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#### Light Waves

###### Reflection of Light • Plane (flat) mirrors produce a reflection.
• Rays from an object reflect off the mirror into our eyes, but we see them behind the mirror.
• The image has these properties:
• The image is the same size as the object.
• The image is the same distance from the mirror as the object.
• A line joining equivalent to the points of the image and object meet the mirror at a right angle.
• The image is virtual; no rays pass through the image and the image cannot be formed on a screen
• Laws of Reflection: Angle of incidence = Angle of reflection
• The incident ray, reflected ray and normal are always on the same place (side of the mirror)
###### Refraction of Light
• Refraction is the bending of light when light travels from one medium to another.
###### Experimental Demonstration
• Using the ray box, pass a ray through a glass slab on a white sheet of paper.
• Mark two points on the incident ray, refracted ray, emergent ray, and draw an outline of the glass slab with a pencil on the paper.
• Then, by connecting the dots, you can produce a diagram like the one below, a protractor is used to find the angles.
• When a ray passes through a transparent parallel-sided material, its passage will look like this: • NOTE: The emergent ray is parallel to the incident ray.
###### Critical Angle
• The angle at which the refracted ray is parallel to the surface of the material.
• If the angle of incidence is greater than the critical angle, there is no refracted ray, there is total internal reflection.
• If the angle of incidence is less than the critical angle, the incident ray will split into a refracted ray and a weaker reflected ray. • Refractive Index, n = Speed of light in vacuum/Speed of light in a medium
• Refractive index = sin i/sin r
• Critical angle = sin-1 1/n
###### Optical Fibers
• Light put in at one end is totally internally reflected until it comes out the other end.
• It is used in communications: Signals are coded and sent along the fiber as pulses of laser light.
• It is used in medicine:  An endoscope, an instrument used by surgeons to look inside the body, contains a long bundle of optic fibers.
###### Thin Converging Lens
• Principal Focus is the point where rays parallel to the principal axis converge with a converging lens.
• Focal Length is the distance from the principal focus and the optical center.
• Principal Axis is the line that goes through the optical center, and the two foci.
• Optical Center is the center of the lens.
• A Real Image can be caught on a screen.
• A Virtual Image cannot be caught on a screen.

Real Image

• When the object is further away from the optical center than F‘ is 1. A ray through the center of the lens passes straight through the lens.
2. A ray parallel to the principal axis passes through the focus on the other side of the lens
3. A ray through F’ will leave the lens parallel to the principal axis.

Virtual Image

• When the object is closer to the optical center than F‘ is • Magnifying Glass
• When a convex lens is used like this – an object is closer to a convex (converging) lens than the principal focus (like the diagram above), the rays never converge. Instead, the appear to come from a position behind the lens.
• The image is upright, and magnified, it is a virtual image.
###### Dispersion of Light • Refraction by a Prism
• When light is refracted by a prism, the incidence ray is not parallel to the emergent ray, since the prism’s sides are not parallel.
• If a beam of white light is passed through a prism, it is dispersed into a spectrum.
• White light is a mixture of colors, and the prism refracts each color by a different amount – red is deviated least and violet most.
###### Light Spectrum ###### Electromagnetic Spectrum • All electromagnetic waves:
• travel at the speed of light; 3 × 108 m/s
• don’t need a medium to travel through (they travel through a vacuum)
• can transfer energy
• are produced by particles oscillating or losing energy in some way
• are transverse waves

Uses