Common Test on Tuesday Next Week

Starter (2 min) The angle of incidence is 67 o. The optical density of the first medium is 1.33 and the optical density of the second medium is Calculate the angle of refraction.

Critical Angle and Total Internal Reflection

Try this!!! The angle of incidence is 75 o. The optical density of the first medium is 1.46 and the optical density of the second medium is Calculate the angle of refraction.

n 1 = 1.00 n 2 = 1.50

n 1 = 1.00 n 2 = 1.50

n 1 = 1.50 n 2 = 1.00

n 1 = 1.50 n 2 = 1.00

From a less optically dense medium to a more optically dense medium θ 1 > θ 2 Bends towards the normal θ 1 < θ 2 Bends away from the normal From a more optically dense medium to a less optically dense medium n 1 < n 2 n 1 > n 2

Critical Angle & Total Internal Reflection

Critical Angle Critical angle is the angle of incidence of the light which refracts at right angle (90 o ) when entering a less optically dense medium. It is the largest angle of incidence for which refraction can still occur. The refracted ray travels along the boundary.

Critical Angle – key points n 1 > n 2 Light enters a less optically dense medium Light speeds up Light bends away from normal Angle of refraction (θ 2 ) = 90 o

How to calculate the critical angle Remember – the light travels into less optically dense medium which means: n 2 < n 1 (the light wave speeds up) θ 2 > θ 1 (the ray bends ‘away’ from the normal) And critical angle is the angle of incidence (θ 1 ) when the angle of refraction (θ 2 ) equals 90 o. Also note that the angle of refraction cannot be greater than 90 o Calculate θ 1 when n 1 > n 2 and θ 2 = 90 o

Examples: 1.Find the critical angle of the water/air interface. (n water = 1.33) 2.Find the critical angle of the glycerol/water interface. (n glycerol = 1.47)

What if you go beyond the critical angle?

Total Internal Reflection When the angle of incidence is larger than the critical angle of the boundary, 100% of the light is reflected and none is refracted. Can only happen when entering a less optically dense medium (i.e. n 1 > n 2 )

PRACTICAL - Finding Critical Angle 1 Use a lightbox to produce a ray of light travelling through a rectangular prism at an angle of about 45 o. Draw a diagram of what is happening in your book and answer the following questions: What happens when the ray enters the prism? What happens when the ray exits the prism? Increase the angle of incidence so the angle of refraction becomes 90 o. Measure this angle of incidence – what is this angle called? What happens when the angle is increased further?

PRACTICAL - Finding Critical Angle 2 Aim: to find the critical angles of water and perspex Method: 1.Shine a ray of light through water (water → air) 2.Adjust the angle to make the angle of refraction 90 o, then measure the angle of incidence 3.Repeat a) and b) using perspex Results: Critical angle of water = Critical angle of perspex =

Textbook Activity 7A and 7B Worksheet #4 Homework Booklet – “Reflection, Refraction and Snell’s Law” (up to Q15)

a)Calculate the size of the angle of refraction. b)Define “critical angle” c)Calculate the size of the critical angle at the plastic / air boundary. d)Define “total internal reflection” Plastic has a refractive index of 1.60 Water has a refractive index of 1.33 Water

Calculate these critical angles. Use the table of refractive index values given in your textbook. 1.ruby/ice interface 2.diamond/ethanol interface 3.ice/flint glass interface 4.Perspex/paraffin oil interface 5.crown glass/water interface

Starter The refractive index of water = 1.33 The refractive index of flint glass = 1.65 a)Calculate the size of the angle of refraction when a ray of light travels from flint glass into water at an angle of 25 o b)Calculate the critical angle of the water/flint glass interface.

n oil = 1.46 n ice = 1.31 oil ice 72 o a)Calculate the size of the angle of refraction. b)Calculate the size of the critical angle of the boundary.

Define: 1.Period 2.Frequency 3.Critical Angle 4.Total Internal Reflection