FISIKA Kelas 11 - Gelombang Cahaya | GIA Academy

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Hello friends, welcome back

to the Gia Academy YouTube channel. I hope my

friends are always healthy and keep up their

spirits.

At night when we use a

car and turn on the lights, we

can see that the light from the car lights

travels straight in the direction of the

light source. At other times, just

after the rain, sometimes we can

see colorful lights in the sky

called rainbows. So, what is

the process by which a rainbow

is formed? We will study the

symptoms of light in

this video.

So, in this video we will learn

about light waves. Keep watching

the video, okay?

So, friends, light is a form of

energy in the form of

electromagnetic waves with a wavelength of

around 380

to 750 nm. These light waves

do not require a medium to propagate

so that light can propagate in a

vacuum. That is why

sunlight can reach the earth even though it

passes through the vacuum of outer

space at a speed of 300 million m/s.

As an electromagnetic wave,

light has the following properties.

First, light can be reflected or

[music] reflection. can be

refracted, experience mixing or

interference,

experience bending, diffraction, can be

broken down, dispersed, and experience

polarization.

So, these are the six properties of light

that we will discuss in

this video. Reflection, refraction, interference,

diffraction, dispersion, [music]

and polarization. Come on, let's discuss them one by one

.

We start from the reflection of light or

reflection. Light reflection is the

reversal of the direction of light because it hits

a surface. The process of

light reflection follows the law of reflection, namely the

incident [musical] ray, the normal line, and the

reflected ray lie on one

flat plane. The angle of incidence is the same as the

angle of reflection. The

second property is the refraction of light or

refraction. Refraction of light is

the bending of the direction of light propagation because it

passes through the boundary plane of two

different propagation media. The process of refraction of

light follows the law of refraction

stated by Willbrad Snellius,

a

Dutch physicist. Snelius put forward the law

of refraction as follows. The incident ray, the

normal line, and the refracted ray lie

on one plane. The ratio

between the sine of the angle of incidence and the sine of the

angle of refraction is constant.

We can write Senelius' second law

in the following equation. Sin

theta 1/ sin theta 2 = n2/n1

[music] = n21 n1v1 = n2v2

n1 lambda 1 = n2 lambda2 with theta 1

angle of incidence theta 2 angle of refraction n1 n2

refractive index of medium v1 v2 speed of

light wave in

lambda 1 lambda 2 wavelength of

light in meters.

According to Snelius, when light is refracted

in a medium of different densities, what

will happen is that the light

coming from the less dense medium [music]

to the denser medium is refracted

closer to the normal line. Rays

coming from a denser medium to a

less dense medium are refracted

[music] away from the normal line. Remember,

distance to closeness brings you closer, closeness to

distance brings you further away. Meanwhile, incident rays

that are perpendicular to the boundary plane are not

refracted, but are transmitted. The

next property of light is interference.

Light interference is a combination

of two coherent light waves

, which have the same frequency and

amplitude, while the

phase difference remains the same.

Interference is of two types.

First, maximum or

constructive interference, which is interference that

reinforces each other. The properties of

this interference are that it produces a

bright pattern if the waves are

in phase or the phase difference is 0 degrees or an

integer multiple of 360 [music]

degrees. Second interference,

minimum or destructive interference.

This means that these interferences weaken each other

.

This interference produces a dark pattern that

occurs if the waves are 180 degrees out of phase

or an odd number of 180

degrees.

There are several experiments to investigate

light interference patterns. First,

Thomas or Fresnel's experiment to

test double slit interference. In

his experiment, Thomas Yang demonstrated

a light source that illuminates two

parallel slits separated by a distance

D, thus producing two

coherent beams of light with a deviation angle theta. The

interference pattern is observed on a screen at

a distance L from the slit and has a

regular pattern, namely a bright pattern for

maximum interference results and a dark pattern

for minimum interference results. In

the center of the screen there is a

central light. The distance from the center light to light or

dark to n is denoted by Yn.

