Reflection and Refraction - General Physcis - Lecture Slides, Slides of Physics

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Part V. Optics

Reflection & Refraction

Images & Optical Instruments

Interference & Diffraction

Drops of dew act as miniature optical systems,with light refracting through the drops to form myriad images of the background flowers －

which themselves are out of focus in the photographer’s camera.

Why does the bee’s image appear at left,and what does this have to do with e-mail and the Internet?

Ans: Total internal reflection; fibre optics.

30.1. Reflection

Conductor:

E

of light drives e to oscillate 

re-radiate 

reflection

Huygens-Fresnel principle :Each point of an advancing wave front is the source of new (spherical) waves.The new wave front is tangent to all these waves.

angle of incidence = angle of reflection

Example 30.1. The Corner Reflector Two mirrors join at right angles.Show that any light ray incident in the plane of the pagewill return anti-parallel to its incident direction.

Ray turned byangle A here

Ray turned byangle B here

A

B

A



Partial Reflection

Continuity of fields at boundary

incident waves always partly reflected.

Least reflection at normal incidence (4% for glass).Anti-reflection coating for lens, solar cells…

Table 30.1. Indices of Refraction

1

2

1

1



 

1

1

2

2

sin

sin

n

n





Snell’s law

1

2

1

2

sin

sin

v t

v t





c

v

n

Example 30.3. CD Music

The laser beam that reads information from a compact disc is 0.737 mm widewhen it strikes the disc, and it forms a cone with half angle

1

It then passes through a 1.20 mm thick layer of plastic with refractive index1.55 before reaching the reflective information layer near the disc’s top surface.What is the beam diameter

d

at the information layer?

At the incidence (lower) interface:

1

2

sin

sin

n





d

D

x

2

tan

D

t



At the information (upper) surface:

^

^

1

sin 27.

tan

sin

mm

mm

^

mm

m



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GOT IT? 30.1.

The figure shows the path of a light ray through three different media. Rank themedia according to their refractive indices.

n

3

> n

1

> n

2

30.3. Total Internal Reflection

Critical angle

c^

for total internal reflection (

refr

^

1

2

sin

c

n

n



n

1

n

2

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Conceptual Example 30.1. A Watching Whale

n water

n

air



internal reflection for

θ

θ

.c

The whale sees the entire world above the surface in a cone of half-angle

θ

c^ ;

beyond that, it sees reflections of objects below the surface.

Planeloads of whale watchers fly over the ocean.What does the whale see when it looks up at them?

GOT IT? 30.2.

The glass prism in figure has

n

= 1.5 and is surrounded by air (

n

What would happen the the incident light ray if the prism is imersed inwater (

n

Beam is both reflected & refracted at thediagonal interface.

Application: Optical Fibre

Typical fibre: glass core of

d

m,

cladded by smaller

n

glass.

total internal reflection Typical transmission ~ km.SemiC laser:

= 850, 1350, 1550 nm

Required bandwidths:

Audio:

kHz

TV:

6 Mhz

Microwave freq:

10

Hz

Light freq:

14

Hz