# Moza M. Al-Rabban Professor of Physics

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Moza M. Al-Rabban Professor of Physics
Lecture 11 Refraction Moza M. Al-Rabban Professor of Physics

Refraction When light is incident on a smooth boundary between two transparent materials (e.g., air and glass), two things happen: Part of the light reflects from the boundary, obeying the law of reflection. Part of the light crosses the boundary, changes direction, and continues into the second medium. This is called refraction.

Willebrord van Roijen Snell
Snell’s Law In a medium in which light slows down, a ray bends closer to the perpendicular.

The Index of Refraction
Light travels through transparent media at a speed less than its speed c in vacuum. We define the index of refraction in a transparent medium as:

Analyzing refraction Draw a ray diagram. Represent the light beam with one ray. Draw a line normal to the boundary. Do this at each point where the ray intersects a boundary. Show the ray bending in the correct direction. The angle is larger on the side with the smaller index of refraction. This is the qualitative application of Snell’s law. Label angles of incidence and refraction. Measure all angles from the normal. Use Snell’s law. Calculate the unknown angle or unknown index of refraction.

Example 3: Deflecting a Laser Beam
A laser beam is aimed at a 1.0 cm thick glass sheet at an angle of 300 above the glass. What is the laser beam’s direction of travel in the glass? What is its direction of travel in the air on the other side? By what distance d is the laser beam displaced?

Example: Measuring the Index of Refraction
A laser beam is deflected at an angle of by a prism. What is the prism’s index of refraction?

Total Internal Reflection
Suppose instead we have a prism with n= At what angle will a beam normal to the long side be deflected? There is no such angle! Therefore, there will be no refracted ray, and the light will be completely reflected. This is called total internal reflection. It occurs only when the 2nd medium has a lower index of refraction than the 1st medium (n2<n1), and when q≥qc. A critical angle is reached when There is no critical angle and no total internal reflection if When q1=qc, q2=900.

Example: Total Internal Reflection
A light bulb is set in the bottom of a 3.0 m deep swimming pool. What is the diameter of the ring of light seen on the pool’s surface? This is the so-called “ring of bright water” seen when looking up from within a pool of water. The outside world is compressed to lie within the ring.

Fiber Optics Total internal reflection makes possible fiber optic light pipes, which can transport light and light-encoded signals over long distances without significant loss.

Clicker Question 1 Light travels from medium 1 to medium 3 as shown.
Which of the following describes the indices of refraction? n3 > n1; n3 = n1; n3 < n1; We cannot compare n3 and n1 without knowing n2.

Image Formation by Refraction

Example: Air Bubble in a Window
Glass A fish and a sailor look at each other through the 5.0 cm thick glass port hole of a submarine. There is a small air bubble half way through the glass. How far behind the glass surface does the bubble appear to the fish? How far behind the glass surface does the bubble appear to the sailor? n = 1.00 Air n = 1.33 Water

Color and Dispersion Experiments with light and prisms show the following: What we perceive as white light is actually a mixture of all colors. White light can be disbursed into colors and, equally important, colors can be combined to produce white light. The index of refraction is slightly different for different colors of light. Glass has a slightly higher index of refraction for violet light than for green or red light. Consequently, different colors refract at slightly different angles.

Dispersion This can be made quantitative by measuring the index of refraction of a transparent material as a function of wavelength and associating wavelengths with colors..

The Dispersion Relation
The cause of dispersion is the oscillations of bound electrons in the transparent medium. In particular, the index of refraction obeys a dispersion relation of the form: Here N is the number of atoms per unit volume, e is the electron charge, me is the electron mass, w0 is the resonant frequency of the bound electrons, and w=2pc/l is the angular frequency of the light. Notice that if w > w0, the index of refraction n can be less than 1, indicating that the phase velocity of light in the medium is greater than c.

Example: Dispersing Light with a Prism
We saw that light incident on a 300 prism is deflected by if the prism’s index of refraction is Suppose this is the index of refraction for deep violet light, and that deep red light has an index of refraction of What is the deflection angle for deep red light? If a beam of white light is dispersed by the prism, how wide is the rainbow spectrum on a screen 2.0 m away>

Rainbows A rainbow is formed by spherical water droplets in which light is refracted twice and reflected once in the droplet. The maximum reflection angle of red light is 42.50, and that of violet light is The net effect is to produce a bright ring, dispersed in wavelength, and centered 1800 away from the direction of the Sun. There is also a two-reflection secondary rainbow with reversed colors outside the main rainbow.

Colored Filters and Colored Objects
When white light passes through green glass and emerges as green light, what happens? Does the glass add “greenness” to the light? No. The glass removes the non-green light from the beam. More precisely, colored glass absorbs all wavelengths except those of one color, and that color is transmitted through the glass without absorption. For example, leaves are green because of chlorophyll, which selectively absorbs red and blue light while reflecting green.

Example: Filtering Light
White light passes through a green filter and is observed on a screen. (a) What happens if a second green filter is placed between the first filter and the screen? What happens if a red filter is placed between the first filter and the screen? (a) Nothing happens because both filters transmit the same light. (b) No light is transmitted, because the red filter blocks what the green filter transmits, and vice versa.

Scattering: Blue Skies and Red Sunsets
At the atomic level, light passing through the atmosphere undergoes Rayleigh scattering, which depends on frequency to the 4th power (or l-4). This preferentially scatters blue light, making the sky blue and removing blue light from beams of sunlight, particularly during sunset when the distance light travels through air is a maximum.

Five Things You Should Have Learned from This Lecture
Transparent media transmit light, but also may reduce the velocity of light as it passes through the medium. Snell’s Law describes how a light beam is deflected as it crosses the interface between one transparent medium and another The index of refraction n=c/v quantifies the reduction in speed of light passing through a transparent medium . When light travels through an interface where the velocity increases, there is a critical angle beyond which it cannot be refracted. This produces the phenomenon of total internal reflection, used in light pipes and binoculars. The index of refraction depends on wavelength. This phenomenon, called dispersion, can be used to separate white light into colors.