From the double slit experiment,

we can calculate the

following quantities. In the bright pattern, the

equation d sin theta = n lambda or

dn/l

= n lambda applies. The number n indicates the order

or number of brightness with the provisions 0

for the central brightness, one for the first brightness

and so on. Meanwhile, in the

dark pattern the equation applies d sin theta = n -

1/2 * lambda or dn/l [music]

= n - 1/2 * lambda. The number n

indicates the order or number of darks starting

with the first dark n = 1, the second dark,

n = 2, and so on. As

we have discussed previously,

the quantities for

this double slit interference equation are D

is the slit distance in meters, theta is the

deviation angle, N is the interference order,

[music] lambda is the wavelength in

meters, L is the distance from the slit to the screen,

also in meters. and Yn is

the distance from the center light to light or dark

to N in meters. The

second interference pattern experiment is

interference in thin layers.

We can observe the interference phenomenon in thin layers in

soap bubbles exposed to sunlight

or thin layers of kerosene

spilled on water, producing

certain colors of light. The

interference pattern in a thin layer occurs

when a ray with an angle of incidence I

hits a thin layer with a thickness of D

and a refractive index of N.

[music]

so that the ray experiences reflection and

refraction with a refractive angle of R so that

we can see the pattern of light colors

.

The difference in optical path length

and the change in the phase of the reflected light

causes

maximum and minimum interference to occur. At

maximum interference or bright pattern the

equation 2nd cos r = m - 1/2 * lambda applies.

Meanwhile, in the minimum interference or

dark pattern, the 2nd cos [music] r = m

lambda applies with the thickness of the thin layer

in meters. R refractive angle [music] n

refractive index of thin film lambda

wavelength of light in air in

meters. Lambda 0 wavelength of light

in a thin layer is in meters and

m is the interference order.

And the last interference pattern experiment

is Newton's ring interference.

Newton's rings are dark circular lines

resulting from the

interference of light by reflection from two

different planes, namely a convex PL lens

and a parallel plane mirror.

This Newton's ring interference pattern occurs when light

comes in a perpendicular direction to an

optical system consisting of a

flat convex lens or plan convex lens

with a large radius R placed

on a parallel plane glass producing a

pattern of dark and light circular lines with a

small radius R [music] and

having an air layer thickness D

between the glass surface and the lens.

The equation resulting from the

Newton's ring interference pattern

is for the bright circle pattern NR²

[music]

= m - 1/2 * lambda r. Dark circle pattern

NR² = m lambda r. And to

calculate the thickness of the air layer

between the glass surface and the lens in the

bright pattern, D = m - 1/2 * lambda/2.

In the dark pattern, D = M * lambda/2 where

D is the thickness of the air, r is the

radius of the ring. Both are in units of

meters, N. The refractive index of a plan convex lens is

lambda, the wavelength of light

is in meters, the interference order is m, and

r is the radius of the lens in

meters. The

next characteristic of light [music] is

diffraction. Light diffraction is

the phenomenon of bending or deflection of the

direction of light propagation when it passes through a

narrow gap or grating so that the

light waves appear to widen at the

edge of the gap. In diffraction, interference also occurs,

forming

dark and light lines on the screen.

Based on the medium or gap it

passes through, diffraction can be divided

into three types. First,

single slit diffraction. In single slit diffraction

, incident light with a

wavelength of lambda enters a

single slit with a width of D, experiences

wave bending and forms a

deviation angle of theta. [music] The incoming light

falls directly on the screen, forming a

pattern of dark and light. The distance between the

slit and the screen is denoted by L

and the distance from the center light to light or

dark to N is Yn. So we can

calculate the magnitude of this diffraction.

For dark patterns the equation d sin

theta = n lambda or dn/l

= n lambda applies. Where n represents the order

or number of the dark pattern. Meanwhile, in the

bright pattern, the equation d sin theta

= n - 1/2 * lambda or dn/l

= n - 1/2 * lambda [music] applies, where n

is the order or number of the bright pattern.

All the quantities in

this single-slit diffraction equation are the same as the quantities for

double-slit interference in the

previous discussion. It's just that the formula for the

light and dark patterns is reversed.

Why is that? In single slit diffraction

it appears that the central bright band is

wider than the slit width. The

other bright [musical] bands become narrower

as they move further from the central light.

But the width of the dark band is almost constant.

