October 26, 2010

Refraction: Snell's Law

October 26, 2010

When light passes from one medium into another, part of the incident light is reflected at the boundary

The remainder passes into the new medium. Unless it is perpendicular, the ray will bend.

(like a pencil in a beaker of water)

This is called refraction

October 26, 2010

Why? Speed of light in a vacuum is close to the speed of light in air because light offers very little resistance.

However: when light travels through water, for example, it slows down.

October 26, 2010

The light bends toward the normal when it goes from a medium that slows light waves down (more dense)

October 26, 2010

Index of Refraction

n = cv

c = speed of light in a vacuumv = speed of light in a mediumn = index of refraction (the ratio)

ratio between the speed of light in a vacuum and speed of light in a trasparent medium

October 26, 2010

Table of indices of Refraction page 61 of old text

Example: Speed of light in a liquid is 2.25 x 108 m/s.What is the a) refractive index of the liquid?

a) c = 3.00 x 108 m/s, v = 2.25 x 108 m/s

Formula: n = c/v

Calculation: n = 3.00 x 108 m/s/2.25 x 108 m/s = 1.33

b) look on page 61 of text, what is the liquid according to the table of indices of Refraction?

October 26, 2010

When the index of refraction refers light passing between two transparent media

relative index of refraction

(n1→2 = n2 ÷ n1 = n2/n1)

October 26, 2010

Laws of Refraction: Sin iSin R

= n

Example:

The angle of incidence is 30.0o and the angle of

refraction is 24.0o. Calculate the index of refraction of the unknown medium.

sin 30o

sin 24o= n

n = 0.500/0.407 = 1.23

October 26, 2010

Java applet for refraction and reflection

http://lectureonline.cl.msu.edu/~mmp/kap25/Snell/app.htm

October 26, 2010

Snell's law is written

n 1sinθi = n 2sinθR

n1 is the index of refraction for the first medium

n2 is the index of refraction for the second medium

October 26, 2010

Use subscripts to indicate the different mediums.

Example

Light travels from crown glass (g) into water (w). The angle of

incidence in crown glass is 40.0o. What is the angle of refraction in water?(use the table to get indices of refraction p.61)

n gsinθg = n wsinθR

1.52 sin 40.0o = 1.33 sinθRsinθw = (1.52)(0.643)/1.33 = 0.735

θw = Sin-1(0.735)

θw = 47.3o

October 26, 2010

Total Internal Reflection and the Critical AngleAs the angle of incidence increases, the intensity of a reflected ray becomes progressively stronger. And... intensity of a refracted ray becomes progressively weaker.

θR

θi

θr

October 26, 2010

Total internal reflection

- occurs when light is leaving a denser medium and is travelling into a less dense medium and θi > the critical angle.

N

October 26, 2010

Total internal reflection

- occurs when light is leaving a denser medium and is travelling into a less dense medium and θi > the critical angle .

The critical angle is a constant between two media.

It is the value of θi when θR = 90o

a) θi < the critcial angle : a refracted ray (partial reflection).

b) θi = the critical angle : critical ray of 90o(to the normal).

c) θi > the critical angle : totally reflected ray (no refraction)

October 26, 2010

Total internal reflection

- occurs when light is leaving a denser medium and is travelling into a less dense medium and θi > the critical angle .

To calculate the critical angle

for any substance set the angle of refraction to 90o . Example: The light ray is travelling from diamond (n 1 = 2.42) to air (n 2 = 1.0003) n1sin ic = n2 sin R

n1 sinθi = n2sinθR

2.42 (sinθi ) = 1.0003 ( sin 90o)

2.42 (sin θi) = 1.0003 (1.0)

2.42(sin θi ) = 1.0003

sin θi = 0.4133 θi = sin-1(0.4133) i = 24.4 o

October 26, 2010

PracticeintextbookP.78‐79#16,17,23,30,36,37,38,39,40,42,45,50(3mediumstherefore2steps)

Notetheanswerstomanyofthequestionsareintheback.

y < -1/3x + 12y < 2x y > 5x < 9

October 26, 2010

Parabolic mirror: http://www.youtube.com/watch?v=Ij1YvJT5tLQ&feature=related

Classroom demo: reflection and images:http://www.youtube.com/watch?v=mYF47078iis&feature=related

October 26, 2010

Challenge of the week:Sherlock Holmes: case of the disappearing jewelrySuspect is believed to be planning a heist.You are hiding behind a desk, and have 3 flat mirrors set up to catch the thief in the act. Show how you might set them up in order to "see" the thief in action

Facts: room is 8.5 x 11 (size of paper)the desk is in one corner and the table with the jewelry is on the other side of the desk

you must use all 3 mirrors (hint: use laws of reflection, ruler and protractor)

October 26, 2010

Collecting data for index of refraction

θi θR n =

Hand in one table per group

n

today's challenge

JELLO CHALLENGE

October 26, 2010