16/05/2016. Book page 110 and 112 Syllabus 3.18, Snell s Law. cgrahamphysics.com 2016

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1 16/05/2016 Snell s Law cgrahamphysics.com 2016 Book page 110 and 112 Syllabus 3.18, 3.19

$diffracts$ $refracts$ reflects

2 Match the words to the objects absorbs transmits emits diffracts disperses refracts reflects Fibre optics Totally internally reflects Mirror Stained Glass

$What are the properties of reflection, refraction and diffraction?$ The change of the direction of a light ray when it enters a glass

The spreading of waves when they pass through a gap is an example of

$reflection, refraction, diffraction A.$ Long radio waves can bend around obstacles such as hills B.

3 What are the properties of reflection, refraction and diffraction? Starter: copy out and complete the gaps i. The change of the direction of a light ray when it enters a glass block from air is an example of ii. The spreading of waves when they pass through a gap is an example of iii. The image of an object seen in a mirror is formed because the mirror causes light from the object to undergo Key words: reflection, refraction, diffraction A. Long radio waves can bend around obstacles such as hills B. Your remote control will still work if you point it away from the TV towards a flat, sold object, e.g. a plate C. The bottom of a swimming pool looks nearer than it actually is

4 What are the properties of reflection, refraction and diffraction? Starter: copy out and complete the gaps i. The change of the direction of a light ray when it enters a glass block from air is an example of refraction ii. The spreading of waves when they pass through a gap is an iii. example of diffraction The image of an object seen in a mirror is formed because the mirror causes light from the object to undergo reflection A. Long radio waves can bend around obstacles such as hills diffraction B. Your remote control will still work if you point it away from the TV towards a flat, sold object, e.g. a plate reflection C. The bottom of a swimming pool looks nearer than it actually is refraction /6 1/6 or 2/6 = RED 3/6 or 4/6 = YELLOW 5/6 or 6/6 = GREEN

5 Aim Describe experiments to investigate the refractive index Know and use Snell s law Know the effect the medium has on the light waves traveling through it Key words Refractive index Snell s Law

$refraction$

6 The big picture Diabetic eye disease Have you ever wondered Can reflection and refraction effects save lives? Eye cancer

7 Changing Speed When light rays enter a more optically dense medium: Speed decreases Wavelength decreases Frequency remains constant Light ray bends towards the normal

8 continued When light rays enter a less optically dense medium: Speed increases Wavelength increases Frequency remains constant Light ray bends away from the normal

9 When light rays enters a medium of different optical density along the normal - it does not change direction. continued

10 Links What is the link between angle of incidence and angle of reflection? What property of the glass block will determine how much the light is refracted? Refractive Index n = sin i sin r Sine of the angle of incidence Sine of the angle of refraction

$How can you find the index of refraction experimentally?$

11 How can you find the index of refraction experimentally? This is a common exam questions You need to describe exactly what you do

12 Answer Practical questions on experiments musts You must discuss: 1. What you would change (independent variable), what you would measure (dependent variable). 2. What instruments would you use and how would you use them? How would you use them carefully. 3. Plot these results on a graph. What does the gradient give? 4. What would you repeat and why? 5. Mention several precautions you will make so that the results are as accurate as possible.

13 Method Determining Refractive Index Experiment Draw around the glass block Mark positions of incident and emerging rays using a pencil or two pins Remove block and draw refracted ray inside the rectangle Measure the incident angle i between the incident ray and the normal Measure the refracted angle between the refracted ray r and the normal Measure the same two angles for different angles of incidence

14 Data Draw a graph of sin i vs sin r Label the axis and draw line of best fit The gradient gives the refractive index Improvements to the method: Take repetitions and find average, mark light rays on paper, use a thinner beam, fix block firmly in position, remove anomalies, use a sharper pencil, use a more precise protractor. Refractive Index of Glass Block using pins

15 Exam Question (5 marks) Explain how a student can use a glass block to find an accurate value for the refractive index of glass

16 Marking scheme MP1. mention ray box MP2. Use of protractor MP3. (vary i to) obtain a range of values; MP4. Statement of equation; n= (sin i) / (sin r ) MP5. a) plot a graph of sin i against sin r OR b) calculate/ work out/ find n MP6. a) find gradient of graph OR b) calculate average n MP7. sensible precaution or improvement Including( use ruler for lines, use thinner beam, single colour, disregard anomalous readings, use sharp pencil, more precise protractor)

17 The speed of light Vacuum Water Perspex Light travels at km/s in a vacuum, as it enters denser media the speed of light decreases. Looking at the chart, which do you think is denser, Perspex or water? Perspex must be denser because light travels slower through Perspex than water. As the speed of light varies depending on the medium, different materials refract light by different amounts Speed of light (thousands km/s)

18 Refractive index Refractive index is a measure of how much a substance slows down light. The higher its value, the slower light travels in that medium. Refractive index for a medium is calculated by comparing the speed of light in a vacuum to the speed in that medium: speed of light in vacuum refractive index = speed of light in medium n = c v The refractive index of air is 1 because light travels at nearly the same speed in air as it does in a vacuum.

19 Using refractive index

$Predicting refraction The refractive index of air is 1 and that of diamond is 2.4. What happens when light goes from diamond into air? Diamond has a higher refractive index than air.$

20 Predicting refraction The refractive index of air is 1 and that of diamond is 2.4. What happens when light goes from diamond into air? Diamond has a higher refractive index than air. higher refractive index light travels more slowly This means light speeds up when it leaves the diamond. The light will bend away from the normal. The angle of incidence is less than or equal to the angle of refraction.

$refraction, θ2, is equivalent to the ratio of velocities in the two media.$

21 Snell s Law The general version of Snell's law states that the ratio of the sines of the angles of incidence, θ1, and refraction, θ2, is equivalent to the ratio of velocities in the two media. sinq 1 sinq 2 = v 1 v 2 = n 2 n 1 n 1 sin θ 1 = n 2 sin θ 2

$Snell s Law - Question Question A light ray enters a block of glass as shown. Determine the angle of refraction.$

22 Snell s Law - Question Question A light ray enters a block of glass as shown. Determine the angle of refraction. Solution n 1 sin θ 1 = n 2 sin θ 2 1 sin 35 = 1.5 sin θ 2 sin θ 2 = θ 2 = sin sin sin =

23 Refraction in Real Life Refraction can make fishing with a spear difficult!

24 Key words Refractive index - measure of the bending of a ray of light when passing from one medium into another. If i is the angle of incidence of a ray in vacuum and r is the angle of refraction, the refractive index n is defined as the ratio of the sine of the angle of incidence to the sine of the angle of refraction; Snell s Law: n 1 sin θ 1 = n 2 sin θ 2

Reflection & refraction

$Reflection & refraction$ 2015 EdExcel A Level Physics 2015 EdExcel A Level Physics Topic Topic 5 5 Reflection & refraction Reflection revision Reflection is the bouncing of light rays off a surface Reflection from a mirror: Normal