Total Internal Reflection

When light travels through one medium and is incident on a second medium some of that light is reflected off of the second mediums surface and some of the light transmits the second medium and becomes refracted – This is known as partial refraction

Review of Refraction Light travelling from one medium to another is both reflected and refracted. Light slows down when it travels from air to another medium due to the presence of molecules. Light will bend: toward the normal when it slows down (angle of refraction is smaller than angle of incidence) away from the normal when it speeds up at the boundary of two media.

Refraction cont’d Light will bend away from the normal when it speeds up at the boundary of two media. As the angle of incidence increases, the angle of refraction will also increase. n1 > n2 Air n2= 1.00 Water n1 = 1.33

Refraction cont’d Light will bend away from the normal when it speeds up at the boundary of two media. As the angle of incidence increases, the angle of refraction will also increase. n1 > n2 Air n2= 1.00 Water n1 = 1.33

Refraction cont’d Light will bend away from the normal when it speeds up at the boundary of two media. As the angle of incidence increases, the angle of refraction will also increase. n1 > n2 Air n2= 1.00 Water n1 = 1.33

Refraction cont’d n1 > n2 Air n2= 1.00 Water n1 = 1.33 Light will bend away from the normal when it speeds up at the boundary of two media. As the angle of incidence increases, the angle of refraction will also increase. will bend away from the normal when it speeds up at the boundary of two media n1 > n2 Air n2= 1.00 Water n1 = 1.33

Refraction cont’d Light will bend away from the normal when it speeds up at the boundary of two media. As the angle of incidence increases, the angle of refraction will also increase. n1 > n2 Air n2= 1.00 Water n1 = 1.33 angle of incidence < angle of refraction

Refraction cont’d Light will bend away from the normal when it speeds up at the boundary of two media. As the angle of incidence increases, the angle of refraction will also increase. n1 > n2 Air n2= 1.00 Water n1 = 1.33 angle of incidence < angle of refraction the angle of refraction will continue to increase as the angle of incidence increases until it becomes 90o

Refraction cont’d Light will bend away from the normal when it speeds up at the boundary of two media. As the angle of incidence increases, the angle of refraction will also increase. n1 > n2 Air n2= 1.00 Water n1 = 1.33 angle of incidence < angle of refraction the angle of refraction will continue to increase as the angle of incidence increases until it becomes 90o

Refraction cont’d Light will bend away from the normal when it speeds up at the boundary of two media. As the angle of incidence increases, the angle of refraction will also increase. n1 > n2 Air n2= 1.00 Water n1 = 1.33 angle of incidence < angle of refraction the angle of refraction will continue to increase as the angle of incidence increases until it becomes 90o

Refraction cont’d Light will bend away from the normal when it speeds up at the boundary of two media. As the angle of incidence increases, the angle of refraction will also increase. n1 > n2 Air n2= 1.00 Water n1 = 1.33 angle of incidence < angle of refraction the angle of refraction will continue to increase as the angle of incidence increases until it becomes 90o

Refraction cont’d The angle of incidence that produces a refracted angle of 90o is called the critical angle.

Refraction cont’d The angle of incidence that produces a refracted angle of 90o is called the critical angle. n1 > n2 Air n2= 1.00 Water n1 = 1.33

Refraction cont’d The angle of incidence that produces a refracted angle of 90o is called the critical angle. n1 > n2 Air n2= 1.00 Water n1 = 1.33 Critical angle (48.8o)

Total Internal Reflection If you increase the angle of incidence past the critical angle, the refracted ray will no longer exit the medium.

Total Internal Reflection If you increase the angle of incidence past the critical angle, the refracted ray will no longer exit the medium The ray will reflect back into the medium n1 > n2 Total internal reflection Air n2= 1.00 Water n1 = 1.33 larger than critical angle

Total Internal Reflection The critical angle occurs when the angle of refraction equals 90º. If you increase the angle of incidence past the critical angle, the refracted ray no longer exits in the second medium. This is called total internal reflection.

Conditions for Total Internal Reflection Light is travelling slower in the first medium than in the second. The angle of incidence is large enough that no refraction occurs in the second medium. Instead the ray is reflected back into the first medium.

Total Internal Reflection If the light is incident at an angle larger than the critical angle qc, total internal reflection will occur. And remember that nr will be 1.0 if the second medium is air.

Total Internal Reflection: Example 1 When light is travelling through glass into air, the total internal reflection will occur at a critical angle of 42o. Find the index of refraction of the glass.

Total Internal Refraction: Example 1 When light is travelling through glass into air, the total internal reflection will occur at a critical angle of 42o. Find the index of refraction of the glass.

Normal Air Glass n = 1.0 n = ? Total Internal Reflection

Total Internal Reflection: Example 1 When light is travelling through glass into air, the total internal reflection will occur at a critical angle of 42o. Find the index of refraction of the glass. Using Snell’s Law…

Total Internal Reflection: Example 2 Use Snell’s Law to calculate the critical angle for total internal reflection of light transmitting through water going to air.

Total Internal Reflection: Example 2 Therefore, the critical angle for light traveling through water into air is 48.8 degrees. This means that any incident ray with an angle greater than 48.8 degrees will entirely reflect off of the surface of the water and not transmit to the air.

Real Life Use of Total Internal Reflection Why Diamonds Sparkle?

Total internal reflection applied Diamonds

Total internal reflection applied Diamonds High refractive index -> very small critical angle (24.4o)

Total internal reflection applied Diamonds High refractive index -> very small critical angle (24.4o) Much of the incident light undergoes total internal reflection inside the diamond

Total internal reflection applied Diamonds High refractive index -> very small critical angle (24.4o) Much of the incident light undergoes total internal reflection inside the diamond Cause of “sparkling” effect

Total internal reflection applied Fibre Optics

Total internal reflection applied Fibre Optics Technology that uses light to transmit information along a glass cable

Total internal reflection applied Fibre Optics Technology that uses light to transmit information along a glass cable Cable has a small critical angle so that light entering it will have an angle of incidence greater than the critical angle

Fibre Optics Made from a glass core, surrounded by an optical cladding which is a covering, like a sleeve The fibre core and the cladding are made of different types of glass The cladding must have a lower index of refraction than the core to facilitate total internal reflection Used in telecommunication and medicine (endoscopes)

Fibre Optics

Triangular Prisms Exhibit total internal reflection. Reflect almost 100% of light internally.

The two orientations can be as follows: 90 degrees 180 degrees

Prisms Used in: Binoculars Periscopes Cameras 39

Retro-reflectors When the ray gets reflected back in the exact same direction as it’s original incident ray it is called a retro reflector. Occurs as a result of two total internal reflections. Eg. Road signs, Reflective strips on clothing, bike reflectors 40

Your Task On a piece of paper: 1) Determine the critical angle of water and plastic using semi-circular containers 2) Carry out the procedure, and record your data 3) Use the critical angle to determine the index of refraction of the material.