Refraction 12.2-12.3 pg 222 – 236

1) Principles of Refraction 2) Snell’s Law & Refractive Index 3) Total Internal Reflection 4) Optical Fibres

• What is this? Who has this at home?

• A new generation of Internet connection – based on optical fibres.

• We will study the Physics of optical fibres in this topic.

You might have learned that (1) Light travels in a straight line (2) Speed of light is a constant In this topic, you will learn that both (1)

and (2) are INCORRECT Moral of the story: keep your mind open,

always be willing to question your teachers, and question what you already think you know.

Definition: refraction is the bending of light as light passes from one optical medium to another.

Similar to reflection, there is an angle of incidence (i) and an angle of refraction (r)

(applet) Can you use refraction to explain why

white light forms a rainbow when it passes through a prism?

Learn Sine and Arc Sine!

Sine and Arc Sine will be needed to do calculations for the topic of refraction

As previously mentioned, there is an angle of reflection (i) an angle of refraction (r)

Snell’s Law is defined as (Sin i)/(Sin r) = constant

[verify Snell’s law using applet]

Snell’s Law is very closely related to the concept of “refractive index”

Symbol for refractive index is n (no units) Refractive Index is a measure of how fast

(or how slow) light passes through a medium

There are 3 different ways to define refractive index:

First way: n = (speed of light in vacuum) / (speed of light in medium)

The first definition is also the “official” definition (in your syllabus)

Hence, n for vacuum or air = 1 n for any other medium >1 Second way: n = (Sin i) / (Sin r)

IMPORTANT light MUST be coming from air (vacuum) and going into the medium (e.g. glass, water, etc.)

Recall Snell’s Law: (Sin i)/(Sin r) = constant Can you see how they are related? The third way will be discussed in the next

sub-topic.

Refractive Index (n) is a property of a material. The same material will always have the same n.

[Not in syllabus]Material Refractive

Index (n)

Vacuum 1 (by definition)

Air 1.00027

Water 1.3330

Plastic 1.4 to 1.6

Glass 1.5 to 1.9

Diamond 2.419

Magic Trick – Disappearing Coin http://www.youtube.com/watch?

v=Slas3ik9Lpo Disappearing Glass & Disappearing Gel

https://www.youtube.com/watch?v=qH1S83Bkttw

If the glass block has a refractive index of 1.5, what is angle x? (hint: need to use arc sine)

60°

x

Step 1: Draw Normal!Step 2: i = 90 – 60 = 30°Step 3: Check: light going from air to medium? Ans: Yes!Step 4: Sin i / Sin r = n[IMPORTANT: presentation of working]Sin (30) / Sin r = 1.5Sin r = Sin (30)/1.5Sin r = (0.5)/1.5 = 0.3333 (4 sf)r = Sin-1 (0.3333)r = 19.5° (final answer?)No!! Qn ask for x not r!!r = 19.47° (2 dp)x = 90 – r = 90 – 19.47= 70.5° (1 dp)

60°

x

30°

r

Always draw normal and measure i and r against normal!

Light entering at 90° doesn’t bend Light bends towards the normal when it is

traveling from lower to higher n Light bends away from the normal when

it traveling from higher to lower n If you need to find n, but light is travelling

from medium to air, use (sin r)/(sin i) instead of (sin i)/(sin r) [Qn: How come can we do this?]

Sometimes the question uses the term “more dense” or “less dense”

This density refers to Optical Density, it is not to be confused with Physical Density (i.e. Mass / Volume)

More dense means higher n, less dense means lower n

When “less dense” think air When “more dense” think water or glass

1) Straight surface Single Ray 2) Curve surface Single Ray 3) Multiple ray These case studies can also apply to

refraction!

Just like previously in refraction, draw a tangent, and consider angles i and r using the tangent.

