Note that as shown in Figure 2.3.9(a), the direction of the ray moves closer to the perpendicular when it slows down. This means that the speed of light is less in medium 2 than in medium 1. In the situations shown in Figure 2.3.9 medium 2 has a greater index of refraction than medium 1. The change in the speed of light is related to the indices of refraction of the media involved. (Some of the incident light will be reflected from the surface, but for now we will concentrate on the light that is transmitted.) The change in direction of the light ray depends on how the speed of light changes. The angles are measured relative to a perpendicular to the surface at the point where the light ray crosses it. The paths are exactly reversible.įigure 2.3.9 shows how a ray of light changes direction when it passes from one medium to another. This is analogous to what happens when a lawn mower goes from grass to footpath. (b) A ray of light moves away from the perpendicular when it speeds up. This is analogous to what happens when a lawn mower goes from a footpath to grass. (a) A ray of light moves closer to the perpendicular when it slows down. The speed of light is greater in medium 1 than in medium 2 in the situations shown here. The change in direction of a light ray depends on how the speed of light changes when it crosses from one medium to another. The incoming ray is called the incident ray, the outgoing ray is called the refracted ray, and the associated angles are the incident angle and the refracted angle, respectively. Here n 1 and n 2 are the indices of refraction for media 1 and 2, and θ 1 and θ 2are the angles between the rays and the perpendicular in media 1 and 2. The law of refraction is stated in equation form as: The exact mathematical relationship is the law of refraction, or Snell’s law, after the Dutch mathematician Willebrord Snell (1591–1626), who discovered it in 1621. For a ray at a given incident angle, a large change in speed causes a large change in direction and thus a large change in angle. The amount that a light ray changes its direction depends both on the incident angle and the amount that the speed changes.
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