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Refraction 1. Explain how refraction is the result of a change of speed 2. Explain what critical angle and T.I.R.. mean 3. Explain how optical fibres use T.I.R. Refraction Waves are refracted when they change speed e.g. when they travel from deep (fast) to shallow (slow) water. Wavelength λ When this happens: 1. Wavelength gets shorter 2. They change direction Watch the simulation in this link for two minutes, then answer the following questions... 1. Do the waves speed up or slow down as they move from the light blue medium to the dark blue medium? They slow down as they move from the light blue to the dark blue medium. 2. Do the waves change direction and what is this called? Yes, the waves do change direction. This is called refraction. 3. Where do the waves change direction? The waves change direction at the boundary. Then they travel in straight lines. 4. What can you say about the wavelength of the waves? The wavelength is shorter in the dark blue medium. 5. Is some of the wave energy reflected as well as refracted? Yes, some of the wave energy reflects (bounces) off the boundary as well as being refracted (changing direction at the boundary.) 1.Place the block in the middle of a sheet of A4 paper. 2.Draw an outline around it 3. Place the block in the middle of a sheet of A4 paper. 4. Draw an outline around it 5. Take the block away 6. Find the middle of the straight edge The NORMAL Line 7. Draw a 90º line through this point – this is the normal line 8.Put the block back 9.Direct a laser beam as shown To Answer: • Is the beam crossing from glass to air or air to glass? • Is the beam bent away from or towards the normal? The NORMAL Line • Is this because it’s slowing down or speeding up? Sand (slow) Concrete (fast) Sand (slow) Concrete (fast) Sand (slow) Concrete (fast) Back to start Sand (slow) Concrete (fast) When waves cross from fast to slow, they are refracted towards the normal. 10.Direct a laser beam as shown To Answer: • Is the beam crossing from glass to air or air to glass? • Is the beam bent away from or towards the normal? The NORMAL Line • Is this because it’s slowing down or speeding up? Sand (slow) Concrete (fast) Sand (slow) Concrete (fast) Sand (slow) Concrete (fast) Back to start Sand (slow) Concrete (fast) When waves cross from slow to fast, they are refracted away from the normal. 11.Gradually make the angle of incidence larger until the angle of refraction is 90º i Note if there is a reflected ray present as well r The NORMAL Line This is called the critical angle. Refraction does not happen after this point. c The NORMAL Line After this, the light is TOTALLY INTERNALLY REFLECTED (T.I.R.ed) The beam is T.I.R.ed* when the incident angle i is larger than the critical angle c. i The reflected angle r is always equal to the incident angle i. r The NORMAL Line * T.I.R.ed = totally internally reflected Copy the 2nd para and 3 diagrams on p.158 Summary c Air into glass Glass into Air Critical Angle T.I.R. Light bent towards the normal Light bent away from the normal Light is refracted at 90º Total Internal Reflection Light speeds up This is the last angle at which refraction happens. (T.I.R. after this.) Light slows down Optical Fibres is guided by T.I.R . . . Light in at this end . . . and comes out here. Optical fibres are used in communications to carry signals. (The signals are pulses of laser light) Optical fibres are used in medicine to look inside the body. An endoscope is made of a bunch of optical fibres to carry light into and out of the body.