Assignment:
I) Introduction:
When a light ray strikes a smooth interface separating two transparent materials (like air, glass, or water), the wave is partly reflected and partly refracted (or transmitted) into the second material. For an example of this, imagine you are outside looking at a restaurant window. You can probably see both the inside of the restaurant (from the refracted light) and some of the street behind you (from the reflected light). Similarly, a person in the restaurant can see some of the street scene, as well as a reflection of the other people in the restaurant.
The goal of this pre-lab is to understand how light is reflected and refracted, and what the general relationships are between the two. You will be using the Bending Light simulation found at ng the Run Now button.
II) Initial Observations:
First, let's get acquainted with the PhET sim that we will be using. The red button on the laser turns the light on.
What do you notice about the angles of the reflected and refracted light? Briefly, give a qualitative description of the following features:
1. What happens to the reflected and refracted rays as you change the angle of the incident light beam?
2. What does changing the index of refraction do to the refracted and reflected light? Be specific.
III) Quantitative Analysis:
Now let's try to find a mathematical expression for what we are seeing. First, notice that there is a protractor and intensity meter that can be found in the lower right of the sim. You can grab each by moving your pointer over top of them and clicking on your mouse while moving them in place.
For convenience (and by convention), try measuring angles as starting from the vertical axis and beam. (The same goes for the refracted beam - but now you will want to measure from the z-axis to the beam.)
1) With air as the top medium and water as the bottom medium, use your protractor to adjust your laser to so it creates an angle of incidence of 30o. What is the angle of reflection? What is the angle of refraction? Using Snell's Law equation, what is the mathematical solution for θ2?
2) As you increase the angle of incidence (move laser lower), what happens to the angle of refraction? Does it move toward the normal or away from the normal? What happens to its intensity?
3) Change the top medium to water and the bottom medium to air. Once again, set the incident angle at 30o. What is the angle of refraction? Compare your answer to the angle of refraction in question 1. Did the angle of refraction increase (move away from the normal) or decrease (move toward the normal)?
4) At what angle of incidence does the refracted light travel perpendicular to the normal, along the boundary layer? What do we call this angle? You won't be able to see this in the simulation, but you can get close. Calculate this angle using Snell's Law.
5) With your angle of incidence set at 30o again, select ‘Mystery A' as the medium in the bottom layer. What do you notice about the angle of refraction compare to air as the bottom medium? Does ‘Mystery A' have a higher or lower indices of refraction than that of water? Solve for the indices of refraction of ‘Mystery A' using Snell's Law.
Mystery A has a higher indices of refraction than water. The refraction is 0.96.
6) Knowing that the speed of light through air is approximately 2.99x108 m/s, what is the speed of light through glass? Through ‘Mystery A'?
7) What happens to the intensity of the reflected and refracted beam as the angle of incidence increases and decreases? What is the value of the refracted beam when the laser is perpendicular to the boundary layer?
8) Consider a simple ray diagram for the objects and lenses below. Determine the images and describe if they are real or virtual, erect or inverted, and enlarged or reduced.
Explain why.
11) A virtual image cannot be displayed on a screen.
12) Aberrations occur only for real images.
13) A diverging lens cannot form a real image from a real object.
14) The image distance for a positive lens is always positive.
15) A negative image distance implies that the image is virtual.
16) An object 4.0 cm tall is placed 20 cm in front of a thin lens of power 20 diopters. Draw a precise ray diagram to find the position and size of the image and check your results using the thin-lens equation.
17) Describe the cause of chromatic aberration in a lens and how it affects the image collected.
Consider your own eyesight. Can you detect any indication of spherical aberration? If so, describe what you see.