Reflection from mirrors Light travels in straight lines. Mirrors are optical devices that divert light using reflection. There are different kinds of mirrors based on their shape: flat mirror (also called a plane mirror) convex mirror (bulges out at the middle) concave mirror (bows inward at the middle)

Incident and reflected rays Light rays can reflect off any surface or any boundary between two materials—such as a classroom wall, or a flat mirror. Incident ray: the incoming light ray that strikes this mirror Reflected ray: the light ray that reflects off the surface of the mirror

Angles of incidence and reflection Normal: an imaginary line drawn perpendicular to the reflecting surface where the light ray strikes it. Angle of incidence: the angle between the incident ray and the normal, θi. Angle of reflection: the angle between the reflected ray and the normal θr. What is true about θi and θr? Stress the idea that the angle of incidence is between the normal and the incident ray (and not between the mirror and the incident ray ).

Law of reflection The angle of incidence always equals the angle of reflection. Remember: measure angles with respect to the normal!

What is an image? An image is an illusion created with light. It is formed by an optical device through reflection or refraction. It is an organization of light rays that has the same spatial pattern as the original light rays coming from the actual object. Lens Image Object

What is an image? ALL light rays from one point on the object that pass through the optical device will converge at that same point on the image. Lens Image Object

Image formation for a flat mirror One other property of reflection: the image in a mirror reverses left and right. Ever tried to write backwards? Tried using a mirror?

Refraction Why does this straw look broken? When light crosses a boundary between materials, it may change direction through refraction. Light reflecting off the straw above the water goes straight to your eye. Light reflecting off the straw below the water bends—refracts—as it passes into the air, so it appears to have come from a different direction.

Reflection and refraction Light may reflect at the boundary between two materials, staying in the original medium. Light may refract as it crosses a boundary between two materials, changing its direction. When light hits glass, it can do both.

What causes refraction? Refraction is a property of all waves. Refraction occurs at a boundary between two materials. Light refracts because it travels at different speeds in different materials.

Visualizing refraction Ray diagrams help us visualize refraction. This diagram shows an incident ray in air and the refracted ray in glass. Be careful not to use the angle between the refracted ray and the interface

Visualizing refraction The normal is a line perpendicular to the boundary at the point where the ray strikes the new medium.

Visualizing refraction Angle of incidence Angle of refraction The angle of incidence lies between the incident ray and the normal. The angle of refraction lies between the refracted ray and the normal.

The direction of refraction When light slows down it bends towards the normal. When light speeds up it bends away from the normal line. Point out that the path of the light ray is reversible.

Focal point Continuously curved surfaces allow a lens to refract the rays to a focal point. A light ray that comes in on the optical axis does not refract. Light rays farther from the optical axis are refracted more.

Focal length The focal length is the distance from the center of the lens to the focal point. Highly curved lenses have short focal lengths. Slightly curved lenses have long focal lengths.

Types of lenses There are two kinds of lenses, converging and diverging. They differ in their shape and how they focus light.

Real and virtual images Object Images can be either real or virtual. A real image is make of real light. It forms where light rays physically meet. It can be projected onto a piece of paper. A virtual image can be perceived by the eye, but can’t actually be projected onto a piece of paper.

Real images Real images form where the light rays from an object actually converge. They can be identified by their ability to be projected onto a screen.

Virtual images object A virtual image cannot be projected onto a screen. Light rays appear to converge at the image location, but in fact do not. Your image in a mirror is virtual. The image appears to be behind the mirror, but there is no light behind the mirror. virtual image