The World Communicates – Focus 4

 Weekly Reading Chapter 4 – Sections 4.1 and 4.2 (Reflection and Refraction)  Class Quiz on Friday 20 th 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

 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

The ‘normal’ is shown in red and is perpendicular to the surface at the point of reflection.

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

 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

 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?

 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.

 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.

 Again parabolic reflectors are used in the transmission of microwaves.

 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.

 Used in safety mirrors – they give a wider field of view and allow viewers to see around corners.

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

 Weekly Reading Chapter 4 – Sections 4.1 and 4.2 (Reflection and Refraction)  Class Quiz on Friday 20 th 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.



 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.

 We have learnt that the speed of light and all other electromagnetic waves is very fast – 3.0 x 10 8 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. MediumSpeed of EM waves Vacuum3 x 10 8 Air2.999 x 10 8 Water2.26 x 10 8 Crown Glass1.97 x 10 8 Perspex2 x 10 8 Diamond1.24 x 10 8

 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  v 1 = velocity of wave in medium 1  v 2 = velocity of wave in medium 2

1. A ray of light enters water from air at an angle of incidence of 40 . i. Draw a ray diagram (to scale) to show the path of the ray through the water. ii. Are the rays refracted towards or away from the normal. 2. 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. MediumSpeed of EM waves Vacuum3 x 10 8 Air2.999 x 10 8 Water2.26 x 10 8 Crown Glass1.97 x 10 8 Perspex2 x 10 8 Diamond1.24 x 10 8

 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: MediumSpeed of EM wavesAbsolute Refractive Index Vacuum3 x 10 8 Air2.999 x 10 8 Water2.26 x 10 8 Crown Glass1.97 x 10 8 Perspex2 x 10 8 Diamond1.24 x 10 8

 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? MediumSpeed of EM wavesAbsolute Refractive Index Vacuum3 x Air2.999 x Water2.26 x Crown Glass1.97 x Perspex2 x Diamond1.24 x

 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.

 Identify the conditions necessary for a wave to be refracted:  Towards the normal  Away from the normal  At an angle of zero degrees.

 Yes - A difficult concept – requires lots of practice!  Next lesson – Refraction Prac + more practice questions

 Sample Data*:  * - thank you Nicole and Melanie Angle of incidence (  )Angle of refraction (  )

Angle of incidence (  ) Sin iAngle of refraction (  ) Sin r A graph of sin r (y-axis) vs sin i (x axis) should be a straight line whose gradient = n 1 /n 2

 The gradient should be used to calculate the refractive index of the perspex.  From the equation of the line the gradient =  We know that the refractive index of air is  Hence:

 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

 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?

 Weekly Reading Remainder of Chapter 4  Assessment Task – 1 st April