Demo Equipment
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Refraction
- Large prisms (triangle? semi-circle?), laser light source, ray light box
Notes: light description
- Speed in vacuum (c) = 3.0×108 m/s
- Light is a traveling wave (v = f λ)
- Light travels more slowly within matter.
Wave nature
- Medium = electric and magnetic fields
- Changing B at one location creates changing E, which creates changing B, ...
- frequency and wavelength describe properties of light
- Visible light = 700 nm (red) - 400 nm (violet)
Sound analogy
- Sound is a traveling wave.
- frequency ⇆ pitch that we hear. There is a measure, and a corresponding perception. For light, frequency corresponds to primary color.
- Different amounts of several frequencies can combine and travel together, and are perceived as differently. For light, combinations of frequencies produce non-primary colors.
Light in matter
- Index of refraction (n) = c / v
- c ≥ v, so n ≥ 1
- Glass has n = 1.5. What is the speed of light in glass? (2.0×108 m/s)
Dispersion
- Different wavelengths might travel at slightly different speeds.
- → index of refraction changes depending on wavelength.
Example: Snell's
- A glass block is submerged in water.
- Light travels through the water, and strikes the glass at a 45° angle to the normal of the glass's surface.
- What angle is the light refracted to within the glass block?
- Glass index = 1.55, water index = 1.33
Solution
- Snell's: ni sin(θi) = nt sin(θt)
- i = water, t = glass
- 1.33 sin(45°) = 1.55 sin(θt)
- sin(θt) = 0.607
- θt = 37.4°
Demo: Total internal reflection
- Semi-circular prism next to white board.
- Ray-light box used to show the path of rays within the prism.
Observations 1
- Shine a ray through the center of the flat side
- Consider the incident ray to be the ray traveling through air before it hits the prism.
- → Transmitted ray = ray traveling within the prism.
- As I make the angle to the normal larger, what happens to the refracted ray? (also gets further from the normal)
- Which is larger, θi or θt? (θi, since ni is smaller)
- Get a student to trace 3 rays, including largest θ.
Observations 1
- Shine rays back along tracing.
- What do you observe? (rays travel back the way they came in part 1)
- Consider incident ray to be the one traveling through the prism.
- → transmitted = ray traveling through air, after the prism.
- Which is larger, θi or θt? (θt, since ni is larger)
- What happens if I make θi a little larger than the largest already traced? (no more light is refracted. Instead, it is all reflected at the flat surface.)
Determining θc
- ni = 1.5, nt = 1, what is θt? (90°)
- Determine θi (42°)
Demo: Dispersion
- Triangular prism
- Ray light box shines white light.
- I angle the ray light box so that the ray which exits the last face is close to the boundary.
- Include a diagram
Observation
- What do you see? (rainbow)
- Which light is bent furthest from the normal to the last boundary? (violet)
- Which color has the highest index of refraction? (violet - nt is same for all colors, θi is the same for all colors, so larger θt→ larger ni)
- Rainbow = "primary" colors. Each point on the rainbow corresponds to a different wavelength of light, decreasing as you go from red to violet.
- All other colors are combinations of the rainbow colors.
- Note: the green in a rainbow is NOT blue + yellow. It is primary.