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Reflection &

Refraction

What Happened??

• Look at the data you collected during the simulation

• Are the angles the same for every scenario?

• Why is there a difference?

Bending• Is this similar to what you saw in the

simulation?

Bending• The change in speed that occurs at an

interface or boundary between two different media

Bending of Light

• Why is there a change in speed?

• Running analogy…

Least Time PrincipleLight always takes the path that requires the least

amount of time

Bending of Light

• The speed of a light wave is dependent upon the optical density of the material through which it moves

Bending of Light

• Optical density of a material relates to the tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave

Energy transformation of the Light

Energy is transmitted into the more dense medium and slows the wave

• The bending of the path of light • Refraction occurs as light passes across the

boundary between two medium

• A synonym for refraction is "bending”

Refraction

• What is going on in this photograph?

Refraction

SFA = Slow to Fast, Away From Normal

If a ray of light passes across the boundary from a material in which it travels slowly into a

material in which travels faster, then the light ray will bend away from the normal line

Refraction of Light

• When light rays enter a medium in which their speed increases (less dense) the rays bend away from the normal.

FST = Fast to Slow, Towards Normal

If a ray of light passes across the boundary from a material in which it travels fast into a

material in which travels slower, then the light ray will bend towards the normal

Refraction of Light

• When light rays enter a medium in which their speed decreases (more dense) the rays bend toward the normal

Refraction of Light• This is caused by the change in speed of light

as it passes from one medium to another

How many times is the light refracted in the picture?

Refraction of Light

• This is caused by the change in speed of light as it passes from one medium to another

Refraction of Light• Which medium is more dense?

Refraction of Light• How does refraction causes the

bottom of a clear lake or pool to appear?

CLOSER !!!!

Recap Questions• What is the cause of refraction in

light waves?

• Describe the path a light ray will take when it travels from a less dense to a more dense medium. Use the NORMAL as your frame of reference.

• What is the main factor that determines the amount of refraction in light waves?

Refraction of Light• The wave speed is always greatest in

the least dense medium

• The wavelength is always greatest in the least dense medium

Refraction of Light• The frequency of a wave is not altered

by crossing a boundary

• However, both the wave speed and the wavelength are changed

Refraction

• The optical density of a material relates to the tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave

• • One indicator of the optical density of a

material is the index of refraction value

Refraction• INDEX OF REFRACTION (n)• Defined as the ratio of the speed of light in

vacuum to the speed of light in the medium

• index of refraction : n = c / v • The speed of light in a given material is

related to this quantity

Every substance has its own specific index of refraction. The next slide has a few examples

Material Index of Refraction  

Vacuum 1.0000 <--lowest optical density

Air 1.0003  

Ice 1.31  

Water 1.333  

Ethyl Alcohol 1.36  

Plexiglas 1.51  

Crown Glass 1.52  

Light Flint Glass 1.58  

Dense Flint Glass 1.66  

Zircon 1.923  

Diamond 2.417  

Rutile 2.907  

Gallium phosphide 3.50 <--highest optical density 

• Index of refraction values (represented by the symbol (n)

• It is the number of times slower that a light wave would be in that material

• A vacuum is given an (n) value of 1.0000

v = c / n n = c / v

Snell’s Law• Snell's Law

• The relationship between the angles of incidence and refraction and the indices of refraction of two media

ni sin θi = nr sin θr

Snell’s Law

light crosses a boundary into a medium with a higher index of refraction, the light bends towards the normal

Light traveling across a boundary from higher (n) to lower (n) will bend away from the normal

Snell’s Law

• Index of refraction ( n ) = c / v material

• Describes the extent to which the speed of light in a material medium differs from that in a vacuum

• ni sin i = nr sin r

• ni = index of refraction of incidence material

• i = angle of incidence

• nr = index of refraction of refractive material

• r = angle of refraction

Find the angle of refraction

ni sin i = nr sin r ni sin i = nr sin r

r = 32o r = 35o

Lumination

• Luminous objects • are objects which generate their own light

Illumination• Illuminated objects • are objects which are capable

of reflecting light to our eyes

• None of us are light-generating objects

• It is only by reflection that we, see

Total Internal Reflection • Total internal reflection, or TIR is the reflection of the

total amount of incident light at the boundary between two medium

• Total internal reflection only occurs with large angles of incidence or if you exceed the critical angle

Total Internal Reflection

• TIR only takes place when f the following conditions are met

• 1. the light is in the more dense medium and approaching the less dense medium.

• 2. the angle of incidence is greater than the critical angle

• **If the ray is at 900 there is no refraction**

                                          

      

                                          

      

                                          

      

                                          

      

Total Internal Reflection

• Critical angle - the minimum angle of incidence for which a light ray is totally reflected within a medium.

