Good AfternoonACTIVITY

Refraction

of

Light

What is Refraction?

Refraction is the bending of light as it

travels from one medium to another.

Refraction

In vacuum, light travels with a speed

c = 3.00 x 108 m/s

In a transparent medium, (ex. water or glass) the speed of light is slower than

its speed in vacuum.

WHY? Ans. interactions between photons

and molecules of the medium

Refraction

What EXACTLY is light doing when it reaches a

new medium?

Air

Water

Incident ray Reflected ray

Refracted ray

Angle of

Incidence

Angle of

Reflection

Angle of

Refraction

Refraction

Air

Water

Normal

Incident ray

Refracted ray

θi

θr

Normal

is a line drawn

perpendicular to the surface

Incident ray is the light ray in 1st

medium (air)

Refracted rayis the light ray in the 2nd medium (water)

Angle of incidence, iis the angle between the incident ray & the normal

Angle of refraction, ris the angle between the refracted ray & the normal

Law of Refraction #1

The incident ray, refracted ray, and normal lie in one play.

Air

Glass

Normal

Higher speed (lower index of refraction)

Lower speed (higher index of refraction)

Refraction

θi

θr

Refracted ray bends

away from the normal

Original path of

light ray

Refraction

Air

Glass

Higher speed (lower index of refraction)

Lower speed (higher index of refraction)

Refracted ray

bends towards the

normalθi

Normal

θr Original path

of a light ray

Refraction

Air

(a) Less dense to denser medium (b) Denser to less dense medium

Air

Glass

Glass

θi

θr

θi

θr

(a) When light ray enters a medium with smaller index of refraction

to a medium with larger index of refraction, it bends TOWARDS the

normal.

(b) When light ray enters a medium with larger index of refraction, it

bends AWAY the normal.

Law of refraction #2

When a ray of light enters a denser medium, it bends TOWARDS the

normal

When a ray of light enters a less dense medium, it bends AWAY from

the normal.

What is “index of refraction”?

The index of refraction (n) for a material is

the ratio of the speed of light (c) in vacuum to

that of the speed (v) in the medium.

The ratio of the two speeds can be compared

cn

v

c

vc

nv

Medium Refracted Index (n)

Vacuum 1.00

Air 1.0003

Water 1.33

Ethanol 1.36

Glycerine 1.47

Crown Glass 1.52

Quartz 1.54

Flint Glass 1.61

Diamond 2.42

Sample Problem

What is the speed of light in a diamond if

its index of refraction is 2.42?

Light travels from air (n = 1) into glass, where its velocity reduces to

only 2 x 108 m/s.

What is the index of refraction for glass?

vair = c

vG = 2 x 108 m/s

Glass

Air

8

8

3 x 10 m/s

2 x 10 m/s

cn

v

Solution:

Answer:

n= 1.50

What is the speed of light in quarts (1.54)?

Solution

V= c/n

V= 3.00𝑥108𝑚/𝑠

1.54

Given

n= 1.54

c= 3.00× 𝟏𝟎𝟖 m/s

v= ?

Answer

V= 1.95x𝟏𝟎𝟖 m/s

Law of refraction1. The incident ray, refracted ray, and normal lie in one plane.

2. When a ray of light enters a denser medium, it bends

TOWARDS the normal.

When a ray of light enters a less dense medium,

it bends AWAY from the normal.

3. Snell’s Law is the relationship between the angle of

incidence and angle of refraction and the angle of refraction of

both materials.

How can we figure out the angle of our

refracted ray?

We use Snell’s Law

Willebrord van Roijen Snell

(1580-1626)

Sample Problem

A ray of light traveling through air is

incident on a smooth surface of water

at an angle of 30° to the normal.

Calculate the angle of refraction for the

ray as it enters the water.

Air

Water

𝒏𝟏sin𝐬𝐢𝐧𝜽 𝒊 = 𝒏𝟐 𝐬𝐢𝐧𝜽 𝒓Formula

𝐬𝐢𝐧𝜽𝒓𝒏𝟏 𝐬𝐢𝐧𝜽 𝒊

𝒏𝟐

Given

𝒏𝟏= 1.0003 (air)

𝒏𝟐= 1.33 (water)

sin 𝜽 𝒊 = 𝟑𝟎°Find 𝜽𝒓 = ?

Solution

sin 𝜽 𝒓 =𝟏. 𝟎𝟎𝟎𝟑 sin 𝟑𝟎°

𝟏. 𝟑𝟑

𝐬𝐢𝐧𝜽𝒓 = 𝟎. 𝟑𝟕𝟔𝟎𝟓𝟐𝟔𝟑𝟏𝛉𝐫 = 𝐬𝐢𝐧−𝟏 𝟎. 𝟑𝟕𝟔𝟎𝟓𝟐𝟔𝟑𝟏Answer:

𝜽𝒓 = 𝟐𝟐. 𝟎𝟗°

Mmmm..…….

Fish and

Chips!!!

Normal

Effects of Refraction

Real depth

Apparent

depth

To an observer standing at

the side of a swimming

pool, objects under the

water appear to be nearer

the surface than they really

are.

It can be shown mathematically that index of refraction is the ratio

of real depth to apparent depth.

Formula:

n=𝑹𝒆𝒂𝒍 𝒅𝒆𝒑𝒕𝒉

𝑨𝒑𝒑𝒂𝒓𝒆𝒏𝒕 𝒅𝒆𝒑𝒕𝒉

Sample Problem

A grizzley bear is seating on a rock in the middle of a calm river,

when she observes a fish directly below. If the apparent deep of a fis

is 0.40m, what is the actual depth of a fish?

Given

n= 1.33 (water)

App. Depth= 0.40m

Find Actual depth=??

Solution

Real Depth= n* apparent depth

Real depth= (1.33)(0.40m)

Real depth= 0.53m

Total Internal Reflection

When light passes at an angle from a medium of higher index to

one of lower index, the emerging ray bends away from the normal.

Water

Air

light

i = r

Critical angle

qc

900

When the angle reaches a certain

maximum, it will be reflected

internally.

The critical angle is defined

as the angle of incidence in

the optically denser medium

for which the angle of

refraction in the optically less

dense medium is 90°.

nSo the question is , how can

you calculate the critical

angle?

Remember, it is when the

refracted ray is equal to 90

degrees

ɵc = sin-1 (𝒏𝟐/𝒏𝟏)

Sample Problem

What is the critical angle for

water-air interface

Given

n1= (water) 1.33

n2= (air) 1.0003

𝜽𝒄 = sin−𝟏𝟏. 𝟎𝟎𝟎𝟑

𝟏. 𝟑𝟑

𝜽𝒄 = 𝟒𝟖. 𝟕𝟕°