Presentation on theme: "Suppose that you hold the transparency in the photograph below in front of a mirror. How will its reflection appear? Is the image: (1) inverted top-to-bottom?"— Presentation transcript:
Suppose that you hold the transparency in the photograph below in front of a mirror. How will its reflection appear? Is the image: (1) inverted top-to-bottom? (2) inverted left-to-right? (3) inverted front-to-back? (4) not inverted at all? Write out the word LIGHT as it will appear in the mirror.
Image Formation: Reflection Plane Mirror: This is the simplest type of image formation we can discuss for reflection. o i ii rr ii d d Mirror How far behind the mirror would the image be? Same distance that the object is in front of the mirror. The path length (distance light travels along path) from the object to the observer must be the same as from the image to the observer. The image formed for a plane mirror is called a virtual image. Virtual image – The light rays never pass through the image. Real image – The light rays do pass through the image Since the incident and reflected angles must be the same, and the total path length for the object and the image must be the same we can conclude that all distances relating the object to the observer and the image to the observer must be the same. The image in the mirror is reversed front to back. Object Location Image Location Observer
Curved Mirror: Curved mirrors are either concave or convex. Concave – Curved towards object – converging mirror Convex – Curved away from object – diverging mirror Focal point Center of Curvature Concave mirror o i f c Principle Axis c f When parallel light rays are incident on the concave mirror, it redirects them all to a single point called the focal point. The focal point is located at a distance ½ the radius of curvature. Light rays from an object will converge at the location of the image. The image will be a real image
Convex mirror o i f c When parallel light rays are incident on the convex mirror, it redirects them all away from the focal point. Light rays from an object will diverge from the location of the image. The image will be a virtual image c f Magnification: h – Object height h’ – Image height p – Object distance from mirror q – Image distance from mirror The Mirror Equation: Relates object distance, image distance and focal point. The following can be determined from geometry.
Refraction: Refraction is another means of changing the direction of light. Refraction – the bending of light rays as it passes the boundary between two different media. Why does the light ray bend? The speed of light changes from one medium to the next. The speed of light is supposed to be a constant how can it change? The absorption and reemission of light from molecule in the medium cause a delay in the travel time through the medium. This corresponds to a lower average speed for light in different media. Different materials have different rates of absorption and reemission. The ratio of the speed of light in a vacuum to the speed of light in a particular medium is the index of refraction. n – index of refraction c – Speed of Light in a vacuum [m/s] v - Speed of Light in a medium [m/s] n > 1 for most materials n = 1 for a vacuum n ~ 1 for air The index of refraction is actually a complex number, with the real part corresponding to the amount of light transmitted and the imaginary part corresponding to absorption.
Let us look at what would be observed for refraction at different boundaries. Air Glass n1n1 n2n2 IncidentReflected Refracted Incident Reflected Refracted When n 2 > n 1 the light ray bends towards the surface normal. When n 1 > n 2 the light ray bends away from the surface normal. 11 22 22 11
What causes these tendencies for a light ray to bend towards or away from the normal? Air Glass n1n1 n2n2 Consider plane waves incident on the interface between air and glass. (n 1 < n 2 ) When one edge of the wavefront enters the new medium, what happens? The edge in the new medium travels slower than the rest of the wave that remains in the initial medium, causing the far edge to rotate about the near edge. Wavefront Wavefront – surface connecting points of equal phase on all waves. The direction of rotation is reversed when n 1 >n 2. The interaction between the electric and magnetic fields of the plane wave and the medium is the reason for the speed change. The result of this interaction is a simple expression that can be used to relate the incident angle to the refracted angle. Snell’s Law of Refraction