EXAM II -- LIGHT

 

All work on this exam must be your own. You may use a calculator and a 3"X5" card of equations. Clearly indicate your final answer for each question. 


 

Section 1: Multiple choice. No partial credit will be given for this section. Circle only one answer for each question.

 
The filament of a headlight is placed at the focal point of a concave mirror, so that

a) light coming from the filament will be focused.

b) light from the filament will form a real image.

c) light from the filament will form a virtual image.

d) light from the filament will emerge parallel.

 

 

2) A ray of light in water enters air. Which of the following does not change?

a) speed at which the light travels.

b) the wavelength of the light.

c) the frequency of the light.

d) none of the above.

 

 

3) Which of the following will not give you information by which you can decide whether a pair of sunglasses is made of polaroid material or not?

a) Rotate the glasses in front of a fluorescent light.

b) Rotate the glasses in front of an LCD.

c) Rotate the glasses in front of glare from the floor.

d) All of the above will provide such information.

 

 

4) When light passes near (but not through) an obstacle it can deviate from its straight-line path. Its path may be changed so that it enters the region behind the obstacle, where geometric optics predicts a shadow. This phenomenon is referred to as

a) dispersion.

b) diffraction.

c) refraction.

d) reflection.

 

 

 

5) Increasing the size of the circular aperture through which light passes

a) will improve the angular resolution of objects viewed through the aperture.

b) will worsen the angular resolution of objects viewed through the aperture.

c) will not change the angular resolution of objects viewed through the aperture.

 

 

 

 

6) Which color of visible light is refracted most when passing from air into fused quartz for which the index of refraction is given by the graph in text?

 

a) red.

b) yellow.

c) green.

d) violet.

7) If a magnifying glass is used to magnify a real object,

a) the focal length of the lens is positive and the image is real.

b) the focal length of the lens is positive and the image is virtual.

c) the focal length of the lens is negative and the image is real.

d) the focal length of the lens is negative and the image is virtual.

 

 

8) Light incident from air onto water experiences total internal reflection

a) for all incident angles.

b) for incident angles larger than the air-water critical angle.

c) for incident angles smaller than the air-water critical angle.

d) for no incident angles.

 

 

9) The index of refraction of a material (other than vacuum)

a) is always less than c (the speed of light in vacuum).

b) is always less than 1.

c) is always greater than 1.

d) is always positive, but may be greater or less than 1.

 

 

10) Two polaroids are crossed, that is the polarizing direction of one is perpendicular to the polarizing direction of the other. Which of the following is true?

a) No light will pass through both, under any circumstances.

b) Nearly all the light that passes through one will pass through the other, under any circumstances.

c) Light will pass through both only if something is placed between them, such as another polaroid.

d) Nearly all the light that passes through one will pass through the other, unless something is placed between them, such as another polaroid.

Section 2: Problems and questions.  Partial credit will be given for this section. Show ALL WORK and JUSTIFY all answers. Be sure your answers include UNITS where appropriate.

 

11) Light is incident from glass on a surface with air, at an angle of 20o.

The index of refraction of the glass is 1.5.

a) Find the angle of reflection and the angle of refraction.

b) Sketch the incident, reflected, and refracted rays, making the angles approximately accurate.

c) For what incident angles will there be no refracted ray?

  

12) A person is wearing glasses, made of diverging lenses with a focal length of 5.3 m (this is the magnitude of f).

a) Suppose an object is 3 m from the lens of these glasses. Calculate the position and magnification of the image.

b) Sketch the ray diagram corresponding to the situation in (a).

c) Fill in the blanks: The image is _______ m _________ (in front or behind) the lens. The image is ________ (erect or inverted) and _______ (larger or smaller) than the image. Be sure your answers to this part agree with both your calculation and your diagram. If not, check for errors (or at least write a comment).

d) Do these lenses correct for near- or far-sightedness?  As always, justify.

 

 

 

13) a) Explain why one does not see interference between the light from two light bulbs.

b) Two slits, each 0.023X10-6 m wide, are 0.75X10-6 m apart. What is the angular distance between the first and second bright fringes to the right of the central maximum, if the slits are illuminated with 526 nm light?

c) What is the difference between the distance traveled by the two beams, as they travel from the two slits to the position of the second bright fringe?



 

14) Light passes through two polaroids. The first has a polarizing direction that makes an angle of 25o from vertical. The other one has a vertical polarizing direction.

a) Unpolarized light passes through these polaroids. What fraction of the initial intensity is passed?

b) Vertically polarized light passes through these polaroids. What fraction of the initial intensity is passed?

c) Describe two ways, other than using polaroids, to produce polarized light.