Light and matter react with each other in different ways. Light, with its numerous wavelengths, travel in space and interact with anything it hits, mainly matter or another light. In general, light and matter interact with 3 ways, transmission, reflection and absorption. From this interaction and the rules of Physics, numerous phenomenons appear in the environment.
When considering these processes, we must look at the properties of light as a wave and matter in the atomic scale. Light waves have their corresponding wavelengths and intensity. This corresponds to a certain energy level. When looking at matter in the atomic scale, we know that atoms contain electrons in a certain orbital in reference to the nucleus. These orbitals have certain energy levels.
Let us consider a single wavelength light and a single atom with a single electron matter. Now, when light with the same energy level as the electron of a certain atom hits the matter, the energy of light is absorbed by the electron, the electron vibrates and releases the energy in other forms other than light (e.g. heat). Now, if the energies do not match, the electron will vibrate for a short period of time and release the energy in the form of light as well. If the object is transparent, the vibration is passed from atom to atom, and finally released on the other side. This corresponds to transmission. But if the object is opaque, electrons on the surface vibrates and releases light on the same side as the source, corresponding to reflectance. Now, if we scan the wavelength of light for the visible range, we can create a profile of how matter will react with light sources of different wavelengths.
Some samples in our everyday environment can demonstrate these processes.
Figure 1. Brightly Colored Leaf
Here we can see how absorption and reflection occurs in everyday objects. The brightly colored leaf reflects red to pink and absorbs the rest. This occurs in almost all objects. It can be also seen in the background image of the wall. The color we see is the reflected wavelengths while everything else is absorbed.
Now, reflection can be classified into 3 more sub-classifications, specular, body and interreflection. Specular reflection, also known as glossy reflection is the reflectance of almost the whole light source due to the angle of incidence. Body or matte reflection is the reflection after absorption has occurred. And interreflection is the reflection of light from a secondary object.
Figure 2. Reflection of the street on the side of a car
Figure 3. Reflection of light off a handkerchief to a wall
In figure 2, we can see all three types of reflection. First, lets examine the strip of metal on the side of the car. The different shades of silver shows the specular reflection (brightly colored silver) and the body reflection (slightly darker silver). Now, interreflection can be seen on the side of the car. Since the car paint is very glossy, the interreflection shows a clear image of the yellow line in the parking lot. Meaning light from the environment hits the yellow line, the yellow line reflects the colors it does not absorb to the car and the car reflects to the camera. This is slightly confusing, so lets take a look at figure 3. In figure three, we see the body reflection of the handkerchief as red and the body reflection of the wall as white. The interreflection can be seen as the red tinge on the wall coming from the red handkerchief.
Now, let us examine some images on transmission.
Figure 4. Transmission of light from an LED through a pane of glass
As discussed earlier, we can see that the transparent object transmitted the light from the side of the light source to the opposite side. Since the pane of glass is highly transparent for almost all wavelengths, we can see the light from the source very clearly and with the same color as the source. Now, most filters with certain colors reflect and transmit the same wavelengths. But there are some objects designed to transmit a different wavelength from the reflected wave.
Figure 5. Front view of a Dichroic Filter
Figure 6. Rear view of a Dichroic Filter
From the front (figure 5), we can see that the filter reflects almost all wavelengths with slight tinges of blue. This is the reflection part. From the rear (figure 6), we can see that the filter transmits red relatively more. This is very useful for museums and galleries where red light (longer wavelengths) can heat and damage the paintings.
Figure 7. Diffraction and interference of light through gratings
Now, let us consider a grating. A grating can be a transparent object with a series of evenly spaced opaque strips. When light hits an opaque strip, each point on the strip acts as a new source of light. When light transmits, light from each new point source interferes with each other producing either bright lights (constructive interference) or dark lights (destructive interference). From optics classes, we know that the equation for grating interference is as follows:
where d is the grating separation, theta m is the angle from the central axis, m is integer specifying the modes and lambda is the wavelength of the incident light. This determines the angles at which constructive interference occurs. As we can see, interference is dependent on the wavelength of light. This is because diffraction gratings are determined by the phase difference due to path difference to create constructive and destructive interference. As we can see in figure 7, light is diffracted differently per wavelength.
So how can we get the transmission, reflection and absorption profiles of different objects? All we need is the color signal of the object and the color signal of the light source on something white. The color signal of the object is thus the reflection of the object with the light source used. So by dividing the color signal of the object by the color signal of the light source on white. This will result to the reflection profile of the object. If we subtract that to 1 (when the reflection profile is normalized), will get the transmission/absorption profile. So how do we know if the profile we get is the absorption profile or the transmission profile? The simplest way to know is to look at the object. If the object is opaque, then it is the absorption profile. If the object is transparent, then the result is the transmission profile. So, lets look at different transmission and absorption of different objects.
Figure 7. Graph of Reflection and Absorption of a Black Wallet
Figure 8. Graph of Reflection and Absorption of a Blue BPI Card
Figure 9 Graph of Reflection and Absorption of a Green Mini-Guitar
Figure 10. Graph of Reflection and Absorption of a 5 Peso Coin
Figure 11. Graph of Reflection and Absorption of a 20 Peso Bill
Figure 12. Graph of Reflection and Absorption of a 100 Peso Bill
This can be verified by confirming the color of the object. We can see that the reflectance and absorption profiles correspond to the object colors. Also, we can see the limitations of the detector in the "noisy" part of the profiles. We can see distinguishable noise in the regions wavelength less than 400 nm and wavelengths greater than 650 nm.
References:
Wikipedia: The Free Encylopedia
The Physics Classroom
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