a light wave magnetic field electric field. the ray model of light taken from since light seems to...

A light wave

Magnetic Field

Electric Field

The Ray Model of LightTaken from www.drake.edu/artsci/physics/L231-234.ppt

Since light seems to move in straight lines, why not follow a light wave along a straight line path and simply draw the line to represent how the light will behave

This line will be perpendicular to the wave front of the light wave

We’ll see how this model helps understand many characteristics of light behavior

The Ray Model of LightLight from an object either results because the

object is emitting light or light is reflecting from the surface of the object

Reflection from a Plane Mirror

The angle of incidence equals the angle of reflection. This assumes the surface is perfectly smooth.

Diffuse Reflection

When the surface is rough, the surface at any point makes some angle w.r.t. the horizontal. The angle of incidence still equals the angle of reflection.

Plane Mirrors

In the left hand picture with a rough surface, you can place your eye anywhere and you will see some reflected rays. On the right hand side, you eye has to be in the correct position to see the reflected light. This is called specular reflection.

Plane Mirrors

A plane mirror provides the opportunity to fool you by making your eye and brain perceive an image.

Plane Mirrors

The image appears to be the same distance behind the mirror as the object is in front of the mirror.

Plane Mirrors

The image is called a virtual image because if you placed a piece of paper at the image location, you wouldn’t see any light.

How Big a Mirror?

Spherical Mirrors

Again, we use the angle of incidence equals the angle of reflection. It is convenient to trace what happens to parallel light rays hitting the mirrors. Remember the definition of convex and concave!!

Spherical Mirrors

Precisely parallel rays do NOT meet at the same point after reflection from the surface of the mirror. Of course, precisely parallel rays only come from objects at huge distances away.

To avoid this problem and to form real images, we need to restrict ourselves to just a very small central region of the mirror.

Spherical Mirrors

In this limited region of the surface, the rays do intersect at the focus.

This picture defines the principal axis, the focal point and the focal length of the mirror. The line CB is a radius of the spherical surface. The focal point is at 1/2 the radius length.

Images in Spherical MirrorsAny ray parallel to the principal axis will pass

through the focal point!

Now we need to look at more rays leaving the same point on the object

Images in Spherical Mirrors

Any ray from the object passing through the focal point will emerge parallel to the principal axis!!


Any ray striking the mirror at right angles will reflect straight back and will pass through the center of curvature!!


Now we want to derive an equation that will express the observations we have just made


The image and object distances are di and do

The image and object heights are hi and ho


The right triangles I’AI and O’AO are similar (angles the same)


€

hohi

=d0

di


Triangles O’FO and AFB are similar

AB ≈ hi and FA = f (focal length)


€

hohi

=OFFA

=do−ff


€

hohi

=do − f

f=dodi

1

do+

1

di=

1

f


€

m=hiho

=−dido

Minus sign means upside down


Image is upright and virtual!

Makeup Mirror


Passenger side outside car mirror

Image is virtual and upright

a light wave magnetic field electric field. the ray model of light taken from since light seems to...

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