6-Nov-10

Chapter 28Reflection

Lecture 31

Midterm 2 – Monday Nov. 15

Reflection• We say light is reflected when

it is returned into the medium from which it came—the process is reflection.

• When light illuminates a material, electrons in the atoms of the material move more energetically in response to the oscillating electric fields of the illuminating light.

• The energized electrons re-emit the light by which you see the material.

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Specular and Diffuse ReflectionReflection from most surfaces is diffuse, with

incoming light scattered in all directions.Reflection from smooth, mirror surfaces is

specular, with light reflecting in one direction.

Mirror

Diffuse reflection Specular reflection

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Check Yourself

What objects in this room radiate light?

Is the projector screen radiating visible light?

Is the screen diffuse or specular reflector?

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Diffuse Surface is Not a MirrorLight rays diffusely scatter in every direction

from every spot on the surface.

See a light ray from the top of your head reflected from this spot.

Also see light ray from the tip of your nose reflected from this spot.

Look at this spot on a diffuse

surface

Diffuse reflection occurs when the sizes of surface irregularities are

A. small compared with the wavelength of reflected radiation.

B. large compared with the wavelength of reflected radiation.

C. Both A and B.D. None of the above.

Law of ReflectionCHECK YOURSELF

Diffuse reflection occurs when the sizes of surface irregularities are

A. small compared with the wavelength of reflected radiation.

B. large compared with the wavelength of reflected radiation.

C. Both A and B.D. None of the above.

Explanation:Diffuse reflection occurs for rougher surfaces.

Law of ReflectionCHECK YOUR ANSWER

6-Nov-10 Physics 1 (Garcia) SJSU

Transparent, Reflecting SurfacesWater and glass are transparent but reflect

some of the light striking their surface.

Can see inside the cab through the right window. Strong reflection of the sun on the left window.

Can see some of the riverbed through the water surface.

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One Way MirrorOne way mirror is just clear glass window.

Bright Room Dark RoomDiscussionRoom (Bright)

ObservationRoom (Dark)

GLA

SS

Reflected light from bright room masks transmitted light from the

darkened observation room

Angle of Reflection

• Angle of incidence– Angle made by the incoming ray and the

perpendicular

• Angle of reflection– Angle made by the reflected ray and the

perpendicular

• Normal – Imaginary line perpendicular to the plane of the

reflecting surface– Lies in the same plane as the incident and reflected

rays

6-Nov-10 Physics 1 (Garcia) SJSU

Law of ReflectionAngle of incidence equals angle of reflection

SPECULARREFLECTION

The law of reflection applies to

A. light.B. sound.C. Both A and B.D. None of the above.

Law of ReflectionCHECK YOURSELF

The law of reflection applies to

A. light.B. sound.C. Both A and B.D. None of the above.

Law of ReflectionCHECK YOUR ANSWER

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Ray Tracing

Light ray from point A is reflected at point B and arrives at point B.

Notice the relationship between point B (where the ray is reflected to) and point B’ (where the ray would go if the mirror wasn’t there).

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Mirror Image Reflection

To observer at point B, the light from point A seems to come from point A’, within mirror.

Law of Reflection

Virtual image• is same size as object, formed behind a mirror,

and located at the position where the extended reflected rays converge.

• is as far behind the mirror as the object is in front of the mirror.

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Plane (Flat) MirrorTracing light rays from original, to mirror, to

eye allows us to construct image.

Original Image

Mirror

HAIR

NOSE

CHIN

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Demo: Pocket Mirror

See full image of your head in a plane mirror that’s half the size of your head.

Original Image

Mirror

Law of Reflection

Plane mirror• Note: the only axis reversed in an image is the

front-back axis.

6-Nov-10 Physics 1 (Garcia) SJSU

Image in the Mirror

Utamaro, Woman Powdering Her Neck

Velazquez, Venus at her Mirror

Helmut Newton,Bergstrom over Paris

Image is seen an equal distance from original within a plane mirror.

Incorrect

Incorrect

Correct

Light reflecting from a smooth surface undergoes a change in

A. frequency.B. speed.C. wavelength.D. None of the above.

Law of ReflectionCHECK YOURSELF

Light reflecting from a smooth surface undergoes a change in

A. frequency.B. speed.C. wavelength.D. None of the above.

Law of ReflectionCHECK YOUR ANSWER

6-Nov-10 Physics 1 (Garcia) SJSU

Non-Flat MirrorsImage in a curved mirror is

distorted from original.Determine image location

and size by ray tracing

Law of Reflection

Shape of mirror forms a different virtual image.• Convex mirror (that curves outward): virtual image is

smaller and closer to the mirror than the object.• Concave mirror (that curves inward): virtual image is

larger and farther away than the object.

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Demo: Convex MirrorsMirrored surface on a sphere or similar

rounded shape. Image is smaller & closer.

MC EscherHand with Reflecting Sphere

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Ray Tracing for Convex Mirror

Image in convex mirror is smaller & closer.

Original

Image

Mirror

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Demo: Concave MirrorsMirror is on the inner curved

surface of sphere or similar rounded shape.

Image size and location depends on object’s distance from the mirror. Telescope

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Ray Tracing for Concave MirrorImage in concave mirror is larger when

original is close to the mirror.

OriginalImageMirror

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Image

Ray Tracing for Concave MirrorImage in a concave mirror is inverted if the original

is far away from the mirror.Image is formed in front of the mirror, not within it.

OriginalMirror

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Reflecting Telescopes

Large, modern telescopes form images using concave mirrors.

Hubble’s mirror

MagellanTelescope

Key Points of Lecture 31Key Points of Lecture 31

Before Wednesday, read Hewitt Chap. 28 (first half).

Homework #22 due by 11:00 PM Tuesday Nov. 9.

Homework #23 due by 11:00 PM Friday Nov. 12

• Specular and Diffuse Reflection• Law of Reflection• Plane Mirror• Ray Tracing• Real and Virtual Images• Curved Mirrors