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The World Communicates – Focus 4

The World Communicates – Focus 4. Many communication technologies use application of reflection and refraction of electromagnetic waves. Class Notices. Weekly Reading Chapter 4 – Sections 4.1 and 4.2 (Reflection and Refraction)

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The World Communicates – Focus 4

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  1. The World Communicates – Focus 4 Many communication technologies use application of reflection and refraction of electromagnetic waves

  2. Class Notices • Weekly Reading Chapter 4 – Sections 4.1 and 4.2 (Reflection and Refraction) • Class Quiz on Friday 20th on outcomes 21 – 29. For study you could complete Chapter 3 Review Questions. • Homework Check – Monday next week • Detecting the Bands • Electromagnetic Waves – Wrap Up • Reflection Prac

  3. Interaction of EM waves with matter • When EM waves (including light) interact with matter several things can happen, the waves maybe: • Transmitted • Scattered • Reflected • Refracted • Absorbed 30. describe and apply the law of reflection and explain the effect of reflection from a plane surface on waves

  4. Reflection of Waves • The Law of Reflection: • The angle of incidence is equal to the angle of reflection: i = r • The incident ray, the reflected ray and the normal are all in the same plane. The ‘normal’ is shown in red and is perpendicular to the surface at the point of reflection.

  5. Reflection from a plane • The “reflections” we are used to seeing occur off plane highly regular surfaces like a mirror. These are called specular reflections.

  6. Diffuse Reflections • Most objects do not have a perfect surface that reflects light uniformly. Most objects when examined closely have irregular surfaces. Light is still reflected bit in a non-uniform way. This enables us to see the surface but not specific images that are being reflected. • This is called Diffuse Reflection. • The Law of Reflection is still obeyed for each ray but because of the uneven surface the normals at each point on the surface are not parallel.

  7. Diffuse Reflections

  8. Reflections

  9. Concave Mirror • Concave mirrors reflect waves converging them at a focus in front of the mirror. Also known as a converging mirror. Some terminology: • The focus is the point where all rays are concentrated after reflection from a converging mirror. • If a concave mirror is thought of as being a slice of a sphere, then there would be a line passing through the centre of the sphere and attaching to the mirror in the exact centre of the mirror. This line is known as the principal axis. • The focal length of the mirror is the distance from the centre of the mirror to the focus

  10. Concave Reflectors - Applications • Concave mirrors produce a magnified image when the object being reflected is close to the mirror • Concave mirrors are often used as make-up or shaving mirrors so that details of the face can be seen more clearly on the magnified image produced by the concave mirror. • The photo on the left shows a concave mirror being used to ignite a block of wood. Why is careful placement of the wood required to make this work?

  11. Concave Reflectors - Applications • If a light source is placed in the focus of a concave reflector the light rays reflected of the mirror will be parallel creating a beam of light.

  12. Satellite Dishes • These dishes collect weak radio waves from Space. • The reflecting dish collects the waves and reflects them to the focus where the detector is located. This strengthens the weak signal.

  13. Microwave Transmission • Again parabolic reflectors are used in the transmission of microwaves.

  14. Convex Mirror • Convex Mirrors spread light out, they reflect light in such a way that it appears that the reflected light is diverging out from a point behind the mirror. Hence they are known as diverging mirrors. • The focus is the point behind the mirror from which the reflected rays appear to diverge.

  15. Convex Mirrors Applications • Used in safety mirrors – they give a wider field of view and allow viewers to see around corners.

  16. Reflection of waves of the ionosphere • Reflection of radio waves from the ionosphere can allow radio waves to be transmitted long distances around the globe.

  17.  For you to do

  18. Class Notices – Week 8 • Weekly Reading Chapter 4 – Sections 4.1 and 4.2 (Reflection and Refraction) • Class Quiz on Friday 20th on outcomes 21 – 29. For study you could complete Chapter 3 Review Questions. • Homework Check – Tomorrow • Detecting the Bands • Electromagnetic Waves – Wrap Up • Reflection Prac Notification will be issued later this week for your first Assessment Task in Week 10.

  19. Refraction – Bending the Light! • http://www.brainpop.com/science/energy/refractionanddiffraction/

  20. Refraction • Refraction is the bending of waves as they pass from one medium to another. • It occurs when the waves are incident on an interface at an angle except for the normal. • Refraction is caused by the change in speed of the waves as they cross the interface. • The density of the medium determined the speed of the waves. As density increases, speed decreases.

  21. But isn’t the speed of light, c? • We have learnt that the speed of light and all other electromagnetic waves is very fast – 3.0 x 108 ms-1 • This is the speed of EM waves in a vacuum. • When EM waves pass through other mediums such as air, water and glass they slow down very slightly as shown in the table.

