Lab 7

Refraction and reflection

In this lab you will explore the behavior of light at the boundary between two transparent media with different indices of refraction.  A fraction of the incident intensity will be reflected, and the rest of the light will be transmitted.  The direction of propagation of the reflected and transmitted light is given by the laws of reflection and refraction.

imageHow much of the light is reflected and how much is transmitted?

The reflectance R is the ratio of the reflected flux to the incident flux, and the transmittance T is the ratio of the transmitted flux to the incident flux.  Energy conservation requires that R + T = 1 (if there is no absorption).
R and T depend on the indices of refraction of the two media n1 and n2, the angle of incidence θi, and the polarization of the incident light.
We distinguish between p-polarization and s-polarization.

Consider, for example, an air-glass interface as shown.  The plane of incidence contains the normal to the boundary and the incident ray.  The electric field vector E of the incident wave is perpendicular to the direction of propagation and can have a component in the plane of incidence, Ep, and a component perpendicular to the plane of incidence Es.  We have E = Ep+ Es.

imageThe reflectance R depends of the polarization and is given for p-polarization by

Rp = ((tan(θi - θt)/tan(θi + θt))2,

and for s-polarization by

Rs = ((sin(θi - θt)/sin(θi + θt))2.

If θ1 + θ2 = π/2, then tan(θ1 + θ2) = infinite  and Rp = 0.  If light is reflected, it will have s-polarization.  The incident angle at which this happens is called the Brewster angle θB.  We then have

n1sinθB = n2sin((π/2) -θB) = n2cosθB.
tanθB = n2/n1.

Explore using this spreadsheet.  Vary n1 and n2 and observe the changes.

Reflection and refraction can result in image formation.  Spherical mirrors form images by reflection.  The mirror equation tells us where the image is formed and if it is real or virtual.

If the magnification is negative, the image is inverted.

Things that always go together for spherical mirrors:

xo and xi are positive for locations in front of the mirror and negative for locations behind the mirror.  R and f are positive for concave and negative for convex mirrors, and f = R/2.

In this lab you will explore refraction at a plane interface and image formation by reflection from spherical surfaces.

Open a Microsoft Word document to keep a log of your procedures, results and discussions.  This log will become your lab report.  Address the points highlighted in blue.  Answer all questions.


Exploration 1

Use an on-line simulation from the University of Colorado PhET group to explore the bending of light.
Link to the simulation: https://phet.colorado.edu/en/simulation/bending-light

Explore the interface!  Click on either the Intro or More Tools tab.  Let red light move from air into water.  (Make sure you select water if you click the More Tools tab.)
For incident angles θi from zero to 80o in 10 degree steps measure the angle of refraction θt and the reflected intensity or reflectance R.
Download this spreadsheet and enter your measured values on sheet 1.

(b)  Design experiments to determine the index of refraction of mystery materials A and B. 

(c)  Design and describe a setup that has the refracted ray bend away from the normal? 

(d)  Click on the prism break tab.  Use red light with a wavelength of 650 nm.  Try to arrange various prisms in such a way, so that the laser beam after total internal reflections moves parallel to the incident beam but in the opposite direction.  Try to use as few pieces as possible.

(e)  Now switch to white light and experiment with various prisms to answer the following questions.


Reflection

Exploration 2:

Explore image formation with spherical mirrors.  Use this interactive simulation.

Investigate 4 different situations and fill out the table on sheet 3 of your spreadsheet.  You choose the radius of curvature R and the object position xo.

case R f xo xi 1/x0 + 1/xi 1/f M image
real?
image
upright?
concave mirror, real image:
|hi| > |ho|
                 
concave mirror, real image:
|hi| < |ho|
                 
concave mirror, virtual image
                 
convex mirror
                 

Experiment

In this experiment you will trace the path of a light ray through a block of glass and measure the index of refraction of the glass.  For an air-glass boundary we can set the index of refraction of air equal to one.  Snell's law therefore yields for the index of refraction of the glass

nglass = sinθair/sinθglass

It is easy to measure θair, but not θglass inside the glass.  But when a light ray enters square block of glass from air making a nonzero angle θair with the normal to the interface, the ray is bent towards the normal as its enters and away from the normal as it leaves the glass block.  The emergent ray moves in the same direction as the incident ray, but is displaced parallel to the incident ray.  The parallel displacement d depends on index of refraction nglass and the width w of the block.  By measuring the displacement d, we can determine nglass.

image    image

 


Procedure:

Click to open the "optics lab" simulation. 

opticslab

It contains a laser, an optical breadboard and several optical components.  The components can be dragged to different positions on the breadboard and the components can snap to the holes on the breadboard which have a spacing of 1 unit = 2.5 cm.  The screen is 4 units wide and 2 units high and the lines on the screen are spaced by ½ unit.  Clicking anywhere on the breadboard you can rotate and zoom the view.

When the simulation opens the laser beam passes through a glass block.  The dimensions of the glass block are 2.8 by 2.8 by 1.4 units.

Data Analysis:

Use your measurements to determine the index of refraction nglass of the glass block.

imageThe expected displacement of a ray passing through the glass block is d.  From the figure on the right we see that

d/L = sin(θair - θglass),
w/L = cos(θglass).

This can be rewritten as

d = wsin(θair - θglass)/cos(θglass),

and applying Snell's law and trigonometric relations

d = w sinθair[1 - cosθair/(nglass2 - sin2θair)½].

This equation can be solved for nglass.

 nglass2 = cos2θair/(1 - d/(w sinθair))2 + sin2θair.

Use your spreadsheet to calculate nglass for each of your 4 measurements.  Paste your spreadsheet table into your log.


Convert your log into a lab report.

Name:
E-mail address:

Laboratory 7 Report

Save your Word document (your name_lab7.docx), go to Canvas, Assignments, Lab 7, and submit your document.