Reflection and refraction

Reflection

imageReflection is the abrupt change in the direction of propagation of a wave that strikes the boundary between two different media.  At least some part of the incoming wave remains in the same medium.  Assume the incoming light ray makes an angle θi with the normal of a plane tangent to the boundary.  Then the reflected ray makes an angle θr with this normal and lies in the same plane as the incident ray and the normal.

Law of reflection:
   θi = θr

imageSpecular reflection occurs at smooth, plane boundaries.  Then the plane tangent to the boundary is the boundary itself.  Reflection at rough, irregular boundaries is diffuse reflection.  The smooth surface of a mirror reflects light specularly, while the rough surface of a wall reflects light diffusely.
The reflectivity of a surface material is the fraction of energy of the incoming wave that is reflected by the surface.  The reflectivity of a mirror is close to 1.

imageProblem:

How many times will the incident beam shown in the figure to the right be reflected by each of the parallel mirrors?

Solution:
After each path between the mirrors the beam gains a distance d in height.
We have d/(1 m) = tan(5o), d = tan(5o) m = 8.75 cm. The beam must therefore pass 11 times between the mirrors to gain a height of 1 m, 6 times towards the right and 5 times towards the left.


Refraction

imageRefraction is the change in direction of propagation of a wave when the wave passes from one medium into another and changes its speed.  Light waves are refracted when crossing the boundary from one transparent medium into another because the speed of light is different in different media.  Assume that light waves encounter the plane surface of a piece of glass after traveling initially through air as shown in the figure to the right.

What happens to the waves as they pass into the glass and continue to travel through the glass? 
The speed of light in glass or water is less than the speed of light in a vacuum or air.  The speed of light in a given substance is v = c/n, where n is the index of refraction of the substance.  Typical values for the index of refraction of glass are between 1.5 and 1.6, so the speed of light in glass is approximately two-thirds the speed of light in air. The distance between wave fronts will therefore be shorter in the glass than in air, since the waves travel a smaller distance per period T.
If f is the frequency of the wave and T = 1/f is the period, i.e. the time interval between successive crests passing a fixed point in space, then λ1 = v1T = cT/n1 and λ2 = v2T = cT/n2, or

λ12 = n2/n1.

imageNow consider wave fronts and their corresponding light rays approaching the surface at an angle.
We can see that the rays will bend as the wave passes from air to glass. The bending occurs because the wave fronts do not travel as far in one cycle in the glass as they do in air.  As the diagram shows, the wave front halfway into the glass travels a smaller distance in glass than it does in air, causing it to bend in the middle. Thus, the ray, which is perpendicular to the wave front, also bends.  The situation is like a marching band marching onto a muddy field at an angle to the edge of the field.  The rows bend as the speed of the marchers is reduced by the mud.

imageThe amount of bending of the light depends on the angle of incidence and on the indices of refraction of glass and air, which determine the change in speed.  From the figure we can see that λ12 = sinθ1/sinθ2.
But λ12 = n2/n1.  Therefore n2/n1 = sinθ1/sinθ2, or n1sinθ1 = n2sinθ2.

This is Snell's law, or the law of refraction.
n1sinθ1 = n2sinθ2.

imageWhen light passes from one transparent medium to another, the rays bend toward the surface normal if the speed of light is smaller in the second medium than in the first.  The rays bend away from this normal if the speed of light in the second medium is greater than in the first.  The picture on the right shows a light wave incident on a slab of glass.
One part of the wave is reflected, and another part is refracted as it passes into the glass.  The rays bend towards the normal.  At the second interface from glass into air the light passing into the air is refracted again. The rays now bend away from the normal.


Problem:

imageThe path of light in air incident on and transmitted through a glass plate is shown in the figure on the right.  The angle of the incident ray to the normal is 45o and equals that of the reflected ray.  The transmitted ray is refracted at an angle of 28o to the normal and exits the glass at an angle of 45o to the normal, an angle equal to that of the incident ray.  What is the index of refraction of the glass?

Solution:
Snell's law: nisinθi = ntsinθt as the ray enters from the air into the glass, we have ni = 1, θi = 45o, and θt = 28o.  We therefore have nt = nisinθ/sinθt = sin45o/sin28o = 1.5.

Problem:

Assume a light signal travels in a straight line through a medium with index of refraction of n = 1.55, such as an optical fiber.  How long does it take the signal to travel 20 cm?

Solution:
The speed of light in the medium is v = c/n. 
v = d/t.  t = d/v = (0.2 m)* 1.55/(3*108 m/s) = 1.03*10-9 s = 1.03 ns.

At a boundary between two transparent media, light is partially reflected and partially refracted.  The ratio of the reflected intensity to the incident intensity is called the reflectance R and the ratio of the transmitted intensity to the incident intensity is called the transmittance T.  Energy conservation requires that R + T = 1 (if there is no absorption).


Sunlight

Sunlight originates at the outer surface of the sun, in a region called the photosphere.  This region has a temperature of ~5800 oC.  The distribution of wavelengths in sunlight is determined by the temperature of the photosphere.  Not all sunlight is visible.  EM waves in the infrared and ultraviolet part of the EM spectrum are also produced in the photosphere.

Sunlight travels from the Sun to the Earth through empty space with the speed of light c.  When it enters the earth atmosphere it is refracted.  The index of refraction of air near sea level is only 1.0003, so the refraction is barely noticeable.  Air molecules, water molecules and dust also scatter some of the light (Rayleigh scattering).  These particles scatter shorter-wavelength light more efficiently that longer-wavelength light.  Scattering by tiny particles is always most efficient when the wavelength of the EM wave approximately matches the size of the tiny particle.  The dimensions of the molecules and dust particles are much smaller than the wavelengths of visible light, so blue light with the shortest wavelength provides the best match.  Blue light is scattered more than red light.  Most sunlight travels directly to our eyes, but the scattered light reaches us by a more complicated path from different directions.  We therefore see the brilliant yellow disk of the sun (direct light) and the fairly uniformly blue sky (scattered light).

As the sun rises or sets, light must travel a long distance through the Earth's atmosphere to reach our eyes.  Most of the blue light is scattered away during this passage through the atmosphere and the direct light from the sun appears red.  Extra dust and ash particles from pollution, forest fires, or volcanic eruptions enhance the Rayleigh scattering and are responsible for unusually red sunrises and sunsets.

imageClouds and fog are composed of relatively large water droplets, larger than the wavelengths of visible light.  All wavelengths in the visible spectrum are scattered very efficiently by these large particles, so that very little direct sunlight reaches our eyes.  The scattered light, however, does not have any particular color, and clouds and fog appear white.

Problem:

imageSuppose there is a cloud made up of some unknown particles that absorb rather than scatter visible radiation.  What color would this cloud appear during the day?

Solution:
The cloud would appear black, since no visible light would reach the observer from the direction of the cloud.


Additional information:

The Physics Classroom: Refraction and the Ray Model of Light  Lessons 1 and 2