’

Where :

i = the angle of incident ray ( from the air to the plan-parallel glass ).

r = the angle of refracted ray ( from the air to the plan-parallel glass ).

i’ = the angle of incident ray ( from the plan-parallel glass into the air ).

r’ = the angle of refracted ray (from the plan-parallel glass into the air).

2. Prism.

Prism is a medium limited by two surface pla- nes which form an angle to each other. The formed angle is called a refracted prism angle. It is symbolized by β.

The incident ray heads for the prism and the ray leaves from the prism are not parallel. It means that any a deflection or a deviation. The magnification of the de flection angle is called the deviation angle.

Look at the figure below ! a beam of light hits the sur face of a prism. The light undergoes two refractions.

The direction and angles of the ray are drawn infigure below !

N N

r’

ⁱ r i’

Explanation of the figure :

β = the angle of refracted of prism

i = the angle incident ray ( air to prism ).

r = the angle of refracted ray ( air to prism ).

i’ = the angle of incident ray ( prism to air ).

r’ = the angle refracted ray ( prism to air ).

δ = the deviation angle.

Based on the figure above, we can obtain that the relation between the deviation angle (δ), the inci- dent ray angle (i), the refracted ray angle (r’), and the refracted prism angle (β) is :

- 3 -

δ = i + r’ -β

In other word, if the refracted ray angle (r’) cuts the prism into isosceles triangle, then the value of the deviation angle becomes minimum. Therefore, it is called a minimum deviation (δ_min). When minimum deviation occurs, the incident angle (i) equals to the refracted angle (r’). Thus, the deviation angle is formulated as follows:

δ_min = 2i – β or δ_min = 2r’ - β

Example :

A stream of ray falls to prism with incident angle of 30⁰ and comes out from prism with refracted angle of 45⁰. If angle of prism refractor is 40⁰, what is the magnitude of deviation angle ? 35⁰

The Happening of the rainbow.

The rainbow happens because there is separa- tion of white light from the sun to be colour series or spektrum by grains of the rain. The rainbow can be seen when you are overshadow the sun and the rain happens infront of you. The rainbow is colourful light radiation that spreads out in the sky. The colour of rainbow are conventionally is said to be red, orange, yellow, green blue, indigo and violet.

Polychromatic light is a light which consists of all kinds of colours, for example :white light.

Monochromatic light is a light which has one wave- length (cannot hang loosely to other lights), for exam- ple : red ray, orange ray, yellow ray and so on.

Light Dispersion is decomposition of polychromatic to be monochromatic light.

3. Lens.

Lens is a transparent object that bounded by 2 bending planes or a bending plane with a flat plane.

Lens if hit by the light, will change direction of the light that strikes it through the event of refraction. Lens con sist of two kinds, namely convex lens and concave lens.

a. Convex Lens.

Convex Lens is a lens that its middle part is thicker than the edge. The convex lens consist of seve ral shapes, namely :

1. Biconvex or Convex – Convex Lens

2. Plankonvex or Convex – Flat Lens

3. Concave – Convex Lens

Pay-attention the figure of shapes of convex lens below !

In lens, a ray can come from two directions, so in the lens there are two focus points. First, focus point isfound infront of the lens (part of lens as the place of incident ray).

Second, focal point is found behind the lens ( part of lens as the place of the ray is refracted ).

Focal point which is infront of the lens is called virtual focal point, while focal point that lies behind the lens is called real focal point.

Pay-attention the figure below !

Infront Back Back Infront

● ● ● ●

F₂ F₁ F₁ F₂

Incident rays that parallel to main axis will be reflected to one point on lens axis called focal point.

Because the incident rays that passing through the convex lens always refracted to one point then the convex lens is called also convergent lens (lens that property is collecting)

Besides, focal point the place of intersection of refrac ted rays always reside in the convex lens backside, hence focus of convex lens is real focus, so focus distance of convex lens is always positive signed. Therefore convex lens is called also positive lens.

Particular rays in convex lens :

1. Incident ray parallel to main axis is refracted through active focus F₁.

Pay-attention the figure below !

2. Incident ray through focal point is refracted paral- lel to main axis.

Pay-attention the figure below !

3. Incident ray through lens center point O is conti – nued without experiencing refraction.

Pay-attention the figure below !

Shape and properties of image are formed by a convex lens can be drawn with the aid of those three par- ticular rays above. Based on location of the object, shape of image and its properties is differetiated as follows :

1. An object is located further than center point of lens curvature (P₂). The image of the object is lo – cated between F₁ and P₁ , forms real image, inver- sed, and minimized.

