Familiarization with light-matter interaction

Several images demonstrating light-matter interaction was taken during the course. Pictures taken were gathered in the perimeter of the NIP. Pictures taken is shown as such:

Specular Reflection

The picture above depicts reflection of light, specifically specular reflection or mirror-like according to wikipedia. This is kind of reflection usually occurs in smooth surfaces like mirrors, metal coated surfaces like the one above also in water or glass. Specular reflection forms images. If the surface is flat, images formed seemed to reverse while if the surface is curved, magnification or demagnification occurs .

Diffuse reflection

If one can see, there is a shade of green in the yellow t-shirt. This is due to the diffuse reflection of the leaf. This kind of reflection occurs mostly in a rough surface. Since the surface is rough, the light rays reflects to all the directions that is why the image is not preserved. However, the energy of the reflected light is preserved.

Absorption

A black object is a prime example of absorption. The reason why it is not advisable to wear black on a sunny day is because the black pigment absorbs all color. This color is then converted in more heat via radiation. And that is what defines absorption. The energy of the photon is taken up by the object and then convert it into other forms of energy.

Transmittance

As wiki said, transmittance is the fraction of the incident light specified that passes through a sample. In this picture, the bottle is filled up with smoke. The smoke forms a colloid that either absorbs the incident light or reflects the incident light diffusely that reduces the incident light the passes through the bottle.

Interference and Diffraction

This is a Titanium dioxide thin film. Because of its small thickness, rainbows are observed. This rainbows are cause a phenomenon called diffraction. Due to interference of the incident light due to thickness variation, light is being separated into it components by this film.

Reflectance(blue)-Absorbance(red) Spectrum of a Blue watch

Reflectance spectrum of a blue watch is obtained and then plotted. In order to get the absorbance, since it is an opaque object, the value of the reflectance at each point is deducted from 1. We can assume that there is no transmittance and only the absorbance of the object minimizes the intensity of the incident light. To check if it's right, think of this: The watch has the color blue because its pigments absorbs the color blue the weakest. Therefore, in reflection, only the intensity of the color blue is dominant. And that dominant color is what we mostly see. Blue has low wavelength and the reflectance peak is at around 400nm. And in absorbance, at around 400nm, it has a low value of absorbance, and thus is weakly absorbs in that color.

Reflectance(blue)-Transmittance(red) of the glass door.

The spectrum is the reflectance of the glass door in the NIP room. It has a certain coating that reduces the intensity of the ambient light. It gives a dimming effect. Here, it reflectance is shown in the blue graph and transmittance in the red graph. Since the door is transparent/translucent, we can assume the there is no absorption present and those light that were not reflected will be transmitted. To get the trasmittance, the value of reflectance is deducted from 1. To check if it is right, think of this: The point of the glass door is to reduce the ambient light from the outside. Therefore, the visible spectrum must be reduced. And in the graph, there is a considerable amount of reflectance inside the visible spectrum. Thus, the value for reflectance is correct. The same logic goes for transmission. if you want to reduce the intensity of ambient light, the transmission in that range must be reduced. In the graph, there is a depression at the visible spectrum.

References:

Wikipedia: Reflection, Absorption, Trannsmittance

Diffraction and Interference: http://electron9.phys.utk.edu/phys136d/modules/m9/diff.htm

AP 187 Handouts and Lectures

Thanks to Gino and Mabi for their camera and intellectual inputs.

Familiarization with Properties of Light Sources

Light, according to Wikipedia, is an electromagnetic radiation of a wavelength visible to the human eye, but in general, Light can be an electromagnetic radiation of any wavelength, regardless if it is visible to the human eye or not. For this entry, the emittance spectra of different light sources will be observed.

Figure 1: Emittance Spectrum for a Blackbody radiator at different temperatures.

Notice that in figure 1, the temperature is directly proportional to the emittance of the blackbody radiator; as the temperature increases, so does the peak of the emittance. Also, as the temperature is increased, the peak seems to be shifted to the left. The emittance spectra of the blackbody radiator is given by

where h is the Planck's constant, lambda is the wavelength of radiation, c is the speed of light and T is the temperature in Kelvins. The range for the wavelength is from 350nm to 750nm, which is the range for visible light. For the case of temperatures about 6500 K and 5400 K, a color of bluish green can be observed.

Below are some of the emittance spectra of common light sources.

Figure 2: Emittance for a Light Emitting Diode.


Figure 3: Emittance Spectra for a lighter that uses butane.


Figure 4: Emittance Spectra for a Mercury Lamp.

These emittance spectra were experimentally gathered using a portable spectrometer. Note that the peaks indicate the wavelength (and hence the color) in which the human eye can see the radiation. For the LED, it is expected to see a bluish-green color, while for the lighter that uses butane, it is expected to observe a yellowish color and for the the mercury lamp, a combination of bluish green and light green is expected.

It is important to at least be familiar with these emission spectra because, depending on the application, certain emittance spectra are required for certain applications.