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) 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

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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

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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!