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      • We know that light is a wave based on how it behaves – it exhibits the same properties of other waves we have examined – it interferes with itself, it follows an inverse-square law for intensity (brightness), and so on.
  1. For light traveling from a more dense to a less dense medium, there is a critical angle * beyond which the light will not pass through the interface. A light ray with an angle of incidence equal to or greater than the critical angle will reflect, following the law of reflection.

  2. The changing of a light ray’s direction (loosely called bending) when it passes through variations in matter is called refraction. Refraction is responsible for a tremendous range of optical phenomena, from the action of lenses to voice transmission through optical fibers.

    • Overview
    • Reflection and refraction

    Light rays change direction when they reflect off a surface, move from one transparent medium into another, or travel through a medium whose composition is continuously changing. The law of reflection states that, on reflection from a smooth surface, the angle of the reflected ray is equal to the angle of the incident ray. (By convention, all angles in geometrical optics are measured with respect to the normal to the surface—that is, to a line perpendicular to the surface.) The reflected ray is always in the plane defined by the incident ray and the normal to the surface. The law of reflection can be used to understand the images produced by plane and curved mirrors. Unlike mirrors, most natural surfaces are rough on the scale of the wavelength of light, and, as a consequence, parallel incident light rays are reflected in many different directions, or diffusely. Diffuse reflection is responsible for the ability to see most illuminated surfaces from any position—rays reach the eyes after reflecting off every portion of the surface.

    When light traveling in one transparent medium encounters a boundary with a second transparent medium (e.g., air and glass), a portion of the light is reflected and a portion is transmitted into the second medium. As the transmitted light moves into the second medium, it changes its direction of travel; that is, it is refracted. The law of refraction, also known as Snell’s law, describes the relationship between the angle of incidence (θ1) and the angle of refraction (θ2), measured with respect to the normal (“perpendicular line”) to the surface, in mathematical terms: n1 sin θ1 = n2 sin θ2, where n1 and n2 are the index of refraction of the first and second media, respectively. The index of refraction for any medium is a dimensionless constant equal to the ratio of the speed of light in a vacuum to its speed in that medium.

    By definition, the index of refraction for a vacuum is exactly 1. Because the speed of light in any transparent medium is always less than the speed of light in a vacuum, the indices of refraction of all media are greater than one, with indices for typical transparent materials between one and two. For example, the index of refraction of air at standard conditions is 1.0003, water is 1.33, and glass is about 1.5.

    The basic features of refraction are easily derived from Snell’s law. The amount of bending of a light ray as it crosses a boundary between two media is dictated by the difference in the two indices of refraction. When light passes into a denser medium, the ray is bent toward the normal. Conversely, light emerging obliquely from a denser medium is bent away from the normal. In the special case where the incident beam is perpendicular to the boundary (that is, equal to the normal), there is no change in the direction of the light as it enters the second medium.

    Light rays change direction when they reflect off a surface, move from one transparent medium into another, or travel through a medium whose composition is continuously changing. The law of reflection states that, on reflection from a smooth surface, the angle of the reflected ray is equal to the angle of the incident ray. (By convention, all angles in geometrical optics are measured with respect to the normal to the surface—that is, to a line perpendicular to the surface.) The reflected ray is always in the plane defined by the incident ray and the normal to the surface. The law of reflection can be used to understand the images produced by plane and curved mirrors. Unlike mirrors, most natural surfaces are rough on the scale of the wavelength of light, and, as a consequence, parallel incident light rays are reflected in many different directions, or diffusely. Diffuse reflection is responsible for the ability to see most illuminated surfaces from any position—rays reach the eyes after reflecting off every portion of the surface.

    When light traveling in one transparent medium encounters a boundary with a second transparent medium (e.g., air and glass), a portion of the light is reflected and a portion is transmitted into the second medium. As the transmitted light moves into the second medium, it changes its direction of travel; that is, it is refracted. The law of refraction, also known as Snell’s law, describes the relationship between the angle of incidence (θ1) and the angle of refraction (θ2), measured with respect to the normal (“perpendicular line”) to the surface, in mathematical terms: n1 sin θ1 = n2 sin θ2, where n1 and n2 are the index of refraction of the first and second media, respectively. The index of refraction for any medium is a dimensionless constant equal to the ratio of the speed of light in a vacuum to its speed in that medium.

    By definition, the index of refraction for a vacuum is exactly 1. Because the speed of light in any transparent medium is always less than the speed of light in a vacuum, the indices of refraction of all media are greater than one, with indices for typical transparent materials between one and two. For example, the index of refraction of air at standard conditions is 1.0003, water is 1.33, and glass is about 1.5.

    The basic features of refraction are easily derived from Snell’s law. The amount of bending of a light ray as it crosses a boundary between two media is dictated by the difference in the two indices of refraction. When light passes into a denser medium, the ray is bent toward the normal. Conversely, light emerging obliquely from a denser medium is bent away from the normal. In the special case where the incident beam is perpendicular to the boundary (that is, equal to the normal), there is no change in the direction of the light as it enters the second medium.

  3. Jun 7, 2023 · From eyeglasses to microscopes, two laws of physics work light-bending magic. A prism causes incoming light to bend by different amounts, depending on the wavelength — or color — of that light. As light passes through the prism and exits on the other side, it spreads out into a rainbow.

  4. May 24, 2024 · We know that light is a wave based on how it behaves – it exhibits the same properties of other waves we have examined – it interferes with itself, it follows an inverse-square law for intensity (brightness), and so on.

  5. Polarization. An age-old debate that has persisted among scientists is related to the question, "Is light a wave or a stream of particles?" Very noteworthy and distinguished physicists have taken up each side of the argument, providing a wealth of evidence for each side.

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  7. Unlike a sound waves, light waves do not need a medium to pass through, they can travel through a vacuum. Light from the Sun reaches Earth through the vacuum of space. A short video...

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