Chapter 18: Ray Optics Lisa & Becky

Ray Model of Light  Light rays travel in straight lines  Light rays cross but do not interact  Light rays travel forever unless they interact with matter  Object is a source of light rays Point source– comes from one point (laser) Point source– comes from one point (laser) Extended source– collection of points (light bulb) Extended source– collection of points (light bulb)  Light sees by focusing a group of rays The eye inverts the object, but the brain flips it back upright The eye inverts the object, but the brain flips it back upright

The Pinhole Camera  A light proof box has a single hole  Film sensitive to light is placed at the back  Each point on the object illuminates a single point on the film  The image is inverted

Types of Reflection  Specular- Reflection off of a smooth surface (mirror)  Diffuse- Reflection off of a jagged surface

Reflection  Angle of incidence = Angle of reflection  Angle of incidence- angle between the ray and the line perpendicular to the surface (normal)  Angle of reflection- angle between the reflected ray and the normal

Plane Mirrors  A plane mirror is a flat mirror  s’ = s  The rays bouncing off the mirror enter our eye, but we see if behind the mirror, at A’

Refraction  The bending of light when it moves from one medium to another  Snell’s Law- the angle of refraction depends on the refractive index (n) of the medium the angles used are always from the normal the angles used are always from the normal

Total Internal Reflection  When all the light reflects back into the medium  Critical angle- the angle the light must refract at in order to have total internal reflection  Critical angle when theta2 = 90 degrees  There is no TIR if n2 > n1

Dispersion  Dispersion- index of refraction varies slightly with color This means it varies slightly with wavelength  N is larger when the wavelength is shorter Violet light refracts more than red light

Thin Lenses  Lens- transparent material that uses refraction of life at curved surfaces to form an image  Ray tracing- the pictorial method to finding an image  Converging lens- Causes the ray to refract towards optical axis Thicker in the middle Thicker in the middle  Focal point- Common point through which initially parallel rays pass  Focal length- distance from the lens to the focal point  Diverging lens- Causes rays to refract away from optical axis Thinner in the middle Thinner in the middle  Virtual image- has the same orientation of the object  Real image- inverted from the image

RAY TRACING!!!!

1.Parallel to optical axis and bends through far focal point 2.Through near focal point and bends parallel to optical axis 3.Through the center of the lens and does not bend Converging Lens Real Image

Converging Lens Virtual Image To draw the rays you follow the same rules as before. However, the rays never meet, so you trace them backwards to find where they will intersect.

Diverging Lens Virtual Image (ALWAYS!) Follow the same rules and trace backwards to find where the rays meet

Concave Mirror Real Image 1.Parallel to optical axis, reflects off and goes through focal point 2.Goes through focal point and reflects parallel to axis 3.Goes to center and reflects at the same angle 1 2 3

Concave Mirror Virtual Image Follow the same rules and extend backwards to see where rays meet

Convex Mirror Virtual Image (ALWAYS!) Follow the same rules and extend backwards to find where rays meet

Now try problem 15 on page 611, and 29 on page 612

Good Luck on the Final! BYEEEE!