Light and Reflection Level 1 Physics. Facts about Light It is a form of Electromagnetic Energy It is a part of the Electromagnetic Spectrum and the only.

Light and ReflectionLevel 1 Physics

Facts about Light It is a form of Electromagnetic Energy

It is a part of the Electromagnetic Spectrum and the only part we can really see

More facts…Speed of light, c, in a vacuum is a constant value

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c = 3x108m

s

Light can be:ReflectedRefractedAbsorbed

Light is an electromagnetic wave where electric and magneticfields fluctuate together.

Reflection of Light

Angle ofIncidence

θi

Angle ofReflection

θr

Incident RayReflected Ray

Mirror

Law of Reflection: The angle of incidence (θi) EQUALS the angle of reflection (θr)

Both angles are measured from a line drawn perpendicularto the surface:

NORMAL

SPECULAR VS. DIFFUSESpecular: Important indetermining properties of mirrors. Rays are always parallel to one another.

Diffuse: Law of reflectionapplies to each ray, but irregular surface reflects light in various directions.

Size of irregularity is ≥ the wavelength of light

Plane MirrorA duck is located 15 cm in front of a vertical mirror. WHERE IS THE OF THE DUCK LOCATED?

MIRROR

Object Distance, Do = 15 cm

On the same side?

Behind the mirror?

On the surface?

Plane MirrorA duck is located 15 cm in front of a vertical mirror. WHERE IS THE OF THE DUCK LOCATED?

MIRROR

Object Distance, Do = 15 cm

Behind the mirror?

Virtual Image

Image Distance, Di = 15 cm

Objectheight ho

Imageheight hi

Virtual ImageTo the eye, it appears that the ray originates from behind the mirror

Rays of light seem to Come from the image, But they DO NOT originateFrom behind the mirror

Since none of the raysactually come from the imageit is called a VIRTUAL IMAGE

Virtual Images continued…Virtual Images are basically images which cannot be visually projected on a screen.

If this box gave off light, we could project an image of this box on to a screen provided the screen was on the SAME SIDE as the box.

You would not be able to project the image of the vase or your face in a mirror on a screen, therefore it is a virtual image.

CONCLUSION: VIRTUAL IMAGES are ALWAYS on the OPPOSITE side of the mirror relative to the object.

Real ImageIf the image can be seen on a screen, it is considered a REAL IMAGE

Real images are always on SAME SIDEof the mirror as the object (for MIRRORS)

The characteristics maybe different from the object:SIZE – reduced, same,

enlargedPOSITION - same side or

opposite sideORIENTATION – right side

up or inverted

Spherical Mirrors

Also called DIVERGING mirror Also called CONVERGING mirror

Concave Mirror - Converging

A converging mirror will focus parallel light rays to a single (fixed)point in front of the surface of the mirror. This point is said to bethe FOCAL POINT. To find this point, you must use light fromINFINITY

Converging MirrorThought to be a slice of a sphere and having a line drawn throughThe exact center of the mirror – Principle Axis.

Since it is technically a sphere, the mirror has a CENTER OF CURVATURE, C

The focal length, f is half the distance of C

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f =C

2

Ray Diagrams

Principal Ray Pineault: Draw a ray from the top of the object parallel to the principal axis and then through f after reflection.

fC

PrincipalAxis

Ray Diagrams

Vice Principal Ray Lanczycki: Draw a ray from the top of the object through the focal point, then parallel to the principal

axis.

fC

PrincipalAxis

Ray Diagrams

Vice Principal Ray Alves: Draw a ray from the top of the object, through C, then back along itself.

fC

PrincipalAxis

Ray Diagrams

fC

PrincipalAxis

Converging Mirror – Image Characteristics

fC

PrincipalAxis

Is the image on the SAME or OPPOSITE side of mirror as object?

SAME SIDE, therefore considered a REAL IMAGE

Is the image ENLARGED or REDUCED?

Is the image INVERTED or UPRIGHT?

Diverging Mirror

Reflected rays will NEVER cross on object side of mirror, therefore, continue reflected rays behind the mirror tolocate the image.

Image is:

REAL or VIRTUAL

SMALLER or BIGGER

INVERTED OR UPRIGHT

Diverging Mirror

A diverging mirror (convex) will ALWAYS form a VIRTUAL IMAGE

Because of its shape, a divergingmirror gives a wider field of view than other mirrors.

The Mirror/Lens EquationIs there any OTHER way to predict image characteristics

besides the ray diagram? YES!

One way is to use the MIRROR/LENS equation to CALCULATE the position of the image.

io ddf

111

Mirror/Lens EquationAssume that a certain concave spherical mirror

has a focal length of 10.0 cm. Locate the image for an object distance of 25 cm and describe the image’s characteristics.

i

iio

d

dddf

1

25

1

10

1111

16.67 cm

What does this tell us? First we know the image is BETWEEN “C” & “f”. Since the image distance is POSITIVE the image is a REAL IMAGE.

Real image = positive image distanceVirtual image = negative image distance

What about the size and orientation?

Magnification EquationTo calculate the orientation and size of the

image we use the MAGNIFICATION EQUATION.

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M = −dido=hiho

M = −16.67

25M = −0.67x

Here is how this works:•If we get a POSITIVE magnification, the image is UPRIGHT.•If we get a NEGATIVE magnification, the image is INVERTED •If the magnification value is GREATER than 1, the image is ENLARGED.•If the magnification value is LESS than 1, the image is REDUCED.•If the magnification value is EQUAL to 1, the image is the SAME SIZE as the object.

Using our previous data we see that our image was INVERTED, and REDUCED.

ExampleAssume that a certain concave spherical mirror has a

focal length of 10.0 cm. Locate the image for an object distance of 5 cm and describe the image’s characteristics.

5

1

5

1

10

1111

i

i

iio

dM

d

dddf

-10 cm

2x

•VIRTUAL (opposite side)•Enlarged•Upright

Characteristics?