  22. Snell’s Law • Snell’s law provides a mathematical relationship between the angle of incidence and angle of refraction of waves crossing an interface. • Where • i = angle of incidence • r = angle of refraction • v1 = velocity of wave in medium 1 • v2 = velocity of wave in medium 2

  23. Practice Questions • A ray of light enters water from air at an angle of incidence of 40. • Draw a ray diagram (to scale) to show the path of the ray through the water. • Are the rays refracted towards or away from the normal. • Light passes from a diamond into the air. If the angle of incidence of the light on the boundary was 15. Determine the angle of refraction.

  24. Refractive index • The refractive index of a material is a measure of the velocity of EM waves in that medium compared to a vacuum. • The absolute refractive index of a medium is determined by the following equation:

  25. Refractive Index • What does it actually mean: • Let’s consider diamond – its refractive index is 2.42 this means that light travels 2.42 times faster in a vacuum compared to diamond. • For Perspex light travels 1.46 times faster in a vacuum compared to in persex and so on..... • Question: What is the unit for refractive index?

  26. Snell’s Law and Refractive Index • Snell’s Law can be re-written using refractive index. • Light rays travelling through air (n=1.00) strike glass at an incident angle of 45. The angle of refraction is 25. Determine the refractive index of the glass. Challenge: Show this mathematically

  27. Snell’s Law • Identify the conditions necessary for a wave to be refracted: • Towards the normal • Away from the normal • At an angle of zero degrees.

  28. Snell’s Law • Yes - A difficult concept – requires lots of practice! • Next lesson – Refraction Prac + more practice questions

  29. Snell’s Law Prac – Some comments • Sample Data*: • * - thank you Nicole and Melanie

  30. Snell’s Law Prac – Some comments A graph of sin r (y-axis) vs sin i (x axis) should be a straight line whose gradient = n1 /n2

  31. Snell’s Law Prac – Some comments

  32. Snell’s Law Prac – Some comments • The gradient should be used to calculate the refractive index of the perspex. • From the equation of the line the gradient = 0.7109 • We know that the refractive index of air is 1.00. • Hence:

  33. Snell’s Law Prac – the discussion • Some confusion re: validity, reliability and accuracy. • VALIDITY – refers to whether the experiment was a fair test, related to how well all the other variables were controlled. This is a valid experiment, all other variables are easily controlled e.g. Same prism used, same light source used, same measuring devices used etc.

  34. Snell’s Law Prac – the discussion • Reliability – related to repetition. Questions to ask: • Was the experiment repeated a number of times and an average taken? • How close were the values from each repetition? • For this experiment – Ask yourself did you repeat the whole experiment several times to obtain a few different values of refractive index and then average them?

  35. Snell’s Law Prac – the discussion • ACCURACY – Two questions to ask yourself here... • How close was your calculated refractive index to the known refractive index of perspex. • Were there any sources of inaccuracy in your experiment and how could they have been eliminated/reduced. E.gerrrors in measurement of angles due to limitation of measuring equipment or spreading out of rays of light.

  36. Validity, Reliability and Accuracy of Secondary Sources • Secondary Information – any information that you do not find out from carrying out experiments. Includes information from: • Books (including textbooks) • Journals (New Scientist, Cosmos etc) • The internet • Teacher

  37. Secondary Sources • VALIDITY – Refers to whether the information is on the topic that is required. • Important when using search engines as not all hits will be equally valid for the research you are carrying out. • Validity can be assessed by considering whether the information from the source relates to the hypothesis or the problem you are solving. • For example the following page shows the results of a google search for GPS....

  38. A student researching the physics behind GPS technology – goggles' “GPS”. The following websites are returned. • Identify (as best you can from the blurbs) potentially valid and invalid sites for her task. • Suggest a method for improving the hit rate of potentially valid web sites?

  39. Secondary Sources • ACCURACY – Refers to whether the information from a secondary source is correct. • Accuracy can be assessed by comparing the information to other sources. • May also be checked by tracing the information back to the source. • E.g. If the information is referring to experimental work conducted by a researcher – finding the scientists original published report of this work could confirm the accuracy of the secondary source.

  40. Secondary Sources • Reliability – refers to whether the information is from a well recognised source. • When looking for reliable source look for: • Recognised sources in the field, organisation or scientists specialising in the area you are researching e.g an astrophysicist or NASA are likely to be more reliable sources of information on space exploration compared to Billy Smith’s Space Blog. • Recent information – scientific knowledge is updated rapidly. However take care – cutting edge science may not be as reliable as the tried and tested science. • Sources with author details and references to other reliable sources. • Consider whether the source may be biased.

  41. Class Notices – Week 9/10 • Weekly Reading Remainder of Chapter 4 • Assessment Task – 1st April

  42. Total Internal Reflection

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