Exam 2 practice problems solutions

At the right face the red light has the smallest

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At the right face, the red light has the smallest angle of incidence, which means it is the least likely to experience total internal reflection. I need to determine the critical angle for the red light. F 75 i1 75 r1 i2 r2
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1 c 1.00 sin 48.3 1.34 In order for the incident angle on the right face to have this value, the refraction angle at the top face must be 90 48.3 41.7    . In order for that to happen the incident angle at the top face must be i1 i1 r1 r1 1 1 r1 r1 i1 i1 sin sin 1.34 sin 41.7 sin sin sin 63.1 1.00 n n n n As long as the incident beam hits the top face at this angle or less, total internal reflection will happen at the right face. 27. The focal length of an iPhone 3GS’s camera lens is 3.85 mm. The camera is able to create a sharp image on the CCD (the sensor that plays the role of film in a digital camera) when the object being focused on is anywhere from 2.0 cm to infinitely far from the lens. In order to deal with objects at different distances from the camera the lens moves closer/further from the CCD. a. Determine the range of motion of the lens. b. Draw a ray diagram for when the object is 2.0 cm from the lens. c. No camera is able to take a sharp picture of an object located closer to the lens than the focal point. Explain why, and include a ray diagram. To determine the range o f motion (meaning, the distances from the CCD the lens can be) I’ll use the thin lens equation, once for the furthest possible object and once for the closest. Since the image forms on the CCD, i d is the quantity I’m interested in: 1 1 i, furthest o 1 i, closest 1 1 1 1 3.85 mm 3.85 mm 1 1 4.77 mm 3.85 mm 20 mm d f d d So, the lens only needs to range from 3.85 mm to 4.77 mm (just 0.92 mm!) to be able to focus on objects 2.0 cm to infinitely far away. A ray diagram is shown here for when the object is 2.0 cm away. When the object is closer to the lens than the focal point the lens produces a virtual image on the same side of the lens as the object. Only real images can be recorded by a physical device, which means camera can’t record images of objects that are this close to the lens. A ray diagram for this case is shown here.
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