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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) - PowerPoint PPT Presentation
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Chapter 18: Ray OpticsLisa & 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) 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 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
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 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 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 point2. Through near focal point and bends parallel to optical axis3. Through the center of the lens and does not bend
1
2
3
Converging LensReal Image
Converging LensVirtual Image
To draw the rays you follow the same rules as before. However, the raysnever meet, so you trace them backwards to find where they will intersect.
Diverging LensVirtual Image (ALWAYS!)
Follow the same rules and trace backwards to find where the rays meet
Concave MirrorReal Image
1. Parallel to optical axis, reflects off and goes through focal point2. Goes through focal point and reflects parallel to axis3. Goes to center and reflects at the same angle
1
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Concave MirrorVirtual Image
Follow the same rules and extend backwards to see where rays meet
Convex MirrorVirtual 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!
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