Physics 240 Second Midterm Exam Your Name: Please write neatly so the grader can follow. Please label the problems in order, and use your own papers as needed. Problem 1 (20 points) The refractive index of a glass is 1.4. (a) What is the critical angle for total internal reflection when light travels from such glass to air? (b) A thin double convex lens (with equal radius of curvature of 10.0 cm for both curved surfaces) is made from this glass. Calculate the focal length of the lens. (c) If an object is placed 25.0 cm away from the left side of the lens, where is the image and what's the magnification? (d) Draw the ray diagram (no need to be very accurate). Problem 2 (20 points) In a Young’s double-slit interference experiment, the light source has a wavelength of 600 nm (1 nm = 10-9 m). The width of each slit is 1.00 m (10-6 m) and the separation between the two slits is 5.00 m. A distant screen is located 1 meter away to observe the intensity pattern. (a) What's the location of the 2nd interference maxima (m=2) on the screen? (b) What's the location of the first diffraction minimum on the screen? (c) Will you see any interference bright fringe missing on the screen? If is, which order? (d) Sketch the intensity pattern you would observe. Problem 3 (20 points)

An object is located 4.0 cm in front of (a) a concave mirror with focal length of 2.0 cm. (b) a convex mirror with focal length of 2.0 cm. Where is the image for each mirror and what's the magnification? Please Show your calculation, and then Draw the ray diagram for each mirror. Problem 4 (20 points) A distant car is approaching you at night. Assuming you can just barely resolve the two headlights, and that the headlights are 1.4 meters apart. Take the wavelength of the light from the headlights as 550 nm, and the diameter of the pupil in your eye as 5 mm. Find the distance between you and the car.

Multiple-choice (20 points) You should circle the best answer, and only one answer should be circled.

1. About the total internal reflection, which of the following statements is NOT true?

(a) It can happen when light travels from a glass to air. (b) It can happen when light travels from water to air. (c) It can happen when light travels in an optical fiber. (d) It can happen when light is reflected at any angle.

2. In a single split experiment, you see a central bright spot with some diffraction pattern on the

screen. Which of the following statements is UNLIKELY to be right? (a) The bright spot becomes larger as you decrease the width of the slit. (b) The bright spot becomes larger as you increase the wavelength of light. (c) The bright spot becomes larger as you move the screen further away. (d) The bright spot becomes larger as you increase the intensity of light.

3. Sending a white light beam into a prism, you see a colorful rainbow-like pattern

because of (a) Diffraction (b) Dispersion (c) Superposition (d) Interference

4. An object is located 4.0 cm in front of a concave mirror with focal length of 2.0 cm. So the image is (a) inverted, 4 cm away from the mirror (b) upright, 2 cm away from the mirror (c) real, 2 cm away from the mirror (d) virtual, 4 cm away from the mirror

5. A soap film has an index of refraction of n, and mercury light of wavelength falls on it at normal incidence. If the thickness of the film is t, which of the following is true in order to have no or minimum reflection? (a) 2t = m(b) 2nt = m (c) 2t = (2m+1) /2 (d) 2nt = (2m+1) /2

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