Refraction Explain how refraction is the result of a change of speed Explain what critical angle and T.I.R.. mean 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. When this happens: Wavelength gets shorter They change direction Wavelength λ Wavelength λ

Do the waves change direction and what is this called? Watch the simulation in this link for two minutes, then answer the following questions... Do the waves speed up or slow down as they move from the light blue medium to the dark blue medium? Do the waves change direction and what is this called? Where do the waves change direction? What can you say about the wavelength of the waves? Is some of the wave energy reflected as well as refracted? They slow down as they move from the light blue to the dark blue medium. Yes, the waves do change direction. This is called refraction. The waves change direction at the boundary. Then they travel in straight lines. The wavelength is shorter in the dark blue medium. Yes, some of the wave energy reflects (bounces) off the boundary as well as being refracted (changing direction at the boundary.)

Place the block in the middle of a sheet of A4 paper. Draw an outline around it

Place the block in the middle of a sheet of A4 paper. Draw an outline around it Take the block away Find the middle of the straight edge Draw a 90º line through this point – this is the normal line The NORMAL Line

Direct a laser beam as shown Put the block back 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? Is this because it’s slowing down or speeding up? The NORMAL Line

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.

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? Is this because it’s slowing down or speeding up? The NORMAL Line

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.

Note if there is a reflected ray present as well Gradually make the angle of incidence larger until the angle of refraction is 90º Note if there is a reflected ray present as well i r The NORMAL Line

This is called the critical angle. Refraction does not happen after this point. After this, the light is TOTALLY INTERNALLY REFLECTED (T.I.R.ed) c The NORMAL Line

The reflected angle r is always equal to the incident angle i. The beam is T.I.R.ed* when the incident angle i is larger than the critical angle c. The reflected angle r is always equal to the incident angle i. * T.I.R.ed = totally internally reflected i r The NORMAL Line

Summary Copy the 2nd para and 3 diagrams on p.158 c Air into glass Light bent towards the normal Light slows down Glass into Air Light bent away from the normal Light speeds up Critical Angle Light is refracted at 90º This is the last angle at which refraction happens. (T.I.R. after this.) T.I.R. Total Internal Reflection

Optical Fibres is guided by T.I.R . . . and comes out here. Light in at this end . . . 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.