11.2 Refraction. December 10, Wednesday, 11 December, 13

Transcription

1 11.2 Refraction December 10, 2013.

2 Refraction Light bends when it passes from one medium (material) to another this bending is called refraction this is because the speed of light changes

3 The Speed of Light the speed of light is 3.0 x 10 8 m/s (300 million!) the particles in a medium slow the waves of light down

4 The Index of Refraction the amount by which a transparent medium decreases the speed of light is indicated by the index of refraction the larger the index, the more the medium decreases the speed of light light travels fastest in a vacuum (no particles, like in space) the speed of light in this medium is assigned an index of refraction of 1.00 air and other gases are extremely close to 1.00 too Denser materials have higher index of refraction.

5 The Index of Refraction the refractive index (n), is determined by comparing the speed of light in the medium (v) with the speed of light in a vacuum (c) index of refraction = speed of light in vacuum speed of light in medium n = c/v

6 The Index of Refraction Practice Example: The speed of light in a sample of glass is 1.91 x 10 8 m/s. The speed of light in a vacuum is 3.0 x 10 8 m/s. What is the refractive index of this glass? index of refraction = speed of light in vacuum n = c/v speed of light in medium n = 3.00 x 10 8 m/s 1.91 x 10 8 m/s n = 1.57

7 How does slowing down bend light? compare light to a line of skaters

8 How does slowing down bend light? The first light waves to enter the water slow down Eventually when the entire wave has entered, the ray straightens out

9 From LOW to HIGH refractive index normal θi air water θr Light is bent towards the normal.

10 From HIGH to LOW refractive index normal θr air water θi Light is bent away from the normal.

11 Snell s Law Snell was a Dutch astronomer and mathematician who identified the relationship between the angle of incidence and the angle of refraction n1sinθ1 = n2sinθ2 n = index of refraction 1 = medium #1 2 = medium #2 θ1 = angle of incidence θ2 = angle of refraction

12 Snell s Law Practice Example: When light passes from air into water at an angle of 60 o from the normal, what it the angle of refraction?

13 From LOW to HIGH refractive index θi = 60 o normal air water θr =?

14 Snell s Law Practice Example: When light passes from air into water at an angle of 60 o from the normal, what it the angle of refraction? n1sinθ1 = n2sinθ x sin(60 o ) = 1.33 x sinθ x sin(60 o ) = sinθ = sinθ o = θ2

15 Total Internal Reflection light can reflect completely off the inside of the wall of a denser medium rather than passing through the wall into a medium with lower index of refraction when light passes from a denser medium to a less dense medium, light refracts away from the normal as θi increases, θr increases as well

16 Total Internal Reflection

17 Total Internal Reflection at the critical angle, the refracted ray follows a path exactly along the surface of the water and does not actually leave the water if the incident ray is increased even more, the ray is completely reflected back in the water

18 Total Internal Reflection

19 Practice Questions all Practice Problems on page 438 all Practice Problems on page 441 all Practice Problems on page 442 SHOW YOUR WORK! page 447 #5, 7, 8, 17, 18, 19