CHAPTER 18
SECTION 1. REFRACTION OF LIGHT
Hook
• Watch → https://www.youtube.com/watch?v=v5SuSB_93FM
Q1. Define the angle of incidence & angle of refraction. Ans → Slide 4.
Q2. What is the relationship between density & speed?
Ans → The lighter the medium (Ex. Air) the higher the speed.
Q3. How does refraction occur?
Ans → It occur through transmission of the light ray from one medium to another.
Q4. Can refraction occur when the incident light ray is perpendicular to the surface?
No, because sin 90 is zero, so no bending will occur in this case according to Snell’s law.
Refraction
• Refraction → It’s the bending of light when it passes through two different media.
• Refraction mostly occur in transparent media Ex. Glass.
• The amount of refraction depends on the index of refraction of each medium & the
angle of incidence.
• Refraction is mainly affected by the medium properties (solid, liquid or gas).
• So, In figure 1. The angle of incidence in all cases is the same, but diamond had the
highest refraction then glass then air is the least.
• For refraction to occur, the incident ray should not be
parallel or perpendicular to the surface to be able to bend.
While the normal line is always perpendicular to the boundary.
Snell’s Experiment of Refraction
• He tried shining a narrow beam of light through glass & measured 2 angles:1. Angle of incidence θ1 → The angle at which the light rays strike a boundary (angle between the incident ray and the
normal).
2. Angle of refraction θ2 → The angle at which the transmitted light leave the surface (angle between the refracted ray
and the normal).
Snell’s Experiment of Refraction
• He found that the incident ray refracts towards the normal when it enters the transparent medium (glass),
then refracts away from the normal when it goes back to air.
• The direction of the light ray when it leaves the glass (back to air θ2’) is equal to before it entered the
glass (θ1).
• So, he found that when light passes from air into glass, sine the angles of incidence and refraction were
related as:-
Snell’s law of refraction → The product of the index of refraction of the first medium (n1) and sine the
angle of incidence (θ1) is equal to the product of the index of refraction of the second medium (n2) and
sine the angle of refraction (θ2).
Index of refraction → A value that determines the angle of refraction of light as it crosses
the boundary from one medium to another.
It describes how fast light will travel through a medium.
Snell’s Law of Refraction
• It’s valid for any two different media.
• The angle of incidence (θ1) is not equal to the angle of refraction (θ2).
• The value of the index of refraction determines if it will bend toward or away from the normal.
• The index of refraction of a medium is inversely proportional to the angle made by this medium and to the speed
(velocity) of light in a medium as well.
•
the index of refraction is also defined as → the ratio of the speed of light in space (C= 3𝑥108 )
and v is the speed of light in a medium.
𝑐
𝑛= 𝑣
• The index of refraction is inversely proportional to V,
theta and the density.
• V, theta & the density of a medium are directly proportional.
Applying concepts
Guided Practice
Exit ticket
Exit ticket
Index of Refraction
• We can conclude another formula for the index of refraction, which is :- The index of refraction of a medium is equal
to the speed of light in vacuum (space) divided by the speed of light in the medium.
• From the formula, the relationship between the index of refraction & the speed is inversely proportional, so the
higher the index, the slower the light will be.
• Frequency is not affected by changing the medium it remains the same.
Critical angle
• Critical angle → It’s the greatest angle at which a ray of light, travelling in one transparent medium, can strike the
boundary between that medium and a second of lower refractive index without being totally reflected within the
first medium.
• It’s also an angle after which, light will refract on the boundary between two media.
• This occur from a medium of high index of refraction to low index of refraction.
• As the angle of incidence increase, the angle of refraction will also increase (directly proportional).
• If the angle of incidence was less than the critical angle, refraction will occur normally but away from the normal.
• If the angle of incidence is equal to the critical angle, the angle of refraction will be 90 degrees as the refracted ray
will be perpendicular to the normal (critical ray).
• If the angle of incidence was more than the critical angle, no more refraction exist and there will be only internal
reflection.
• Total internal reflection → Occur when light moves from a region of high index of refraction to a region of low index
of refraction (theta incident > theta critical), so all the light will reflect to the high index medium.
Critical angle
Θ1 < θc
Θ1 = θc
Θ1 > θc
Critical angle
Evaluation
Real life applications
Real life applications
1. Mirage
• They result from the Sun heating the roads.
Ex. On a hot sunny day, you can see a pool of water and as you approach it you find it
dry.
• The speed of light & the index of refraction for a gaseous medium can slightly change
with temperature.
• Therefore, the hot road heats the air & produces a thermal layering of air causing light
traveling toward the road from the car to gradually bend upward, which results in the
appearing mirages.
Real life applications
1. Mirage
• Figure 6. Light waves reflected from the car on the road are refracted when the air near
the surface of the road is hotter than the air above it.
• Cooler air → high index (n), low speed & high density
• Hotter air → Low index (n), high speed & low density
• They are faster near the surface of the road as it has low index, so the speed will be
high as well as the density of the hot air is low.
REAL LIFE APPLICATIONS
2. OPTICAL FIBERS
Cladding layer should be made out
of a material that has a lower index
of refraction than the core, so
comparing both materials from the
table, we should choose float glass
for the cladding and quartz for the
core.
•
THEY REPRESENT AN APPLICATION FOR TOTAL INTERNAL REFLECTION.
•
THEY’RE AS THIN AS A HAIR & CARRY INFORMATION IN THE FORM OF
LIGHT.
•
THE LIGHT TRAVELING THROUGH THE TRANSPARENT FIBER ALWAYS HITS
THE INTERNAL BOUNDARY (CORE) OF THE OPTICAL FIBER AT AN ANGLE
GREATER THAN THE CRITICAL ANGLE (Θ1 > ΘC).
•
ALL LIGHT IS REFLECTED & NONE IS TRANSMITTED THROUGH THE
BOUNDARY.
•
LIGHT PULSES IN THE FIBER OPTIC CABLES CARRY LARGER AMOUNTS OF
INFORMATION OVER LARGER DISTANCES THAN OTHER FORMS OF
COMMUNICATION.
•
CLADDING → IT’S THE OUTER MATERIAL OF EACH FIBER.
Real life applications
3. Dispersion of Light
• The separation of white light into a spectrum of colors.
• Refraction lead to this dispersion of light.
ROYGBIV
High wavelength
Low frequency
Low dispersion
Low index of refraction
Low wavelength
High frequency
High dispersion
High index of refraction
Real life applications
3. Dispersion of Light
• Rainbow → A spectrum formed when sunlight is dispersed by water droplets in the
atmosphere.
• Although each droplet produces a spectrum, an observer positioned between the Sun &
the rain will see only a certain wavelength of light from each droplet depending on
relative positions of the Sun, the droplet & the Sun.
• Second-order-Rainbow → Formed sometimes as a faint rainbow & has a reversed order
of colors.
• It’s caused by double reflection.
Real life applications
Real life applications
Real life applications