## Chapter Objectives and Summaries

### CHAPTER 17 Reflection of light

LEARNING OBJECTIVES
Knowledge of subject matter
• Distinguish between diffuse and regular reflection.
• Know the laws of specular reflection
• Describe the properties of reflection in mirrors (plane, concave, convex)
• Use the following formulae in simple situations:
(a) L(i) = L(r)
(b) 1/do + 1/di = 1/f; or 1/v + 1/u = 1/f
(c) hi/ho = di/do; or hi/ho = v/u = M
• State the meaning of the terms associated with mirrors: (a) object distance (b) image distances (c) focal length (d) principal focus (e) principal axis (f) magnification (g) centre of curvature

SCIENTIFIC PROCESSES

• Analyse passage of light from mirrors.
• Derivation of the mirror formula
• Communicate optical information by means or ray diagrams.
• Identify the use of lenses and mirrors in every day life
• Write up laboratory reports on images formed in mirrors and/or lenses

COMPLEX REASONING PROCESSES

• Analyse the passage of light to find images for a combination of optical devices

### CHAPTER 17 SUMMARY

• Light is a form of electromagnetic energy that does not require a medium for its propagation.
• The speed of light is 3 x 108 m s-1 in a vacuum and less in other media.
• When light reflects from a plane mirror the angle of incidence equals the angle of reflection. Also, the incident ray, the reflected ray, and the normal all lie in the same plane.
• The image of an object in a plane mirror lies the same distance behind the mirror as the object is in front. The image is: upright, the same size, virtual, and laterally inverted.
• Regular reflection is the reflection from a smooth surface such as a mirror. Diffuse reflection occurs when rays are reflected from a irregular surface - rays are reflected in all directions and no image is formed.
• Curved or spherical mirrors are parts of spheres whose centre is the centre of curvature of the mirror.
• Concave or converging mirrors are curved inwards and focus parallel light.
• Convex or diverging mirrors are curved outwards and diverge parallel light.
• The line from the centre of the mirror through the centre of curvature is the principal axis.
• The focus is the point where light parallel to the principal axis converges (for a concave mirror) and appears to have come from after reflection for a convex mirror. The focal point lies half way between the mirror and the centre of curvature. The focal point for a concave mirror is real and is virtual for a convex mirror.
• For a concave mirror light originating at the focus reflects parallel to the principal axis.
• To find the image of an object in a mirror requires the drawing of two rays that intersect, or appear to intersect, after extrapolation the reflected rays. The common rays used are: one through the focus which reflects parallel to the principal axis; one parallel to the principal axis reflects back through the focus; and one through the centre of curvature reflects back through the centre of curvature.
• Images are described using the terms: position; nature - real or virtual; size; and upright or inverted.
• Virtual images form behind the mirror. Rays of light do not actually pass through them and the image can not be focused on a screen.
• Real images are formed when the rays of light actually pass through them. They form in front of the mirror.
• The Mirror formula ; .. 1/f = 1/v + 1/u
shows the relationship between the location of the object, the image and the focal length.
• Magnification (M) is the size of the image compared to the size of the object. M = v/u = Hi/H0
• Spherical aberration is the inability of a concave mirror to focus parallel light to a point.
• Curved mirrors have many uses including: shaving and make-up mirrors; reflectors for car headlights, searchlights, torches, satellite dishes, and in large optical telescopes. Convex mirrors are used where wide fields of view are required: security mirrors and for `blind corners'.