SNC2D PHYSICS 5/20/2013. LIGHT & GEOMETRIC OPTICS L Total Internal Reflection (TIR) (P ) Total Internal Reflection (TIR)

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1 SNC2D PHYSICS LIGHT & GEOMETRIC OPTICS L (P ) Sometimes, such as in the case of fibre optics or laser light travelling from water into air, the light does not pass from one medium to another but stays within the medium. Why? May 20, DPHYS - Total Internal Reflection 1 Recall that when a light ray travels from water (the slow medium) into air (the fast medium) at an angle, it speeds up and bends (refracts) away from the normal. As a result, the angle of refraction (R) is greater than the angle of incidence (i). May 20, DPHYS - Total Internal Reflection 2 1

2 What happens if the angle of incidence keeps getting larger? Eventually, the angle of refraction would reach 90E the refracted light ray grazes along the boundary between the two media. The angle of incidence that produces an angle of refraction of 90E is called the critical angle. CRITICALANGLE(2 C ) angle of incidence (i) that produces an angle of refraction (R) of 90E May 20, DPHYS - Total Internal Reflection 3 PRACTICE 1. Calculate the critical angle for light travelling from lucite (n L = 1.97) into water (n W = 1.33). 2 C = 42E May 20, DPHYS - Snell's Law 4 PRACTICE 2. Calculate the critical angle for light travelling from diamond (n D = 2.42) into ice (n I = 1.31). 2 C = 33E May 20, DPHYS - Snell's Law 5 2

3 NOTE! When the angle of incidence is greater than the critical angle, the light ray is no longer refracted. Instead, it is reflected back into the water. In a way, the light is trapped inside the water. This is called total internal reflection. May 20, DPHYS - Total Internal Reflection 6 PRACTICE 3. What is the critical angle for light travelling from water to air? 2 C = 48.8E May 20, DPHYS - Total Internal Reflection 7 PRACTICE 4. Is there a critical angle for light travelling from air to water? no because i > R May 20, DPHYS - Total Internal Reflection 8 3

4 NOTE! Total internal reflection only occurs when: 1. Light is travelling from a slow medium into a fast medium. i.e. L n 1 > n 2 May 20, DPHYS - Total Internal Reflection 9 NOTE! Total internal reflection only occurs when: 2. The angle of incidence is greater than the critical angle for the material. i.e. L i > 2 c May 20, DPHYS - Total Internal Reflection 10 TOTAL INTERNAL REFLECTION(TIR) reflection of light at the boundary between two transparent media only occurs when: Î n 1 > n 2 7 the light is travelling from a slow medium into a fast medium (i.e. the light speeds up and bends away from the normal so R > i) Ï i > 2 c 7 the angle of incidence is greater than the critical angle for the medium May 20, DPHYS - Total Internal Reflection 11 4

5 Applications of TIR Diamonds One of the features that make diamonds so attractive in jewellery is the fact that they sparkle. This sparkling is due to the cut of the diamond faces, which, combined with the high index of refraction for diamond (2.42), result in the total internal reflection of light. May 20, DPHYS - Total Internal Reflection 12 Applications of TIR Diamonds The high refractive index means that diamonds have a very small critical angle (24.4E). So a great deal of incident light undergoes total internal reflection inside the diamond. A light ray can bounce around several times inside the diamond before eventually exiting through the top. This causes the sparkling effect that makes diamonds so appealing. May 20, DPHYS - Total Internal Reflection 13 Applications of TIR Fibre Optics Total internal reflection explains how light travels through a fibre-optic cable. Fibre optics, used extensively in the communications industry, is a technology that uses light to transmit information along a glass cable. As such, the light must not escape as it travels along the cable. May 20, DPHYS - Total Internal Reflection 14 5

6 Applications of TIR Fibre Optics Because the fibre is made of glass, which has a higher index of refraction than the surrounding medium, the cable has a small critical angle. So light enters the cable with an angle of incidence greater than the critical angle and, as a result of TIR, is completely reflected inside the fibre. May 20, DPHYS - Total Internal Reflection 15 Applications of TIR Triangular Prism A triangular prism also exhibits total internal reflection. The critical angle for glass is about 41E. If a prism is oriented in such a way that the angle of incidence is greater than 41E, total internal reflection will result. Prisms are much more useful to reflect light than mirrors because a prism reflects almost 100% of the light internally. May 20, DPHYS - Total Internal Reflection 16 Applications of TIR Triangular Prism Mirrors reflect most incident light but lose a little through absorption. Also, the silvered surface of mirrors deteriorates over time. For these reasons, most optical devices, such as cameras and binoculars, use prisms instead of mirrors. May 20, DPHYS - Total Internal Reflection 17 6

7 Applications of TIR Mirages Both total internal reflection and refraction play a role in forming a mirage. A mirage is an image of a distant object produced as light refracts through air of different densities. Since the light rays pass through layers of air with progressively lower indices of refraction, eventually the light is totally internally reflected. May 20, DPHYS - Total Internal Reflection 18 Applications of TIR So what looks like a puddle of water from the distance is actually an image of the sky being reflected into the eyes of the observer. NOTE! TIR is used in various optical devices or phenomena including: diamonds fibre optics triangular prisms (periscopes, binoculars, ) mirages May 20, DPHYS - Total Internal Reflection 19 Mirages DYK? The mirage on the left is classified as an inferior mirage. The mirage on the right is classified as a superior mirage. Can you see the difference? May 20, DPHYS - Total Internal Reflection 20 7

8 Mirages DYK? MIRAGE image of distant object produced as light refracts through air of different densities two types inferior and superior May 20, DPHYS - Total Internal Reflection 21 Mirages DYK? INFERIOR MIRAGE cool (more dense) air on top of warm (less dense) air TIR occurs image is formed in the ground SUPERIOR MIRAGE warm air on top of cool air TIR occurs image is formed in the sky May 20, DPHYS - Total Internal Reflection 22 U Check Your Learning 1. Explain why total internal reflection is only possible for light travelling from water (n = 1.33) into air (n = 1.00), and not for light travelling from air into water. You may want to use a diagram in your answer. in order for TIR to occur light must be travelling from a slow medium into a fast medium (i.e. n 1 > n 2 ) May 20, DPHYS - Total Internal Reflection 23 8

9 U Check Your Learning 2. What causes a diamond to sparkle? Use a diagram to help explain your answer. TIR i > 2 c May 20, DPHYS - Total Internal Reflection 24 U Check Your Learning 3. With the aid of diagram explain how a mirage is formed. see notes May 20, DPHYS - Total Internal Reflection 25 U Check Your Learning WIKI(PHYSICS) O... 2DPHYS - WS5 (Total Internal Reflection) O... 2DPHYS - QUIZ3 (Refraction & TIR) May 20, DPHYS - Total Internal Reflection 26 9