Category Archives: Optics

Diffraction

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Diffraction is the bending of waves around obstacles, or the spreading of waves as they pass through an opening, most apparent when looking at obstacles or wavelengths having a size of the same order of magnitude as the wavelength. Typically, the smaller the obstacle and wavelength, the greater the diffraction. Taken to the extreme, when a wave is blocked by a small enough opening, the wave passing through the opening actually behaves like a point source for a new wave.

You can observe diffraction quite easily… I’m sure you’ve heard a noise from a room with an open door even when your ears aren’t in a direct line from the sound source… this is a result of diffraction of the sound waves around the door opening (along with some reflection of sound as well).

Thomas Young’s Double-Slit Experiment is a famous experiment which utilized diffraction to prove light has properties of waves. Young placed a single-wavelength light source behind a barrier with two narrow slits, allowing only a small portion of the light to pass through each slit. Because the two light waves travel different distances to the screen on which they are projected, you can see effects of both constructive and destructive interference, phenomena that occur only for waves!

Question: The spreading of waves into the region behind an obstacle is known as _______.

Diffraction Question

Answer: diffraction

Question: Which wave phenomenon is represented in the diagram?

Answer: diffraction

Intro to Refraction

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When a wave reaches the boundary between media, part of the wave is reflected and part of the wave enters the new medium. As the wave enters the new medium, the speed of the wave changes, and the frequency of a wave remains constant, therefore, consistent with the wave equation, Wave Equation, the wavelength of the wave must change.

Question: When a wave enters a new material, what happens to its speed, frequency, and wavelength?

Answer: Speed changes, frequency remains constant, and wavelength changes.

The front of a wave has some actual width, therefore if the wave does not impinge upon the boundary between media at a right angle, not all of the wave enters the new medium and changes speed at the same time. This causes the wave to bend as it enters a new medium in a process known as refraction.

To better illustrate this, imagine you’re in a line in a marching band, connected with your bandmates as you march at a constant speed down the field in imitation of a wave front. As your wavefront reaches a new medium that slows you down, such as a mud pit, the band members reaching the mud pit slow down before those who reach the pit later. Since you are all connected in a wave front, the entire wave shifts directions (refracts) as it passes through the boundary between field and mud!

Snell's Law

The index of refraction (n) is a measure of how much light slows down in a material. In a vacuum, all electromagnetic waves have a speed of c=3*108 m/s. In other materials, light slows down. The ratio of the speed of light in a vacuum to the speed of light in the new material is known as the index of refraction (n). The slower the wave moves in the material, the larger the index of refraction: Index of Refraction

Question: A light ray traveling in air enters a second medium and its speed slows to 1.71 x 108 m/s. What is the absolute index of refraction of the second medium?

Answer: IndexRSoln

Indices of Refraction

The amount a light wave bends as it enters a new medium is given by the law of refraction, also known as Snell’s Law. Snell’s Law states that Regents Physics Snell's Law, where n1 and n2 are the indices of refraction of the media, and theta corresponds to the angles of the incident and refracted rays, again measured from the normal. Light bends toward the normal as it enters a material with a higher index of refraction (slower material), and bends away from the normal as it enters a material with a lower index of refraction (slower material).

Not only does index of refraction depend upon the medium the light wave is traveling through, it also varies with frequency. Thankfully, its variation is typically fairly small, and the Regents Physics Reference Table even provides you a table of indices of refraction for common materials at a set frequency.

Snell's Law Question

Question: A ray of monochromatic light having a frequency of 5.09 × 1014 hertz is incident on an interface of air and corn oil at an angle of 35° as shown. The ray is transmitted through parallel layers of corn oil and glycerol and is then reflected from the surface of a plane mirror, located below and parallel to the glycerol layer. The ray then emerges from the corn oil back into the air at point P.

Calculate the angle of refraction of the light ray as it enters the corn oil from air.

Answer: Snell's Law Answer

Question: Explain why the ray does not bend at the corn oil-glycerol interface.

Answer: The indices of refraction are the same for corn oil and glycerol (the speed of the wave does not change at the corn-oil / glycerol interface).