Optics

Mirrors and Lenses

Reflection

• We describe the path of light as straight-line rays
• Reflection off a flat surface follows a simple rule:
• angle in (incidence) equals angle out (reflection)
• angles measured from surface “normal” (perpendicular)

incident ray

exit ray

reflected ray

�Reflection Vocabulary

• 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 Vocabulary

• Virtual Image–
• “Not Real” because it cannot be projected
• Image only seems to be there!

Virtual Images in Plane Mirrors

Hall Mirror

• Useful to think in terms of images

mirror only

needs to be half as

high as you are tall. Your

image will be twice as far from you

as the mirror.

LEFT- RIGHT REVERSAL

AMBULANCE

Curved mirrors

• What if the mirror isn’t flat?
• light still follows the same rules, with local surface normal
• Parabolic mirrors have exact focus
• used in telescopes, backyard satellite dishes, etc.
• also forms virtual image

Concave Mirrors

• Curves inward
• May be real or virtual image

View kacleaveland's map

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Taken in a place with no name (See more photos or videos here)

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"Have you ever approached a giant concave mirror? See your upside-down image suspended in mid-air. Walk through the image to see a new reflection, right-side-up and greatly magnified. In the background you see reflected a room full of visitors enjoying other

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

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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. ��

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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.��

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

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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. ��

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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.��

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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.��

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

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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 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. ��

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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.��

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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 at f, no image is formed. The reflected rays are parallel and never converge.��

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

What size image is formed if the real object is placed at the focal point f?

Convex Mirrors

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• Curves outward
• Reduces images
• Virtual images
• Use: Rear view mirrors, store security…

CAUTION! Objects are closer than they appear!

Refraction

• Light also goes through some things
• glass, water, eyeball, air
• The presence of material slows light’s progress
• interactions with electrical properties of atoms
• The “light slowing factor” is called the index of refraction
• glass has n = 1.52, meaning that light travels about 1.5 times slower in glass than in vacuum
• water has n = 1.33
• air has n = 1.00028
• vacuum is n = 1.00000 (speed of light at full capacity)

Refraction at a plane surface

• Light bends at interface between refractive indices
• bends more the larger the difference in refractive index

Convex Lenses

Thicker in the center than edges.

• Lens that converges (brings together) light rays.
• Forms real images and virtual images depending on position of the object

The Magnifier

Concave Lenses

• Lenses that are thicker at the edges and thinner in the center.
• Diverges light rays
• All images are�erect and reduced.

The De-Magnifier

How You See

• Near Sighted – Eyeball is too long and image focuses in front of the retina
• Near Sightedness – Concave lenses expand focal length
• Far Sighted – Eyeball is too short so image is focused behind the retina.
• Far Sightedness – Convex lense shortens the focal length.

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Cameras, in brief

In a pinhole camera, the hole is so small that light hitting any particular point

on the film plane must have come from a particular direction outside the camera

In a camera with a lens, the same applies: that a point on the film plane

more-or-less corresponds to a direction outside the camera. Lenses have

the important advantage of collecting more light than the pinhole admits