Reflection
Chapter 13
Reflectance
- Light passing through transparent medium is transmitted, absorbed, or scattered
- When striking a media boundary can also be reflected
- Ratio of light reflected to light incident on surface is reflectance of surface
Reflection
- Diffuse
- light reflected but scattered by irregular surface
- paper, flat paint, snow
- Regular
- light reflected by polished surface without changing orientation between rays
- images can be seen
- shiny metals, mirrors, glossy paint, smooth water
Laws of Reflection
- The angle of incidence is equal to the angle of reflection
- The incident ray, the reflected ray and the normal to the surface are all in the same plane
- True for regular and diffused reflection
Mirrors
- Plane mirror: flat reflecting surface, usually a coating of silver on plane glass
- Spherical mirror: small section of surface of sphere: can be convex or concave depending on which side is silvered
Image Types
- Virtual Image: light rays don't actually pass through image point but appear to have come from there
- Real Image: formed by converging rays of light that actually pass through image point
Images Formed by Plane Mirrors
- Always a virtual image, always upright (right side up) and same size
- Reversed left and right
- Appears as far behind mirror as object is in front of mirror
- Thick mirrors can produce multiple images due to reflections at each boundary
Curved Mirror Terminology
- Center of curvature: center of original sphere
- Vertex: center of mirror
- Principal axis: line drawn through center of curvature and vertex
- Secondary axis: any line drawn through center of curvature to mirror
- Principal focus: point on principal axis where rays parallel to principal axis converge, or appear to diverge from
- Aperture: diameter of mirror
- Focal Length: distance from principal focus to vertex
Converging (Concave) Mirrors
- Converge or focus light
- For light parallel to principal axis, light focused at principal focus
- Parallel rays not parallel to principal axis are focused in focal plane
- Focal length = 1/2 radius of curvature
Spherical Aberration
- Large aperture mirrors exhibit spherical aberration where parallel rays don't converge at same point
- Can be corrected by making mirror parabolic
- Parabolic reflectors used in flashlights, radio communication, car headlights, telescopes
- Case 1: Object at infinite distance; incoming rays are parallel, focus at F; image is point
- Case 2: Object farther than C; image is real, inverted, reduced, located between C and F
- Case 3: Object at C; image is real, same size, inverted, at C
- Case 4: Object between C and F; image is real, enlarged, inverted, located beyond C
- Case 5: Object at F; all rays are reflected parallel, no image formed (reverse of case 1)
- Case 6: Object closer than F; image is virtual, upright, magnified, behind mirror
Ray Diagrams
- Used to locate and describe image produced by object/mirror combination
- Draw principal axis, mirror, C and F, object
- Draw any two of three important rays from point on object to mirror and along reflection
- Ray from object through F will reflect parallel to principal axis
- Ray through C will reflect back on itself
- Ray from object to mirror parallel to principal axis will reflect through F
- For virtual images, use extensions of reflected rays drawn behind mirror
- Point where rays intersect is location of point on image corresponding to point on object
- Usually use arrow for object; draw rays from point
- For a more complete discussion with animation,
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- Convex mirrors diverge light
- Form only virtual, upright, reduced images, located between V and F behind mirror
- Diverging mirrors have negative focal lengths because F is behind mirror
Mirror Equations