Refraction: bending of light at the interface of 2 different materials

Refraction In the Refraction lab you saw the laser light change angles when it traveled from air into water or oil. Light changes directions because it changes speed: it actually travels slower in water, glass, oil, etc.

Refraction of light Light direction changes when it slows down

Refraction To remember how light bends, use the “little red wagon” analogy. When traveling from concrete (fast material) onto thick grass (slow material), the front wheel hits the grass and slows down, causing the front axle to rotate. This makes the wagon change directions.

Refraction Wagon wheel turns at the interface

Refraction and Snell’s Law Snells Law: the “equation” for refraction n in sinΘ in = n out sinΘ out n = index of refraction = property of the material n water= 1.3 n glass = 1.5 n diamond = 2.4 n = speed of light in air / speed of light in other material n air = 1.0

Refraction & snell’s law Example: Light traveling in air hits a glass window at an incident angle of 30 degrees. What is the refracted angle in the glass? n in sinΘ in = n out sinΘ out 1.0 sin 30 = 1.5 sin Θ out Θ out = 19.5 degrees

Total Internal Reflection Total internal reflection happens when light is in “slow” material which is surrounded by “faster” material. An example is light traveling in plastic which has air surrounding it. Light leaving the plastic will bend away from the perpendicular. At a certain incident (incoming) angle (the critical angle) the outgoing light is at 90 degrees or would be traveling right along the interface between the 2 materials.

Total Internal Reflection, idea of the “critical angle” Picture of the “critical angle”

Total Internal reflection Idea of the “critical angle”

Total Internal reflection At the critical angle (incident) the outgoing angle is 90 degrees. Happens only when the light is trying to go from slow material (larger index of refraction) to faster material (smaller index of refraction)

Critical angle for light going from water into air Use Snell’s law with the outgoing angle = 90 degrees

Deeper or Shallower When you look at a fish swimming in a stream, does it look bigger or smaller? Does it look shallower or deeper in the water?

Fish under water It looks bigger to the “fisherman”

Reflection of Light by a flat mirror Light reflects “symmetrically” so that the incident light angle = reflected light angle

Image in a flat mirror The image you see in a flat mirror appears “behind” the mirror a distance equal to the distance the object is in front of the mirror

Image in a flat mirror Object distance = image distance

Image in a flat mirror Image is the same size as the object for a flat mirror