Refraction Total Internal Reflection Dispersion

Activity: Watching a filling bucket 1.Place a bucket on the floor and put an object in the centre of the bucket. 2.Students start off closer to the bucket and walk backward until they just can NOT see the object. 3.Fill the bucket slowly with water and observe!

Refraction Light travels fastest (3 x 10 8 m/s) through empty space, which is a VACUUM (the absence of a medium). Light travels more SLOWLY through transparent media like water, glass and plastic, COMPARED TO AIR. When light travels from one medium to another, its SPEED changes. This causes the light to BEND, or REFRACT, at the boundary between the two media.

Wave Model

Ray Model ALWAYS MEASURE ANGLES FROM THE NORMAL!

Activity: Student Wavefront Refraction 1.Move to an open area. 2.Half the class links arms to represent a wavefront. 3.They walk slowly from a fast to slow “medium” to a few different angles. 4.They repeat the procedure, but going from a slow to fast “medium”. 5.The other have of the class makes observations. 6.The two halves switch and repeat.

Rules of Refraction When travelling from a less dense (fast) medium into a more dense (slow) medium, light bends TOWARDS the normal. Example: from air into glass. ANGLE OF REFRACTION GETS SMALLER When travelling from a more dense (slow) medium into a less dense (fast) medium, light bends AWAY from the normal. Example: from water into air. ANGLE OF REFRACTION GETS BIGGER

The Index of Refraction (p. 437) Optical density is a physical property of a TRANSPARENT medium called the INDEX OF REFRACTION (n). The refractive index of the speed of light in a vacuum is assigned a value of ONE. The refractive index of a transparent medium (n) is determined by comparing the speed of light in the medium (v) with the speed of light in a vacuum (c) using the following formula: n = c/v Refractive index has NO units!

Wave Energy at Boundary When a wave encounters a boundary between two media with different WAVE SPEEDS, some of the energy of the incident wave is TRANSMITTTED and REFRACTED. The rest of the energy is REFLECTED. (ap-physics.david-s.org/wp-content/uploads/2012/11/reflection.png)

Wave Energy at Boundary The SMALLER the angle of incidence, the more energy is transmitted and less is reflected. Use the following simulation to demonstrate this effect (can also be used for total internal reflection) phet.colorado.edu/sims/html/bending-light/latest/bending-light_en.html

medium 1 medium 2 medium 1 medium 2 For each diagrams below, Label the normal, incident ray, reflected ray, refracted ray, angle of incidence ( i ), angle of reflection ( r ), and angle of refraction ( R ). Which medium is faster? Which medium is denser (ρ represents density)? Which has the higher index of refraction? incident ray reflected ray refracted ray i i r r R R normal Example 1 Example 2 v 1 > v 2 ρ 1 < ρ 2 n 1 < n 2 v 1 < v 2 ρ 1 > ρ 2 n 1 > n 2

Total Internal Reflection (p. 442) If a wave travels from a slow to a fast medium, the refracted ray bends AWAY FROM the normal. Therefore, at a certain angle of incidence, the refracted ray will be PARALLEL to the boundary between the two media and the angle of refraction will be 90 o. When this happens, all of the incident energy is REFLECTED and none of the energy is TRANSMITTED. The minimum angle at which this happens is called the CRITICAL ANGLE.

Total Internal Reflection (p. 442) (

Dispersion (p. 440) In certain media, the speed of light depends on the WAVELENGTH of the light travelling in it. This means that the INDEX OF REFRACTION will also depend on the wavelength of the light. Therefore, the ANGLE of refraction will also be different. If WHITE LIGHT is incident on a medium, the white light will “SPLIT UP” into the different colours of the visible spectrum. This phenomenon is called DISPERSION.

Dispersion (p. 440) Here, the speed of light in the glass decreases as the wavelength of light increases. This means the index of refraction increases as the wavelength increases.

Refraction is the Bending of Light Light moves (transmits) through transparent media A medium is more dense than a vacuum and will slow light down Each different transparent medium has a different optical density or refractive index (n = c/v) The higher the n value the more it slows light (see page 437)

Applications of Refraction Mirages: Rainbows: Apparent Depth: Fibre Optic Cables