Therefore, if d sin theta = n lambda,

what occurs in single slit diffraction

is minimum interference or

dark bands. The

second type of diffraction is

grating diffraction. A grating is a barrier that

has many gaps with

the same width and distance between the gaps

. A beam of light

passed through a grating produces a

sharper bright band

than double-slit interference

and single-slit diffraction. [music]

Some equations that we can

use to find the magnitude of the

diffraction grating are first we can

calculate the grating constant or the distance between the

slits with the equation D = 1/n. N

represents the number of lines per cm.

Next is the equation for calculating the

magnitude of the light and dark patterns.

Where the equation is the same as the

equation for double slit interference.

In light patterns, the equation d sin

theta = dyn/

n lambda applies. And in the dark pattern, d

sin theta = dn/l

= n - 1/2 * lambda.

And the last type of diffraction is

diffraction of resolving power in optical instruments. The

resolving power of an optical instrument is the ability of an

optical instrument to produce separate images

of two adjacent objects.

If an optical instrument has a

diaphragm diameter D, [music] then two

light sources S1 and S2 can still be

separated precisely on the screen

to form images S1 A' and S2' [music]

with resolving power DM'. The intersection of the two

lights at the lens diaphragm

will form a separation angle or

minimum resolution angle theta M and the distance of the object

from the lens is L. So, to

calculate the resolving power of this optical tool

, we can use the

following equation. DM = 1.22

l/d.

And to calculate the minimum resolution angle

is theta M = 1.22

lambda/d. where L is the distance of the object

from the lens and D is the aperture diameter of

both optical instruments in meters.

Until [music] here, friends can

understand it, okay?

Next we will discuss the

fifth property of light, namely dispersion.

Light dispersion is the process of breaking down

white or polychromatic light into

colored or monochromatic light.

Dispersion occurs when light passes through

mediums with different refractive indices. In the

following image we can see that

white light passing through a prism

is broken down into a spectrum of colors, namely

red, orange, yellow, green,

blue, indigo and violet. This

shows that white light is actually a

combination of the

seven colors mentioned above, called

polychromatic light. Meanwhile, light that

only consists of one color is called

monochromatic light.

In the event of light dispersion by a

prism, a dispersion angle is formed which

is the width of the [music] spectrum

produced by the prism, the size of which

depends on the difference between the [music]

deviation angles of the purple and

red colors. The magnitude of this dispersion angle can be

calculated using the equation pi = nu -

nm * beta where pi is the

dispersion angle, nu is the refractive index of violet light, NM is the

refractive index of red light, and beta is the

peak angle or refractive angle of the prism.

And the last property of light is

polarization. Light polarization is the

phenomenon of partial absorption of the direction of

vibration of light waves. Light can be

polarized due to the following events:

reflection, refraction, and reflection,

double refraction or twin refraction,

selective absorption, scattering. And

this is an illustration of the path of

light polarization.

In light polarization, we can

determine the intensity of transmitted light

using two

polaroids. The first Polaroid [music]

is called a polarizer which functions

to pass polarized light with a

vibration direction according to the easy axis P1. The

second polaroid is called an analyzer, which

functions to analyze the light

passed through the polarizer. When

the analyzer is rotated, then [music]

when the easy axis P2 is parallel to the

easy axis of the polarizer P1, the brightest light will be seen

. Next, the light

will dim and will appear dark when

P1 and P2 are perpendicular to each other. If the angle

formed between P1 and P2 is

theta, then the intensity of light

passed through the analyzer is I2 = I1

cos² theta = 1/2 I0 [music] cos² theta

with I0 the initial light intensity I1

the intensity of the polarized light coming out

of the polarizer and I2 the intensity of the

polarized light coming out of

the analyzer all three in

watts/m².

So, friends, that was

our entire discussion about the

properties of light, namely reflection

, refraction, refraction,

combination, interference, bending,

[music]

diffraction, dispersion, and

polarization. Friends can

understand it, right?

