Holography
- Gabor reasoned that light waves radiating from various points on an illuminated object would interfere with each other.
- A photographic plate could be used to record the interference pattern.
- After development, if the film has light passed through it, the original image should be produced.
- The above described process results in a hologram.
- Hologram – the 3D image of an object produced in the way described above.
- White light can be used but the image is not sharp because white light is incoherent.
- Incoherent light – Large range of frequencies are present.
- Lasers solved the problem of incoherent light.
- Holograms are produced with laser light by:
- One beam is split into two.
- One of the two falls on the object to illuminate the object.
- The other beam is directed at the photographic film as a reference beam.
- Light reflects off of the object and interferes with the reference beam. All is recorded on film.
- When illuminated by the same laser light, the image is seen.
- Application.
- Used with ultrasound to create 3D images of internal organs.
- Holographic pictures on credit cards.
- Used to record universal bar codes on products.
Polarization
- A single ray of light passing through a calcite crystal breaks into two.
- Caused by light polarization.
- This tells us that light must be a transverse wave since only transverse waves can be polarized.
- Polarized light – has all the magnetic field in one plane and all the electric fields in a plane perpendicular to it.
- Un-polarized light passing through a filter becomes polarized.
- If this polarized light is allowed to fall on another filter, then if the filters are oriented in the same way, the light will pass through the second filter unaffected.
- If the second is oriented 90o to the first, then no light goes through the second.
- Transverse waves can have the particles of the medium move in many different directions with respect to the energy transfer.
- If this motion is standardized, we have a polarized wave.
- Longitudinal waves can have their particles move in one way only with respect to the energy transfer, therefore, they cannot be polarized.
- A calcite crystal splits a beam into two beams polarized perpendicular to each other.
- One beam obeys the law of refraction in that the refractive index is a constant 1.66.
- The other beam does not obey the law of refraction , and will have a refractive index between 1.49 and 1.66 depending upon the angle of incidence with respect to the planes of atoms within the crystal.
- Each beam then travels a different path and at a different speed within the crystal.
- There are other crystals besides calcite, that respond to polarized light. An example is herapathite
- Edwin Land – produced Polaroid by imbedding herapthite in plastic.
- Herapathite absorbs light polarized in a plane parallel to its long axis and transmits the rest.
- The plastic is stretched to cause all the crystals to line up in the same direction.
- When light reflects off of surfaces, it tends to be polarized parallel to the surface.
- Photoelastic material – Materials that become doubly refractive when subjected to mechanical stress
- When these are placed between polarizing filters, they show the strain patterns present.
- Used to stress test models of structures to be built.
- Light from the sun that is scattered by the sky is polarized. The greatest polarization is in the direction perpendicular to the sun.
- To improve pictures, polarizing filters are used.
- Some animals have eyes that are sensitive to polarized light. Some birds that fish, ants, horseshoe crabs, and honeybees are examples.
Waves and Light
- By 1900, the wave theory of light was the accepted theory.
- Wave theory explains such things as reflection, refraction, diffraction, dispersion, interference, and polarization.
- Two questions remained.
- What is light’s medium?
- What is the nature of the wave’s vibrations?
- “Either” was proposed as the medium, but Michelson and Morley showed that it did not exist.
- Therefore, the wave theory must be modified to explain propagation without a medium.
- This modification is discussed in Physics Second Course
- The second question was answered by Maxwell.
- He explained that light waves are electromagnetic waves.
- Electromagnetic waves – Waves that travel as a series of alternating electric and magnetic fields.
- Greater details are left to Physics Second Course.
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November 21, 2013