Pay-attention the figure below !

- 4 -

2. An object is located on center point of lens curva- ture (P₂). The image of the object is located on P₁ forms real image, inversed and has the same size.

Pay-attention the figure below !

3. Object is located between F₂ and P₂. The image of the object is located behind P₁, forms real image, inversed, and enlarged.

Pay-attention the figure below !

4. An object is located between F₂ and O. The image of the object is located between F₂ and P₂ , forms virtual image, upright, and enlarged.

5. Object is located on focal point (F₂)Be not formed image, because there is no intersection particular rays.

( The refracted rays are parallel )

Pay-attention the figure below !

In everyday life a convex lens is often used to help human need, such as glasses lens, camera,micros cope, Magnifying Glass (Loupe), and projector.

b. Concave Lens.

The inverse of convex lens is concave lens.

Concave lens is a lens that its middle part is thinner than its edge. The concave lens is also consist of several shapes namely : - Biconcave or Concave-Concave.

- Planconcave or concave – flat.

- Convex – Concave.

In the concave lens, incident rays parallel to main axis will be refracted spreading seemed comes from one point. Means the concave lens has property spreading light that strikes it, so the concave lens is called also Divergent Lens.

Pay-attention the figure below !

The concave lens has active focal point (F₁), that is located infront and passive focal point (F₂) that is located behind the lens. So focus distance of the concave lens (f) always negative marked. Therefore, the concave lens is called also Negative Lens.

The particular rays of the concave lens :

1. Incident ray parallel to main axis is refracted seemed comes from active focal point F₁.

Pay-attention the figure below !

2. Incident ray seemed goes to passive focal point F₂ is refracted parallel to main axis.

Pay-atttention the figure below !

3. Incident ray that goes to optic center point of lens is continued without refraction.

Pay-attention the figure below !

Shapes and properties of image of the object that is formed by the concave lens can be drawn by the aid of those three particular rays above as follows:

1. The object is located infront of P₁.

The image formed is virtual, upright, minimized, and located between O and F₁.

Pay-attention the figure below !

2. The object is located between P₁ and F₁

The image formed is virtual, upright, minimized, and located between F₁ and O.

Pay-attention the figure below !

3. The object is located between F₁ and O

The image formes is virtual, upright, minimized, and located between F₁ and O.

Pay-atttention the figure below !

- 5 -

From the drawing of image above can be con- cluded that properties of image by the concave lens al ways virtual, upright, minimized,& located between F₁and O.

In everyday life a concave lens is many used by human for glasses lens and telescope.

In lens holds also relationship of focus distance object distance,and image distance with the lens same as hold in the mirror as follows :

and M = │

│ x =│

│ x

But necessary to be considered that in calcula- tion holds the following rules :

a. f has positive value (+) for convex lens.

b. f has negative value (-) for concave lens.

c. S_o has positive value (+) for real object (object is located infront of the lens).

d. S_o has negative value (-) for virtual object (object is located behind the lens).

e. S_i has positive value (+) for real image.

f. S_i has negative value for virtual image.

Exercise :

1. An object is put 20 cm infront of a convex lens. If focus distance of the lens is 15 cm, what is :

a. Image distance from the lens ? 60cm

b. Image magnification 3

c. Properties of image ?

2. An object of height 10 cm is located infront of a concave lens with distance of 30 cm. If focus dis – tance of the lens is 20 cm, calculate :

a. Image distance !

b. Image magnification !

c. The height of image !

Power of Lens.

The power of lens is ability of lens in collecting or spreading the ray it is received.

The power of lens is formulated :

P =

In which : P = power of lens by the unit is Dioptri (D)

f = focus distance of the lens in meter unit. Example :

A glasses of concave lens with focus distance of 40 cm. What Dioptri is the power of the lens ?

Solution :

Given : f = - 40 cm = - 0.4 m

Asked : P = ............?

Answered : P =

- 2.5 Dioptri.

Exercise Power of Lens :

1. Distance of focal point of convex lens is 50 cm.

What is the power of the lens ? 2 D

2. A child wears a minus glasses of minus 2, what is the focus distance of the glasses lens used by the child ?

-0.5 cm

c. Combination Lenses.

Combination lenses is the combination of se- veral lenses which pressed together of each other, so that the distance between them can be neglected ( d ≈ 0 ).

For combination lenses holds the following equations

= Σ or = ..........