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Phys 214, Autumn 2004
Light and Color

Old Exam Questions for Midterm 2


a) What is the shortest length a flat mirror can be for you to be able to see yourself from head to foot when you are standing in front of it  (it is perpendicular to the floor as you are)?  How far off the ground (if it should be) must it be?  (Draw a figure showing it.)

b)            How would you curve a mirror so that you look thinner?  Draw a “ray” diagram showing this.  There is no one correct answer.  Your reasoning counts.

c)             Is it possible to make a real image with a mirror?  Define real image.


Blackbody radiation:   How does the light intensity that a hot body emits vary with its temperature?  As the hot body heats up its color changes.  Please explain how the color changes by drawing emission curves and liken them to filters. 

Interference: Assume the crests of the waves in the picture  below are labeled by the lines. At which of A,B,C is there a "double crest",  a "double trough", or a "nodal point", that is one which does not vary with time?


Interference: Laser light of 400 nm wavelength passes through a pair of narrow slits, separated by 0.02 mm and onto a screen, 2 meters away. 

A)Where is the brightest spot on the screen?

B)How far is the next brightest spots from this one?

C) If you were to use a grating with the same spacing between lines as distance between slits above, how would the image differ?

D)        If you used white light, what would happen with the image on the screen?

E)         If you got a grating with lines closer together, what would that do to the image on the screen?

Polarization: Two polarizing sheets are oriented so that no light passes through the combination.

a)      Where would you place a third polarizing sheet and how would you orient it to allow the maximum amount of light to pass through the combination of three sheets?

b)      Explain why this orientation works.  Show either with figures and explain in words or use an equation and show numerically.


Raytracing:  Find the location of the image of an object placed 1.5 focal lengths from an ordinary converging lens by using a ray diagram showing the three principal rays.


b.                        Is the final image larger, smaller, or the same as the object?  Is it real or virtual?  Is it inverted or upright?

Refraction, reflection: When a beam of orange light (l = 600 nm) goes from air to leaded glass (n = 2.0) at an incident angle of 45°, where 25% of the light is reflected.

A:  What is its wavelength in the glass? 

B:  Does this light still appear orange?  Why?

C:  What is the angle of the ray in the glass relative to the normal of the glass surface?

D:  If this glass is 7 cm thick, how thick does it appear to be when you look into it directly?

E: At what angle would the amount of reflected polarized light be an optimum?  (set up the equation, solve for extra credit) What is this angle called.

F.  If the ray were originating in the glass and trying to make its way out of the glass, what maximum angle of incidence could be used for light to escape?  What happens if this angle is exceeded? (set up the equation, solve for extra credit)

Polarization: How can polarized sunglasses reduce the glare of sunlight reflected from shiny surfaces better than unpolarized sunglasses? Explain, include which direction the glasses are polarized.

Extra Credit: Why does a bright spot appear in the center of the shadow of a sphere formed by a very concentrated source of light? Explain it in a way a friend who has not taken this course might understand.

Other questions:

1. Blue laser light is incident on the surface of glass which has an index of refraction of 3/2. Approximately what is its speed and wavelength in the air and in the glass?

2. If the light in Prob. 1 is incident normally, roughly, only 4% of its intensity is reflected. Which of the following are different in the glass: direction, period, frequency, amplitude, polarization, color?

3. Laser light of 500 nm wavelength passes through a pair of very narrow slits, separated by .05 mm and onto a screen, 1 meter away. Describe the pattern on the screen. Compare with the pattern from a grating that has 200 narrow slits per centimeter.

4. How would the grating pattern in Prob. 3 change if the color of the light were white instead of blue?

5. I have four polaroid sheets, the direction of each is rotated 45 deg. from the preceeding one as shown. If initially unpolarized light is incident on sheet A, what fraction of the original intensity is transmitted? What if only sheets A and C are present? What if only sheets C and D are present?

6. Why is skylight partially polarized?


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