# Mirrors and Lenses.  How can we see the word on the front of this vehicle written correctly?  Why is it written the way it is?

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Mirrors and Lenses

 How can we see the word on the front of this vehicle written correctly?  Why is it written the way it is?

 Because light waves travel in straight lines an arrow called a ray can be used to show a light wave’s path and direction.  We use rays in diagrams to show the direction of light that is reflected by mirrors or refracted by lenses.  Review:  What is reflection?  What is refraction?

 A plane mirror is a mirror that has a flat surface.  When you look in a mirror you see a virtual image. A virtual image is an image through which light does not travel.

When you look in a plane mirror, your image is:  Right side up  The same size as the object being reflected  Reversed left to right

 A concave mirror is a mirror that curves in. (Hint to remember: You go into a cave.)  The image formed by a concave mirror depends on three things: the optical axis, focal point, and focal length of the mirror.  The focal point is the point in space at which light incident to the mirror and traveling parallel to the principal axis will meet after reflection.  Watch video: http://www.physicsclas sroom.com/Class/refln/ u13l3a.cfm#vocab http://www.physicsclas sroom.com/Class/refln/ u13l3a.cfm#vocab

Two rules of reflection for concave mirrors:  Any incident ray traveling parallel to the principal (optical) axis on the way to the mirror will pass through the focal point upon reflection.  Any incident ray passing through the focal point on the way to the mirror will travel parallel to the principal (optical) axis upon reflection.  These two rules of reflection are illustrated in the diagram below.

 A concave mirror can form a virtual image (as a plane mirror does), or a real image.  A real image is an image through which light passes.  Watch the video at the website to see a real image formed.  http://www.physicsclas sroom.com/class/refln/ u13l3d.cfm http://www.physicsclas sroom.com/class/refln/ u13l3d.cfm

 What does the image look like if the object is more than one focal length from the mirror?  The image will be an upside-down, real image. It’s size will depend on it’s distance from the mirror.  What does the image look like if the object is less than one focal length from the mirror?  The image will be a right-side-up, virtual image that appears larger than the actual object.

 What happens to the image if the object is placed at the focal point of the mirror?  The incident rays reflect off parallel to the optical axis and no image is formed.  Headlights and flashlights use concave mirrors to reflect a powerful beam of light from a light bulb by placing the bulb at the focal point of the mirror.  Lighthouses also use concave mirrors.

 Convex mirrors are mirrors that curve outward, like the back of a spoon.  How does the image appear in a convex mirror?  The image formed is a right-side-up, virtual image that is smaller than the actual object.

 Convex mirrors are useful because they make images of large areas.  Convex mirrors are often used for security in stores and factories.  They are also frequently used as side mirrors on cars and trucks.

 What are some uses for lenses?

 This lens is thinner in the center than around the edges.  It is a diverging lens because when it refracts light rays it spreads them outward.

 This lens is thicker in the middle than at the edges.  It is a converging lens because it causes light rays to refract and meet at a focal point.

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