reflection and refraction of light - physics and physical...

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Physics 1051 - General Physics IIOscillations, Waves and Magnetism

Physics 1051 – Lecture 30

Reflection and Refraction of Light

07/08/10Physics 1051 – Bill Kavanagh

2 Physics 1051 - General Physics IIOscillations, Waves and Magnetism

Lecture 29 - Contents

25.0 Introduction to Reflection/Refraction of Light

25.1 The Nature of Light

25.2 The Ray Model of Light

25.3 The Wave Under Reflection

25.4 The Wave Under Refraction

25.5 Dispersion and Prisms

25.7 Total Internal Reflection

25.8 Context Connection – Optical FibersLeft for reading...



25.0 Introduction to Reflection/Refraction of Light

● This discussion is analogous to the discussion of reflection and transmission across a boundary of a light string to heavy string. [Have a look at Chapter 13 and Lecture 6]

● In this version, transmission is called refraction.● The string version the of the wave was

restricted to a 1 d string.● For light, the wave (ray) is free to move in 3D.



25.1 The Nature of Light

● We have been using either particle or wave models to describe physics

● Particle model was used to describe light by Newton.− Explained reflection and refraction.

● Wave model was later used to describe light by Huygens.− Explained reflection and refraction



Particle or Wave?

● Young did an experiment that could only be described by wave model.− It was the double slit experiment. (will explore later)− Wave model explained interference behaviour.

◦ Maxwell's mathematical theory also backed up wave model.

● Hertz did an experiment that could only be explained by the particle model.− It was the photoelectric effect experiment.− Particle model explained why electrons are ejected

from a material when certain light is incident on it.



It's Both! - A summary.

● Depending on the case; light acts as a particle OR a wave.

● When light interacts with matter (e.g. light ejecting electrons from a metal – photoelectric effect), it acts as a particle, otherwise it is propagation and interference is explained as a wave.

● Refraction and reflection can explained by either.



25.2 The Ray Model of Light

● We will explain light using a model of rays● Ray: a line pointing along the direction of

a single wave (e.g. an extension of )● Wave front: the surface connecting points

of equal phase.● Plane wave: collection of waves coming

from a bunch of sources that include any point on a plane that are all emitted in phase. The wave front is a geometric plane.

c

See Figure 251



Wave meets Barrier

When opening is relatively big, ray moves on in straight line.

When opening is relatively small, rays spread out in all directions (diffraction).

When opening is relatively very small, rays spread out in all directions (perfect diffraction).

See Figure 25.2



25.3 The Wave Under Reflection

● Unless a surface is perfectly absorbing, it reflects.● Specular reflection: reflection from a smooth

surface. Reflected rays are in various directions. ● Diffuse reflection: reflection from a ruff surface.

Reflected rays are in the same direction.

(b) Diffuse Reflection

(a) Specular Reflection

See Figure 25.3



Law of Reflection

− is angle between the normal and reflected ray− is angle between the normal and incident ray

“The angle of reflection equals the angle of incidence.”

Quantity Type Scalar

SI Unit Degrees/Rad1'=1

1'

1

Note: Number in subscript indicates medium. Here 1 in both angles indicates same medium.

See Figure 25.5



25.4 The Wave Under Refraction

● When a light ray enters a medium (e.g. crosses a boundary) it experiences a change in direction. This is known as refraction.

● The incident, reflected, and refracted rays lie in the same plane and angle of refraction determined by

● Refracted b.c. speed changes across boundary.● Light travels at its maximum speed in a vacuum.

sin2

sin1

=v2

v1

=constant

2



Refractive index

− is speed of light in a vacuum− is average speed of light in a medium− is refractive index of medium


SI Unit _n= cv

c

v

n

n≡ speed of light inside a vacuumaverage speed of light inside amedium



What about light doesn't change under refraction?

● We already know that speed of light changes in a medium.

● As a wave travels from one medium to another, the frequency does not change.− Thus the speed and wavelength change.

● Reason is similar to the fact that wave frequency going from heavy to light string must be constant otherwise the wouldn't be connected.

See Figure 2511



Law of Refraction (Snell's Law)

− is angle between the normal and incident ray− is angle between the normal and refracted ray− is refractive index of 1st

medium− is refractive index of 2nd

medium


SI Unit _n1 sin1=n2 sin1

1

2

n2

n1See Figure 25.9



Indices of Refraction for Various Substances

Refractive indices actualy depends on the wavelength of light!

See Table 25.1



Example

Problem 25.12, page 860 A light ray initially in water enters a transparent substance at an angle of incidence of 37º, and the transmitted ray is refracted at an angle of 25.0º. Calculate the speed of light in the transparent substance.



25.5 Dispersion and Prisms

● The refractive index for a light ray entering a medium depends on the medium AND the wavelength of light.

● Dispersion: is the dependance that index of refraction has on wavelength of the light.

● The angle of refraction, when light enters a medium, depends on wavelength of the light.

● Because of dispersion different wavelengths of light, when entering a medium, spread out and exit medium in different directions.



Prisms and Rainbows Explained!

Physics takes credit for Rainbows! Water droplets act as little prisms!

See Figure 25.14

See Figure 25.17

See Figure 25.15



25.7 Total Internal Reflection

● Total Internal Reflection: occurs when light is contained in a medium because it does not refract through a boundary from within (only reflects).

● To find the defining equation we will use Snell's Law and from the above definition we know that:

● We call the incident angle critical angle

2=90º

1=c

See Figure 25.22



Deriving Associated Law

● Start with Law of Refraction

n1 sin1=n2 sin1

n1 sinc=n2 sin90º

n1 sinc=n2

sinc=n2

n1

Total internal reflection only happens only when light approaches a boundary from higher to lower index of refraction. (since sin < 1)

See Figure 25.22



25.8 Context Connection – Optical Fibers

● The short version of the optical fiber storey:− It is a fiber engineered so that light is totally

internally reflected.− The light is thus restricted to the fiber− It can travel around bends!

● Read pages 855-857

reflection and refraction of light - physics and physical...

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