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Light

Investigation 1 – Concept Day

 

 

 

 

 

 

 

Light: Investigation 1

Concept Day

SLIDE LIGHT-1-1

In this Investigation, we will discuss light in terms of its transmittance, absorption, and reflection. We will not introduce the concept of wavelengths of light in this Investigation but will discuss it later. We will also discuss the spectrophotometer in further detail. You may have used this instrument in the lab previously.

Note: Remember that light is composed of small moving particles called photons. Since photos move, they have kinetic energy. Therefore, light is a form of energy.

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SLIDE LIGHT-1-2

  • In this simple slide, we see three options light has when it interacts with an object. Another option, refraction, is a property of light passing from one medium to another. Refraction will be discussed in detail in Investigation 4.
  • Reflected light essentially bounces off the surface of an object. We will see in a later slide that reflection of light is not limited to solid objects. Transmitted light passes right through an object, whether solid, liquid, or gas. Finally, absorbed light enters an object; it is not reflected. However, neither does it pass through the object.

Note: You should be familiar with each of these examples of the interaction of light with objects around you. We will briefly discuss these interactions in the next few slides.

  • This slide ends by asking the question, “Which of these forms of light would cast a shadow?” What do you think?

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SLIDE LIGHT-1-3

Note: This is a good slide for a discussion of the question posed in the previous slide, “Which of these forms of light (reflected, transmitted or absorbed) would cast a shadow?”
  • Notice that the only light that passes into this room through the window is where it is transmitted through the panes of glass. Light is not transmitted by the wooden grids between the panes of glass or the material of the wall around the window. Light that strikes these other surfaces is either absorbed or reflected.
  • The window seal where light passes through the glass may appear a bit warmer to the touch than areas in complete shadow. This would be due to the absorption of light energy (more on this later). On the other hand, the very fact that we can see the lighted areas of the widow seal and floor in this photograph tells us that some light is reflected from these surfaces and into our eyes.  

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SLIDE LIGHT-1-4

Note: This slide promotes further thoughts on the nature of light transmittance, absorption, and reflection.

  • “If you were invisible, would you absorb light, transmit light, or reflect light?” “If you were invisible, why would you have a shadow?”
  • If you were invisible, you would transmit all of the light that struck you, like a pane of glass. A perfect pane of glass would not cast a shadow (see previous slide). What is wrong with this picture?

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SLIDE LIGHT-1-5

  • This simple slide of Mt. Hood in Oregon shows a spectacular example of the reflection of light. Light reflecting off of a very still water surface produces an almost mirror-like image of the mountain. Not only can we see the light reflected directly from the mountain, sky, and trees in the top half of the photograph, but we also see light reflected from these objects that is then reflected a second time off the water surface of the pond.  

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SLIDE LIGHT-1-6

  • This slide addresses not only the concept of reflection and absorption of light by different colored objects but also injects the important idea that light is energy. Light has the kinetic energy of moving photons.
  • Photons from the Sun originally obtained their energy from nuclear fusion reactions in the Sun’s deep interior. These photons flood the Earth with enormous quantities of energy and light, some of which is absorbed and some reflected back into space.
  • In this slide, the white block at the left absorbs very little light – almost all of it is reflected. The black block on the right, however, absorbs much of the light energy that strikes its surface. In fact, black itself is the color we perceive when little light is reflected from an object’s surface. There are no “black wavelengths” of light. Rather, black is perceived in the absence of reflected light.
  • As light is absorbed by the black block, so is the energy the light contains. This energy is transferred to the molecules that compose the block. This causes them to vibrate and move more, which in turn causes heat. This is one reason that an asphalt pavement can become intensely hot under the summer Sun compared to a light gray concrete pavement.  

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SLIDE LIGHT-1-7

  • Spectrophotometers are extremely useful instruments in a vast variety of scientific research studies. They can be very expensive.
  • Some spectrophotometers can use light in the UV spectrum in addition to the visible spectrum used by the spectrophotometers in LabLearner labs. Others may have carefully controlled heated cuvette compartments so that chemical reactions can be carefully followed with great accuracy.  

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SLIDE LIGHT-1-8

  • This slide depicts the use of a “blank” to calibrate the spectrophotometer. A blank is a sample that consists only of the solvent that the solute of other samples will be dissolved in. Very frequently, the solvent is water, but this is not always the case. If, for example, a solute is insoluble in water but dissolves readily in isopropyl alcohol, we could use pure isopropyl alcohol for the blank.
  • The blank essentially “tells” the spectrophotometer that there is no solute in the sample and therefore the amount of light that is transmitted through the sample should be read as 100%. On the other hand, since the blank is transmitting 100% of the light that is shined on it, it also tells the spectrophotometer that no light is being absorbed, so the absorbance reading of the instrument is set to zero.
  • In a spectrophotometer, a “detector” detects the amount of light that is transmitted through the sample.  

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SLIDE LIGHT-1-9

  • This slide depicts what happens when a blanked spectrophotometer is used to analyze a sample that contains dissolved solute. Remember that this sample consists of dissolved solute in the exact same solvent used to blank the spectrophotometer.
  • As can be seen, solute molecules absorb some of the light that enters the sample cuvette. As a result, the absorbance reading is no longer zero. Notice that as the absorbance goes up, the percent transmittance decreases.  

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SLIDE LIGHT-1-10

  • One very important use for a spectrophotometer is to determine the amount or concentration of solute in a solution. This slide is a simple reminder for you of the concept of molecular concentration.
  • Water is the solvent in this experiment. The solute is a red-colored molecule. By adding a specific mass of the solute to a specific volume of solvent, solutions of varying concentrations can be prepared. In this example, we have prepared solutions of 2.5g/ml, 5g/ml, and 10g/ml. The first beaker contains only water so the concentration of solute is 0g/ml. We will use this sample as the blank in the following slide to construct a concentration curve.  

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SLIDE LIGHT-1-11

  • This final slide depicts an experiment with a spectrophotometer to create a concentration curve. First, the spectrophotometer is blanked with the pure water sample (0g/ml). Next, the absorption of the three samples (2.5g/ml, 5g/ml, and 10g/ml) are determined.
  • The resulting absorption readings are plotted on the y-axis of a graph against the known concentration of each sample on the x-axis. The points of the curve are joined and a best-fit line is drawn. This is a concentration curve.
  • The value of a concentration curve is to be able to determine the concentration of a sample solution containing an unknown amount of the solute. To do this, the absorption of the unknown sample is determined. This absorption is then located on the y-axis of the graph and read across to the concentration curve and then down to the x-axis. Where that absorption intersects the x-axis gives an estimate of the concentration of the unknown sample. As one can imagine, spectrophotometers and concentration curves are used in laboratories every day.
  • Examples of unknown sample absorptions are given on the lower left of this slide for classroom practice. Look at the absorbance data and the concentration curve above it to estimate the concentration of each of the three samples.