Optics Mirrors and Lenses. Light Light can be a wave or a particle.Light can be a wave or a particle. Individual particles of light are called photons.Individual

OpticsOpticsMirrors and LensesMirrors and Lenses

LightLight

• Light can be a wave or a particle.Light can be a wave or a particle.

• Individual particles of light are called photons.Individual particles of light are called photons.

• Photons have no mass, they are a bundle of energy.Photons have no mass, they are a bundle of energy.

• The higher the frequency the greater the wave has.The higher the frequency the greater the wave has.

UCSD: Physics 8; 2006

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Light IntensityLight Intensity

• Intensity is the the amount of light covering a given Intensity is the the amount of light covering a given surface area.surface area.

• Intensity is a measure of power, so it is the amount of Intensity is a measure of power, so it is the amount of power over a certain amount of space. power over a certain amount of space.

• As light gets further away it spreads out more, which As light gets further away it spreads out more, which means the amount of power on a given area is less means the amount of power on a given area is less and there fore the intensity is less.and there fore the intensity is less.

– (don’t worry I am not making you do the math here)

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Light IntensityLight Intensity

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ReflectionReflection• We describe the path of light as straight-line raysWe describe the path of light as straight-line rays• Reflection off a flat surface follows a simple rule:Reflection off a flat surface follows a simple rule:

– angle in (incidence) equals angle out (reflection)– angles measured from surface “normal” (perpendicular)

surface normal

sameangleincident ray exit ray

reflected ray

Reflection VocabularyReflection Vocabulary• Real Image – Real Image –

–Image is made from “real” light rays that converge at a real focal point so the image is REAL

–Can be projected onto a screen because light actually passes through the point where the image appears

–Always inverted

Reflection VocabularyReflection Vocabulary

• Virtual Image– Virtual Image– –“Not Real” because it cannot be

projected –Image only seems to be there!

Hall MirrorHall Mirror

• Useful to think in terms of Useful to think in terms of imagesimages

“image” you

“real” you

mirror onlyneeds to be half as

high as you are tall. Yourimage will be twice as far from you

as the mirror.

LEFT- RIGHT REVERSALLEFT- RIGHT REVERSAL

Curved mirrorsCurved mirrors

• What if the mirror isn’t flat?What if the mirror isn’t flat?– light still follows the same rules, with local surface normal

• Parabolic mirrors have exact focusParabolic mirrors have exact focus– used in telescopes, backyard satellite dishes, etc.– also forms virtual image

Two types of curved mirrorsTwo types of curved mirrors

• Convex: Means the mirrors surface bulges outward.Convex: Means the mirrors surface bulges outward.– Rays are reflected away from the center.

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Two types of curved mirrorsTwo types of curved mirrors

• Concave: Means the mirrors surface curves inwards.Concave: Means the mirrors surface curves inwards.– Rays are focused towards a middle point.

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Parts of the mirror imageParts of the mirror image

• We can see an image where the rays meet.We can see an image where the rays meet.• This is called a focal point. This is called a focal point. • The distance from the mirror surface to the focal point The distance from the mirror surface to the focal point

is called the focal length.is called the focal length.• For convex mirrors the image is virtual.For convex mirrors the image is virtual.• For concave mirrors the image is real.For concave mirrors the image is real.

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Curved MirrorsCurved Mirrors

• The line running perpendicular through the middle of The line running perpendicular through the middle of the mirror is called the principal axis.the mirror is called the principal axis.

• If we were to make a complete circle for the mirror If we were to make a complete circle for the mirror the very center of the circle would be at “C” (center of the very center of the circle would be at “C” (center of curvature)curvature)

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Objects in mirrorsObjects in mirrors

• Where you place an object relative to a mirror Where you place an object relative to a mirror determines the following:determines the following:– Is the image real or virtual– Is the image larger or smaller than the true object.– Is the image inverted (up side down) or up right.

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For a real object between f and the mirror, a For a real object between f and the mirror, a virtual image is formed behind the mirror. The virtual image is formed behind the mirror. The image is upright and larger than the object. image is upright and larger than the object.

For a real object between f and the mirror, a virtual image is formed behind the mirror. The position of the image is found by tracing the reflected rays back behind the mirror to where they meet. The image is upright and larger than the object.


For a real object close to the mirror but outside of the center of curvature, the real image is formed between C and f. The image is inverted and smaller than the object.

For a real object between C and f, a real image is formed outside of C. The image is inverted and larger than the object.





For a real object at C, the real image is formed at C. The image is inverted and the same size as the object.

For a real object at C, the real image is formed at C. The image is inverted and the same size as the object.




For a real object at f, no image is formed. The reflected rays are parallel and never converge.

For a real object at f, no image is formed. The reflected rays are parallel and never converge.

Mirror EquationMirror Equation

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We can use math to determine focal length and magnification.

Mirror Equation: 1/f = 1/di + 1/do

f = focal length, di = distance to image do = ditsance to object

MagnificationMagnification

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You can find out by how much the image was enlarged or Shrunk by simply comparing the two heights.

Magnification = hi / ho

It also happens to be the same as –di / do

Practice ProblemPractice Problem

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An object is placed 12 cm form a mirror and the image appears 20 cm away. What is the focal length?

An image appears at a distance of 14m and the mirror has a Focal length of 10m. How far is the object from the mirror?

RefractionRefraction• Light also goes Light also goes throughthrough some things some things

– glass, water, eyeball, air

• The presence of material slows light’s progressThe presence of material slows light’s progress– interactions with electrical properties of atoms.

• The “light slowing factor” is called the The “light slowing factor” is called the index of refractionindex of refraction• Light bends at interface between refractive indicesLight bends at interface between refractive indices

– bends more the larger the difference in refractive index

n2 = 1.5

n1 = 1.0

A

B

Convex LensesConvex Lenses

Thicker in the center than Thicker in the center than edges. edges. – Lens that converges

(brings together) light rays.

– Forms real images and virtual images depending on position of the object

The Magnifier

Concave LensesConcave Lenses

• Lenses that are Lenses that are thicker at the edges thicker at the edges and thinner in the and thinner in the center. center. – Diverges light rays

– All images areupright and

smaller.

The De-Magnifier

Lens EquationsLens Equations

• The lens equation is the exact same as the mirror equation.The lens equation is the exact same as the mirror equation.

• 1/f = 1/d1/f = 1/dii + 1/d + 1/doo

• The magnification equation is the same too.The magnification equation is the same too.

• m = hm = hii / h / h00

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Optics Mirrors and Lenses. Light Light can be a wave or a particle.Light can be a wave or a particle. Individual particles of light are called photons.Individual