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LSU PHY 2113 Test 1 - Chao Wang
questions and answers
When light passes through a spherical wave, what do the rays do? - The rays diverge When light passes through a plane wave, what do the rays do?? - They remain parallel Define the law of reflection. - The incident ray, the reflected ray, and the normal to the surface all lie in the same plane. The angle of reflection theta r equals the angle of incidence theta i What is specular reflection? - When paralellel light strikes a smooth, plane surface and the reflected rays are PARALLEL to each other. --surface is shiny and smooth; like a mirror What is diffuse reflection? - Surfaces are not entirely smooth. The irregular surface reflects light in various directions, spreading out the light. Describe the properties of an image formed by a plane mirror. - The image is: --Upright --The same size as the object(i.e. the heights of the object and the heights of the image are equal) --virtual image --Located as far behind the mirror as the object is located in front of it (meaning Do= Di) What is a virtual image? - None of the rays actually emanate from the image
What is a real image? - All rays emanate from an image What are rays? - lines that are perpendicular to the wave fronts and point in the direction of the velocity of the wave. What is the mirror equation? - 1/ distance of the image from the mirror + 1/ distance of the object from the mirror = 1/ the focal length What is the radius of curvature (R)? - The distance from the center of curvature to the mirror What is the magnification equation?? - m = image height/ object height = - distance of image from mirror/ distance of object from mirror Describe the effect of spherical aberration. What does this effect result in? - This effect occurs in spherical mirrors when rays lie too far from the principal axis and will not be reflected through the focal point. --Basically a spherical mirror does not bring all rays parallel to the principal axis to a single impact point after reflection. This results in blurred images. What is the angle of incidence? - The angle that the incident ray makes with respect to the normal to the surface. How can spherical aberration be minimized? - By using a mirror whose height is SMALL compared to the radius of curvature What is the angle of reflection? - The angle that the reflected ray makes with the normal Describe the three rays you draw to locate the position and size of an image formed by a CONCAVE mirror - Draw each of the rays from the top of the object. Ray 1:
Describe the image that is formed by a concave mirror when the object is between C and F? - The image is: --Larger --Inverted --Real How do convex and concave mirrors differ in how they reflect light in relation to the principal axis? - Concave mirrors reflect light rays TOWARD the principle axis. Convex mirrors reflect light rays AWAY from the principal axis. Describe the three rays you draw to locate the position and size of an image formed by a CONVEX mirror - The procedure is the same as for a concave mirror. BUT REMEMBER The focal point and center of curvature lie BEHIND the mirror. Ray 1: --This ray leaves the object traveling parallel to the principal axis. After reflection from the mirror, the ray appears to originate from the focal point of the mirror Ray 2: --This ray leaves the object and heads toward the focal point. After reflection, the ray travels parallel to the principal axis. Ray 3: --This ray leaves the object and travels toward the center of curvature. The ray strikes the mirror perpendicularly and reflects back on itself. What is the focal length (f)?? - The distance between the focal point and the mirror Describe the image that is formed by a convex mirror - The image is: --Upright
--Virtual --Smaller than than the object True or false. A convex mirror always forms a virtual image. - True What is the equation for the focal length of a concave mirror? A convex mirror? - Concave: F = 1/2R Convex: F = - 1/2R Remember R is the radius of curvature for spherical mirrors
What is the magnitude of the focal length? - It is 1/2 the radius of the curvature. or 1/2R Is the focal length positive or negative for a concave mirror? - Positive When is the ONLY time that it is valid to use F = 1/2R? - Only valid for rays that lie close to the principal axis (for paraxial rays) Is the focal length positive or negative for a convex mirror? - Negative What is the sign of the object distance if the object is IN FRONT of the mirror (real object)? - Positive What is the sign of the object distance if the object is BEHIND the mirror (virtual object)? - Negative Where is the focal point of a concave spherical mirror? convex spherical mirror? - Concave: A point on the principal axis, IN FRONT of the mirror Convex: A point on the principal axis, BEHIND the mirror.
Answers (a) and (b) are incorrect. Figure 25.9b represents a top view of the person in front of the two mirrors and has been repeated in the margin for convenience. It is a straightforward matter to understand two of the images that she sees. These are the images that are normally seen when one sits in front of a mirror. Sitting in front of mirror 1, she sees image 1, which is located as far behind that mirror as she is in front of it. She also sees image 2 behind mirror 2, at a distance that matches her distance in front of that mirror. Each of these imag - A person is sitting in front of two mirrors that intersect at a 90° angle. As Figure 25.9a illustrates, the person sees three images of herself. (The person herself is only partially visible at the bottom of the photo.) These images arise because rays of light emanate from her body, reflect from the mirrors, and enter her eyes. Consider the light that enters her eyes and appears to come from each of the three images identified in Figure 25.9b. The following table shows three possibilities for the number of reflections that the light undergoes before entering her eyes. Which one is correct? 55 degrees. The law of reflection states that the angle of incidence equals the angle of reflection. The angles of incidence and reflection are measured with respect to the line perpendicular to the surface. Since the first and last mirrored walls off of which the light reflects are perpendicular, the sum of the initial angle of incidence and the final angle of reflection is equal to 90 degrees. Therefore, the final angle of reflection is 55 degrees. - The drawing shows a light ray undergoing multiple reflections from a mirrored corridor. The walls of the corridor are either parallel or perpendicular to one another. If the initial angle of incidence is 35°, what is the angle of reflection when the ray makes its last reflection? If a clock is held in front of a mirror, its image is reversed left to right. From the point of view of a person looking into the mirror, does the image of the second hand rotate in the reversed (counterclockwise) direction? - Yes. Since the image is reversed, the second hand appears to rotate in the counterclockwise direction. Images in plane mirrors are always reversed left to right and right to left. open it up to produce a more gently curving shape
Since spherical abberation can be minimized by using a mirror whose height is small compared to the radius of curvature, opening the surface up to produce a more gently curving shape will bring the top ray closer to the focal point F after reflection. - Refer to Figure and the related discussion about spherical aberration. To bring the top ray closer to the focal point F after reflection, would you open it up to produce a more gently curving shape or bring the top and bottom edges closer to the principal axis? Is it possible to use a convex mirror to produce an image that is larger than the object? - No. Can the image formed by a concave mirror ever be projected directly onto a screen without the help of other mirrors or lenses? If so, specify where the object should be placed relative to the mirror. (b) Repeat part (a) assuming that the mirror is convex. - Yes, if the distance between the object and the mirror is greater than the focal length of the mirror. (b) No, because the image is always behind the mirror. Solution: (a) If the object distance is greater than the focal length of the mirror, then the image is real and can be projected directly onto a screen. (b) It is not possible for a convex mirror to project an image directly onto a screen because the image is always virtual. Suppose you stand in front of a spherical mirror (concave or convex). Is it possible for your image to be (a) real and upright (b) virtual and inverted? - No and no Since upright images are always virtual and inverted images are always real, neither (a) nor (b) is possible. An object is placed between the focal point and the center of curvature of a concave mirror. The object is then moved closer to the mirror, but still remains between the focal point and the center of curvature. Do the magnitudes of (a) the image distance and (b) the image height become larger or smaller? - The image distance and image height become larger. By drawing two ray diagrams, one for each location of the object, one can see that both the image distance and image height become larger.
