Home Optics Experiments PE-0300 Reflection and Transmission

Keywords:

  • Reflection Law
  • Fresnel Laws
  • Reflectance
  • Transmittance
  • Polarization
  • Anti-Reflection Coating
  • Brewster Angle
  • LED Characterisation
  • Green Laser Properties

 

Basic experiment

Intended institutions and users:

Physics Laboratory

Engineering department

Electronic department

Biophotonics department

Physics education in Medicine

 

Introduction

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PE-0300 Reflection and Transmission

Reflection of light on surfaces is a familiar phenomenon of daily life. Therefore it is no surprise that the reflection law is one of the well known optical laws and was first stated by Ibn Al Haitham beginning of the 10th century. Little  is commonly known about polarization of light and its connection with reflection and refraction. This connection is formulated in the Fresnel Laws which are usually expressed as transmittance and reflectance as a function of the index of refraction and angle of reflection. The four formulas which were deduced by Augustin Fresnel in 1821 contain the complete theory of reflection, refraction and polarization of isotropic materials.

The fundamental understanding of reflection and transmission is essential for the design of laser mirror, sun glasses and a lot more. By dielectric coating such components are made either to optimise or to suppress reflection.

Within this experiment the reflection law which is verified using a metal coated mirror. The next part covers  the quantitative verification of the Fresnel Laws on a specially shaped glass plate using polarized light. Finally the spectral performance of a dielectric coated mirror is investigated using a white LED and a grating.

 

PE-0300 Reflection and Transmission

As source a white light LED and a “green” laser are used. Both sources are characterised by measuring the optical power versus the injection current. In case of the LED, a collimator is used to obtain an almost parallel light beam. Furthermore, the spatial intensity distribution of the LED can be measured when using the provided goniometer.

The light of either a white light LED or “green” laser is strongly polarised by means of polarizer 1 and 2 and hits the probe plate. When using the LED, a collimator lens creates an almost parallel light beam. The optical plate is attached to the goniometer plate B and can be rotated by 360°. The photodiode along with a focussing lens and the  polarizer 3 are attached to the arm  of goniometer A. By turning of B a defined angle of incidence is set. The arm of goniometer A is turned in such a way that the signal detected by the photodiode becomes maximum. In this way the Snell’s or reflection law is verified. To verify the Fresnel’s equations, the polarisation state of the incident light is set to either “s” (perpendicular) or “p” (parallel) with respect to the plane of incidence which is spanned by the vector of the incoming and reflected beam.

Optical components, especially mirrors belong to the most important means in photonics. Meanwhile a variety of technologies exist to tailor the spectral behaviour of optical surfaces like short pass, long pass or even ultra narrow band pass mirror. The aim of this experiment is the spectral characterization of such a mirror. As light source a white light LED and for the spectral resolution a transmission grating is used. The dielectric mirror is attached to a turntable (C) to measure the spectral response also for different angle of incidence. The grating is attached to the goniometer B where it is kept at a angle of zero degrees with respect to the direction of the probe light beam. The arm of goniometer A is turned to the first or second order of the grating. The first measurement is carried out without the dielectric mirror to record the spectrum of the white light emitting LED. The second measurement is performed including the dielectric mirror. Normalising the values of the second measurement to the first measurement the spectral transmittance of the dielectric mirror is obtained.

PE-0300 Reflection and Transmission

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