Reflection

When a wave changes media, some of it is reflected back.

The amount of reflection depends on how different the media are.

Fixed End

Direction of Wave Pulse

Light reflected from a plane (flat) mirror follows the Law of Reflection:

These angles are measured from the normal, a line perpendicular to the mirror.

Angle of Incidence = Angle of Reflection

Ɵi = Ɵr

Reflective Surface

Normal Line

Incident Light Ray

Questions:Why can you not see yourself in all flat

surfaces?

The paper is rough and gives irregular reflections.

Ray DiagramsWhen you look at an image in a plane mirror it is:

The same size.The same distance behind the mirror as you

are in front of it.Right side up and laterally inverted.Also, the reflected light has the same speed,

wavelength and frequency as the incident light.

When drawing ray diagrams we draw two rays of light coming from the object, reflecting and then heading to the observer.

Questions:What minimum length of mirror do you need to

obtain a full length view of yourself?

You need a mirror half your height.

Curved MirrorsConcave mirrors are curved (“caved”) inwards

Convex mirrors are curved outwards

Since the surfaces are curved, the normal is different at different points.

Reflective Surface

NormalNormal

Normal

Reflective Surface

Normal Normal

Principle axis: straight line perpendicular to the mirror

Centre of curvature (C): the point that represents the center of the sphere

Focal point (F): point where all of the reflected rays meet

Focal length (f): distance from the focal point to the mirror

Concave

Mirrors

Consider the situation where an object is beyond the center of curvature:Remember: any ray of light traveling parallel to the principal axis will reflect through the focal point.

The image appears both smaller and inverted.

Now consider if the object were between C and F:

The image is inverted and larger.

Finally consider the case where the image is within the focal point, F:

The image is larger and upright.

Convex

Mirrors

Convex mirrors reflect light from their outside surface. Because of this the reflected rays always diverge.

Since there is no true focal point it is placed behind the mirror

Consider an object at any point in front of a convex mirror.

The image is always smaller and upright!