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Investigation 4 – Concept Day

 

 

 

 

 

 

 

Light: Investigation 4

Concept Day

SLIDE LIGHT-4-1

  • In this Investigation, we will turn to another way in which light interacts with objects – refraction. Refraction of light occurs when it moves from one medium (air for example) into another (water, for example). Different media slow the speed of light waves to different extents. This results in a bending of light at the interface between the two media.

Note: Like many other aspects of light, you already have a wealth of experience with light refraction in everyday life. This will make it easier for you to understand the scientific concepts that help explain why refraction occurs.

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SLIDE LIGHT-4-2

  • This slide appears twice in this presentation. It shows a laser beam shined onto the surface of a liquid in a tank. At the bottom of the tank, the beam interacts with a small mirror.
  • We will end the presentation by returning to this slide and identifying the different examples of both refraction and reflection that occurs in it. At this point, it can simply be said that refraction bends light as seen where the laser interacts at the air/liquid interface.

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SLIDE LIGHT-4-3

  • This slide is similar to ones used in previous Investigations in this CELL. It serves as a review of the concepts of reflection, absorption, and transmittance. In addition, it introduces the concept of light refraction.
  • Refraction occurs when light is bent when moving from one medium to another. If the second medium does not absorb the light, it will emerge from the second medium and will often be refracted a second time as shown here.

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SLIDE LIGHT-4-4

  • This slide shows a very good example of refraction in everyday life. In this case, the two mediums that light travels through are air and water. Notice that solid objects, when they reflect light to our eyes through different media, exhibit refraction. You should keep this in mind during this Investigation because we will study refraction by observing actual light beams being refracted rather than light being reflected from objects.

Note: Can you think of other examples of refraction that you have observed in the past. Some examples of refraction are listed below, can you name others?

  • A spoon in a glass of water
  • A prism
  • A rainbow
  • Any lens (hand lens, microscope lens, lens in human eye)
  • An old glass window that appears rippled refracts light and distorts the image

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SLIDE LIGHT-4-5

Note: The green box insert is included simply for reference. It shows reflection of light.

  • In this slide we see light being refracted as it moves from Medium 1 into Medium 2. Notice that the beam of light is bent at the interface. To determine the extent or degree of bending, a perpendicular dashed line is drawn. The angle of incidence is the angle between the light beam in Medium 1 and the perpendicular line. Notice that the angle of incidence for refraction and reflection (green box) are essentially the same. It is the angle at which the light beam strikes the surface.
  • The angle of refraction is determined by measuring the angle between the light beam in Medium 2 and the perpendicular.

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SLIDE LIGHT-4-6

  • This slide explains the property of a substance known as the index of refraction (also called the refractive index). It also includes a formula for calculating the index of refraction of a medium.

refractive index

  • Don’t be intimidated by this equation because of the Greek letter phi (ɸ) and the mathematical term sin. In fact, this may be an exceptionally good opportunity to use a somewhat complicated-looking equation to dispel the notion that the math component of science is difficult. In fact, this equation is as simple as dividing two real numbers such as 8 by 2.
  • Walking through this formula using the steps on the slide should be done with an eye on its simplicity. In all our calculations, for example, ni will always be 1 as this is the index of refraction of air. Phi is simply used instead of the word “angle” and sine (or sin) is only a keystroke on a calculator!

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SLIDE LIGHT-4-7

  • This slide gives an example of refraction and a sample calculation of the index of refraction of Medium 2. The equation is presented in red text.
  • The first thing we need to plug into the equation is the angle of incidence and the angle of refraction. In this instance, these are 45o and 39o, respectively.
  • Next, the sin of each of these numbers is obtained by entering them into a calculator and hitting the sin key. Representations of these calculator steps is shown on the right.
  • Next, we insert these two sins into the formula and also replace the index of refraction of ni with the numeral 1, which is the ni of air.
  • Next, we do the simple division of 0.707 by 0.629 and get the answer. It’s that simple!

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SLIDE LIGHT-4-8

  • This slide gives you additional practice by solving another index of refraction problem. The calculator images with the appropriate sins are included to the right.

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SLIDE LIGHT-4-9

  • This slide shows a close-up of the objective lenses of a microscope similar to those in the LabLearner lab. Since glass lenses refract light, the refractive index of all objective lenses is always stamped on them for reference. You may wish to confirm this when in the lab.

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SLIDE LIGHT-4-10

  • This slide shows how a prism uses the principle of refraction to separate the mixed waves in white light into its separate component wavelengths.

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SLIDE LIGHT-4-11

  • Finally, we return to this slide of a laser beam being shined into a tank of liquid and striking a mirror at the bottom. The direction of the laser beam is now shown by the arrows on it – the beam comes in from the left.
  • The first refraction event is at the air/liquid interface. One can easily see that the angle of refraction is smaller than the angle of incidence. If we measured these two angles with a protractor, we could easily calculate the index of refraction of the liquid.
  • The second refraction event at the liquid/air interface as the laser beam leaves the tank of liquid. Notice that in this case, the angle of incidences is smaller than the angle of refraction, the reverse of the first refraction event. You might notice visually that the angles formed at the two refraction events are very similar… they are, in fact, identical.
  • A reflection event occurs at the bottom of the tank where the laser beam strikes a mirror. Notice that the angle of incidence and the angle of reflection are equal as discussed in Investigation 2 due to the Law of Reflection.
  • Finally, there is a somewhat less obvious reflection event. This is the point where the laser beam first strikes the liquid surface. Although most of the beam is transmitted into the liquid, a small component of it is reflected off the surface and back into the air, in which case, the angle of reflection is identical to the angle of incidence. This is a bit hard to see in this example.
  • One could look at this slide for some time and ponder the other geometric relationship in play!