11.1Refraction of Light

LEARNING GOALS§ I will be able to describe what refraction is§ I will be able to state and use the rules of refraction§ I will learn key words: refraction, refracted ray, angle of refraction, index of refraction, dispersion

https://www.youtube.com/watch?v=jQDRNb-E-cY

Refraction§ light travels in a straight line and at a constant speed in the same medium

§ when light travels from one medium to another (e.g. from air to water), light rays refract (bend) and both the direction and speed change

Refraction§ the bending of light as it travels, at an angle, from a material with one refractive index to a material with a different refractive index

Marching Band AnalogyWhy does light refract?

§ when one end of the band slows down, the direction of travel of the entire band changes

Imagine…

A  beam  of  light  was  made  up  of  people  walking  in  formation…..like  

a  marching  band!!

Imagine…Light  traveling  in  air  is  like  the  

marching  band  walking  on  grass.

Light  traveling  in  glass  is  like  the  marching  band  walking  in  mud

Air Glass

Air Glass

Entering  a  more  dense  medium  causes  the  light  beam  to  slow  down

Air Glass

Beam  of  light  moving  towards  an  air-­‐glass  interface

Air Glass

The  parts  of  the  light  that  hit  the  air-­‐glass  interface  first  slow  down  and  first  causing  the  line  to  bend.

Air Glass

The  beam  of  light  is  now  travelling  in  a  different  direction

What  happens  if  the  light  travels  from  a  more  dense  medium  to  a  less  dense  medium  (i.e.  glass  to  

air)?

What  happens  if  the  marching  band  walks  from  mud  to  grass?

OR

Air Glass

A  beam  of  light  travels  towards  a  glass-­‐air  interface

Air Glass

The  parts  of  the  light  that  hit  the  air-­‐glass  interface  first  speed  up  and  first  causing  the  line  to  bend.

Air Glass

The  beam  is  now  travelling  in  a  different  direction.

Refraction and Raysrefracted ray§ the ray that is bent upon entering a second mediumangle of refraction (R)§ the angle between the normal and a refracted ray

Direction of the Refracted Ray§ when a light ray travels from a medium in which its speed

is faster (air) to a medium in which its speed is slower (water) the refracted ray bends toward the normal

fast to slow – towards the normal

Direction of the Refracted Ray§ when a light ray travels from a medium in which its speed

is slower to a medium in which its speed is faster, the refracted ray bends away from the normal

slow to fast – away from the normalArcher Fish Video

Dispersion§ the process of separating colours by refraction

§ white light (all wavelengths of visible light) refracts twice:1. when it enters the prism2. when it leaves the prism (separates into a spectrum of colours)

Dispersion§ each colour of light travels at a slightly different speed§ only in a vacuum (space with no matter) do all colours travel at

the same speed: 3.00 x 108 m/s§ e.g. blue light bends more than red light (blue light travels

slower than red light)

Roy G. Biv

How much of the light refracts?

§ speed of light in a vacuum is

§ the speed of light in any other medium is less than that e.g. the speed of light in water is 2.26 x 108 m/s

3.00 x 108 m/s

Index of Refraction§ the ratio of the speed of light in a vacuum to the speed

of light in a given medium

Disappearing Glass Video

Index of Refraction

§ the index of refraction is always greater than 1§ as the speed of light decreases due to the medium, the index

of refraction increases

Sample ProblemCalculate the speed of light in fused quartz. The index of refraction for fused quartz is 1.46.

Equationn = c

v

n = index of refractionc = speed of light in a vacuumv = speed of light in a medium

Rearrange the equation!!!

Givenn = 1.46c = 3.00 x 108 m/sv = ?

v = c n

v = 3.00 x 108 m/s 1.46

v = 2.05 x 108 m/s The speed of light in fused quartz is 2.05 x 108 m/s

c

vn

If the speed of light in water is 2.26 x 108 m/s, calculate the index of refraction for water.

Equationn = c

v

n = index of refractionc = speed of light in a vacuumv = speed of light in a medium

Givenn = ?c = 3.00 x 108 m/sv = 2.26 x 108 m/sn = 3.00 x 108 m/s

2.26 x 108 m/sn = 1.33 The index of refraction for

water is 1.33.

https://www.youtube.com/watch?v=f8oV4RBYR9U

Refraction and Fish?!?!?!?!

Homework§ Read p. 449-455 in the textbook.§ Complete “Section 11.1 Review” Worksheet