• Total internal reflection - the100% reflection of light that strikes the boundary between two media at an angle greater than the critical angle.

Total Internal Reflection• Optical fibers - a transparent fiber that can transmit

light down its length by means of total internal reflection.

• The light travels through the core by constantly bouncing off the cladding.

• The cladding does not absorb any of the light.

The maximum possible angle of refraction is 90-degrees

if the angle of refraction were greater than 90 degrees, then the refracted ray would lie on the incident side of the medium - that's just not possible

The maximum possible angle of refraction is 90-degrees

if the angle of refraction were greater than 90 degrees, then the refracted ray would lie on the incident side of the medium - that's just not possible

Critical Angle

critical = sin-1 ( nr / ni )

the critical angle is defined as the angle of incidence which provides an angle of refraction of 90-degrees

For the water-air boundary, the critical angle is 48.6-degrees

Critical Angle

The following slides can be optional

The Rainbow• The conditions for a rainbow are • 1. that the sun must be shining on one part of the

sky and • 2. water droplets in a cloud or a falling rain must

be in the opposite part of the sky

The Rainbow• The water droplets refract the light twice and

reflect the light as well

• AT this first refraction the light is dispersed into its spectral colors

• (red bent the least, violet the most)

The Rainbow• The second refraction acts like a prism, where

refraction at the second surface increases the dispersion already produced at the first surface

The Rainbow• Everyone sees their own personal rainbow• It is based on your location

Atmospheric Refraction

• Mirage: a floating image that appears in the distance and is due to the refraction of light in the Earth’s atmosphere

The image appears upside down, to an observer just as if it were being reflected from a surface of water

• ** the light is not reflected; it is refractedUndeflected wave fronts would travel at one

speed; the greater speed near the ground, however the causes the light ray to bend upwards

Atmospheric Refraction

• After the sun has set one would think it was still setting, this is because light is refracted by the earth’s atmosphere

The density of the atmosphere changes gradually, the refracted rays bend to produce a curved path

When the sun is near the horizon, the rays from the lower edge are bent more than the rays from the upper edge; this makes the sun appear elliptical instead of round

Dispersion in a Prism

• Since different frequencies of light travel at different speeds in transparent materials, they will refract differently and bend at different angles

• Dispersion - the separation of light into colors arranged according to their frequency

The end

Reflection• Reflection: • when a wave reaches a boundary, some or all of

it bounces back into the first medium

• Totally Reflected • waves are reflected back rather than

transmitted into another object

• Partially Reflected • some energy is transmitted into a new medium

Reflection• Luminous objects emit light in a variety

of directions• your eye only sees the very small

portion of rays coming towards it• Ray = a narrow beam of light

The Law of Reflection• Normal = • imaginary line drawn perpendicular to

the surface

The Law of Reflection• angle of incidence = • The angle made by the incident ray and the

normal• angle of reflection = • The angle made by the reflected ray and the

normal

The Law of Reflection• angle of incidence = angle of reflection

The Law of Reflection

Summary Questions

.

1. Differentiate between the angle of incidence and the angle of reflection2. What is meant by the normal to the surface?3. What is the law of reflection

Pop Quiz 1.  Consider the diagram at the right. Which one of the angles (A, B, C, or D) is the angle of incidence? Which one of the angles is the angle of reflection?

2. A ray of light is incident towards a plane mirror at an angle of 30-degrees with the mirror surface. What will be the angle of reflection?

Answer: 60 degrees.

Note the angle of incidence is not 30 degrees!

Answer: B

Mirrors• light reflects off surfaces in a very predictable

manner - law of reflection• Rays of light are reflected from mirrors in all

directions                                                      

The Law of Reflection is Always Observed (regardless of the orientation of the surface)

Mirrors• Virtual images are images which are

formed in locations where light does not actually reach

Mirrors• Virtual Image are formed through reflection

that can be seen by an observer but can not be projected on a screen

• light from the object does not actually come to a focus

Mirrors• Your eyes cannot differentiate

between a real image and a virtual one

Your image in a plane mirror is?

ALWAYS

VIRTUAL !!!

Specular• Specular reflection• Reflection off of smooth surfaces such as

mirrors or a calm body of water

Diffuse• Diffuse reflection • When light is incident on a rough surface

and is reflected in many directions.• What constitutes a rough surface for some

rays may be a polished surface for others.

Diffuse vs. Specular• What is the main type of reflection occurring

here?

Diffuse vs. Specular• What type of reflection is occurring in picture

A? B?

• A B