Total Internal Reflection Learning Objectives: All- understand what total internal reflection is Most- Understand how to work out critical angle Some- Know what endoscopy is P1 E – Part 2 • Rebounding Light Optical Fibre • I have in front of me a light source. • Does any one know the name of the light source? • This is known as a fibre optic or an optical fibre. • What is different about this light source? • In an optical fibre the end of the light is bright because the light is bouncing around inside the fibre until it gets to the end. • Draw an optical fibre and show me how you think the ray of light travel along it. – Can Do Task – 1 Point How does light travel? • Light Travels in a straight line. • Reflection is found when light rebound of a mirror. • Refraction happens when light travels from one material to another. The ray of light is no longer straight. • Activity 1. • Set up your ray box. • Draw a line on the piece of paper, showing light travelling in a straight line. • Put a mirror in front of the light beam and draw the light being reflected • Put a piece of glass in front of it and show the light being refracted. • Label your lines, Reflection and Refraction. Labelling up • Label your diagram, with the normal, perpendicular to the mirror, measure your angle of incidence and your angle of reflection using the protractor. Labelling Up. Draw the Normal again, measure your angle of incidence, measure the angle of refraction. Was your angle of incidence larger or smaller than the angle of refraction. Refraction • Light normally travels in straight lines, but it can bend at the boundary between two materials with different densities. Reflection or Refraction • Decide which is which from below. • Reflection, Angle of Incidence, Angle of Reflection, Angle of Refraction A D E B C Critical Angle • Waves going from a dense medium (e.g. Perspex) to a less dense medium (e.g. air) speed up at the boundary. • This causes light rays to bend when they pass from glass to air at an angle other than 90°. • Beyond a certain angle, called the critical angle, all the waves reflect back into the glass. We say that they are totally internally reflected. Critical Angle • Can do Task – Calculate the critical angle of your block of perspex. – 3 Points • Draw your block and draw your normal. • Draw a line 30 degrees from the normal. • Draw a line 60 degrees from the normal. • Put your light source on the thirty line and move it to the sixty slowly. • Stop when the light ray is refracted by 90 degres. • Record the angle on your piece of paper. You will only be awarded the three point task if you have got the correct angle. The Critical Angle Critical Angle • Have a go at calculating the critical angle for your piece of glass. • This is the angle of incidence when the angle of refraction becomes 90 degrees. • Once you have done this use the prisms to try and complete the sheet to figure how a periscope works. The angle of incidence at which maximum refraction occurs is called the critical angle, c. For water this is about 49°. For angles greater than this total internal reflection results: Total Internal Reflection internal, optical, little, bent, light, An -------- fibre is a thin rod of high-quality glass. Very ------ light is absorbed by the glass. ------ getting in at one end undergoes repeated total ---------- reflection, even when the fibre is -----, and emerges at the other end. Up Periscope • If you have two right angled prisms, you can make a periscope which reflects the light at right angles around a corner. • In your groups, have a go at making a persicope with your two perspex blocks. • Show me how it works. Submarines – 5 Min • Explain how total internal refraction might be used in submarines. • They are used in telecommunications to transmit telephone conversations and data • Whilst traditional copper wires can be used (and indeed are for the first 500 m or so from your phone) they cannot carry as much information and suffer from magnetic interference Endoscopy • Optical fibres are used in endoscopes that allow surgeons to see inside their patients. • The endoscope is a long flexible tube that can be swallowed. • It has a camera and light inside it.
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