So that friends understand better, let's

solve the following example problem. The

first question, it is known that the

wavelength of the incident double-slit interference is

7,500 [music]

angstroms which we convert to meters

to 7.5 * 10^ -7 m. Slit width 0.2

mm = 2 * 10^ -4 m. The distance from the slit to the screen is

1 m and the distance from the center light to the

outermost light is 7.5 cm. This means yn = 7.5

cm = 7.5 * 10^ -2 m. The question is the number of

bright lines on the screen or n? Well,

we know the equation used to

solve the problem of the light pattern in

double slit interference is dn/l

= n lambda. Then we enter all the

quantity values ​​that have been converted

to meters. 2 * 10^ -4 * 7.5 *

10^ -2/1

= n * 7.5 * 10^ -7. We do the

arithmetic operation until we get n = 15/7.5

* 10^1 = 20. So the number of bright lines

on the screen is 20. The correct answer is

D.

Second question. It is known that the wavelength

incident on the thin membrane is 589.3

nm. We don't need to convert

to meters because the answer choices

in the question are also in nanometers.

The refractive index of soapy water is n = 1.33.

Since what occurs is the first bright pattern

, the interference order

is 1 and the refraction angle is perpendicular

to the thin film, [music] so r = 0

degrees. What is asked in this question

is the thickness of the membrane or d. To

answer this, we recall the

thin film interference equation in the

bright pattern, namely 2nd cos r = m - 1/2 *

lambda. So to find the value of d =

[music] m - 1/2 * lambda/2n

cos r. We plug in the known values ​​of the quantities

in the equation. D = 1 - 1/2 *

589.3/2

* 1.33

* 1 and we do the arithmetic operation

until we get D = 110.8

nm. [music] So the thickness of the soap water film

is 110.8

nm. The correct answer is A.

Next question. It is known that the radius of the circle of

Newton's ring is small R = 1 mm =

1 * 10^ -3 m. The radius of the convex plane or

r is 4 m. And the refractive index of the lens n =

1. Because the pattern that occurs is the

first light, then m = 1. [music]

What is asked is the wavelength of light

or lambda. We know this problem is

related to

Newton's ring interference whose bright circle pattern equation

is NR² = m - 1/2 * lambda

r. So to find lambda we

can enter the value of the

known quantity. 1 * 10^ -3^ 2 = 1 - 1/2 *

lambda * 4. Lambda = 1 * 10^ -6/2.

Lambda = 5 * 10^ -7 m = 5,000 angstroms.

So the correct answer is C.

Fourth question. It is known that a grating in

the fraction of 1500 lines/cm is passed by a

beam of light from a lamp which

produces a bright pattern of n = 1. If the

first order deviation angle is 30

degrees, we are asked to determine the

wavelength of light from the lamp.

In grating diffraction, the equation

used is d sin theta = n lambda

with d = 1/n. [music]

So the equation becomes 1/n sin theta

= n lambda. We enter the value of the

known quantity. 1/1500

sin 30 degrees = 1 lambda. So

we get lambda = 0.0033

cm = 3.33

* 10^ -6 m. So, the wavelength of

light from the lamp is 3.33

* 10^ -6 m. The correct answer is A.

Next question. It is known that the refracting angle

of the prism is 10 degrees. The refractive index of

red light NM = 1.60

and violet NU = 1.64.

This question asks us to determine the size of the

angle of dispersion in a prism. To

calculate the angle of dispersion of a prism, we

use the equation pi = nu - nf * beta.

Then we enter the known value of the quantity

. Pi = 1.64

- 1.60

* 10 degrees. So we get pi =

0.04 * 10 degrees = 0.4 degrees. So

the angle of dispersion of the prism is 0.4

degrees. The correct answer is D.

Last question. It is known that the angle between the

polarizer and analyzer is 60 degrees

and the initial natural light intensity is 124

watts/m²

with I1 only 50%. This means that I1 in

this case is only 1/2 I0. What is asked

in the question is the intensity of light

coming out of the analyzer or I2.

Well, we can calculate I2 at

this polarization with the equation I2 = I1

cos² theta. Since I1 = 1/2,

then I2 = 1/2 cos² theta. We enter

known numbers. I2 = 1/2 * 124 *

cos² 60 degrees. So we get I2 =

124/2

* 1/2

= 15.5 W/m². [music]

So the intensity of light coming out

of the analyzer is 15.5 W/m².

The answer is A.

Okay, guys. Thus

our discussion about light waves. Don't

forget to keep watching about

light waves. Don't forget to keep watching the

latest videos on our channel, OK?

See you in the next video.

[music]

Yeah.