Special case: semi-circular glass block [applet]

Math rule: if a line passes through the center of a circle, it will always be perpendicular to the tangent

Note: Semi-circular glass block is a possible lab experiment for light

Note: the terms “real depth” and “apparent depth” are not in syllabus But you need to know that the image is more shallow than the object

Eye

Object

Image

[Not in syllabus] Photographers: What is a fish eye lens? Why is it called a “fish eye lens”? Qn: There is a device inside your home

(esp if you live in HDB flat) which is also a fish eye lens. What is it?

Ans: Doorviewer

[Not in syllabus]

Source: idigitaldarwin.wordpress.com

1) Draw a ray diagram of light going from glass to air, n = 1.5, i = 45°

What happened? 2) Find the angle of incidence when r =

90° (What did you realize about sin 90?) There is a special name for this angle, it

is called the critical angle, or c

Definition: the angle of incidence in an optically denser medium for which the angle of refraction in the optically less dense medium is 90°

Third definition of refractive index (n): n = 1 / Sin c Question: what happens when i > c?

Important: Something which only occurs when light is going from more dense (i.e. higher n) to less dense (lower n)

TIR never occurs when light is going from less dense (lower n) to more dense (higher n)

Definition: The complete reflection of a light ray inside an optically denser medium at its boundary with an optically less dense medium

[applet demo]

TIR is a case of reflection! That means angle of incidence = angle of reflection applies

When ray is going from higher n to lower n, always check for TIR!

Remember the two conditions for TIR: From more dense medium to less dense

medium Angle of incidence > Critical Angle

If the glass block has n = 1.5, complete the ray diagram below:

75°

n = sin i / sin r1.5 = sin 75 / sin rr = 40.09°[What happens at the top surface?][Check for TIR!]n = 1/sin cc = 41.8[Is i>c? Yes]

75°40.1°

49.9°

Periscopes and Binoculars can be constructed either using mirrors (reflection) or using prisms (total internal reflection)

Prisms are preferred and is the better method. Qn: Why? Ans: There is double reflection in mirrors due

to thickness of glass (remember your lab expt) Important: Textbook Pg 234 has an error!

Reward for someone who can spot and explain the error.

Source: sensorcentral.com

(see textbook Pg 235) Higher carrying capacity Less signal degradation Lightweight Lower cost Can be used as endoscopes for medical

purposes

McGyver overcomes the light sensors trap https://www.youtube.com/watch?

v=O50bOf_Z2cg The Engineer Guy: How Fiber Optic

Cables work http://www.youtube.com/watch?

v=0MwMkBET_5I

[Not in syllabus] Qn) Why does light bend when it goes

into a different medium? (i.e. what is the actual explanation behind Snell’s law?)

Ans: speed of light changes when medium is changed

Qn) Why does a change in speed result in the light being bent?

[Not in syllabus] Recall that the n of diamond is very high There are many ways to describe &

valuate a diamond. One way is called the “brilliance” of the

diamond What is “brilliance” and how is it related

to refraction?

3 definitions to memorize in this subtopic! Refraction - is the bending of light as

light passes from one optical medium to another.

Total Internal Reflection - The complete reflection of a light ray inside an optically denser medium at its boundary with an optically less dense medium

Critical Angle - the angle of incidence in an optically denser medium for which the angle of refraction in the optically less dense medium is 90°

Snell’s Law: (Sin i)/(Sin r) = constant 3 equations for refractive index (n): n = (speed of light in vacuum) / (speed of

light in medium) n = (Sin i) / (Sin r), provided light coming

from air and going into the medium n = 1 / Sin c, where c is critical angle

Refraction as bending of light due to change in medium

Less dense to more dense – bend towards the normal

More dense to less dense – bend away from the normal

Use of Snell’s Law, Sine and Arc Sine to solve refraction problems

Critical Angle & Total Internal Reflection Optical Fibres – how it works and its

advantages

If you think you have not understood: (a) Principle of Refraction / Snell’s Law –

put a colored paper clip on your exit card (b) Total Internal Reflection / Critical

Angle / Optical Fiber – put a gold paper clip on your exit card