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5.55: Mirrors

Difficulty Level: Basic Created by: CK-12

What does this picture show? Is it a photo of identical twin sisters, or is it just one girl looking in a mirror? The picture shows a single girl and her mirror image.

How Mirrors Form Images

A mirror is typically made of glass with a shiny metal backing that reflects all the light that strikes it. When a mirror reflects light, it forms an image. An image is a copy of an object that is formed by reflection or refraction. Mirrors may have flat or curved surfaces. The shape of a mirror’s surface determines the type of image it forms. For example, some mirrors form real images, and other mirrors form virtual images. What’s the difference between real and virtual images?

  • A real image forms in front of a mirror where reflected light rays actually meet. It is a true image that could be projected on a screen.
  • A virtual image appears to be on the other side of the mirror. Of course, reflected rays don’t actually go through the mirror to the other side, so a virtual image doesn’t really exist. It just appears to exist to the human brain.

Q: Look back at the image of the girl pointing at her image in the mirror. Which type of image is it, real or virtual?

A: The image of the girl is a virtual image. It appears to be on the other side of the mirror from the girl.

Plane Mirror

The mirror in the opening photo is a plane mirror. This is the most common type of mirror. It has a flat reflective surface and forms only virtual images. The image formed by a plane mirror is also right-side up and life sized. But something is different about the image compared with the real object in front of the mirror. Left and right are reversed. Look at the man touching his hair in the Figure below . He is using his left hand to touch his hair, but his image (on the right) appears to be touching his hair with the right hand. All plane mirrors reverse left and right in this way. The term mirror image refers to how left and right are reversed in an image compared with the object.

Diagram of plane mirror

Concave Mirror

Some mirrors have a curved rather than flat surface. Curved mirrors can be concave or convex. A concave mirror is shaped like the inside of a bowl. This type of mirror forms either real or virtual images, depending on where the object is placed relative to the focal point. The focal point is the point in front of the mirror where the reflected rays meet. You can see how concave mirrors form images in the Figure below and at the following URL. Concave mirrors are used behind car headlights. They focus the light and make it brighter. Concave mirrors are also used in some telescopes. http://www.splung.com/content/sid/4/page/concavemirrors

Diagram of concave mirror

Convex Mirror

The other type of curved mirror, a convex mirror, is shaped like the outside of a bowl. Because of its shape, it can gather and reflect light from a wide area. As you can see in the Figure below , a convex mirror forms only virtual images that are right-side up and smaller than the actual object. You can see how a convex mirror forms an image in the animation at this URL: http://physics.slss.ie/resources/convex%20mirror.swf

Diagram of convex mirror

Q: Convex mirrors are used as side mirrors on cars. You can see one in the Figure below . Why is a convex mirror good for this purpose?

A: Because it gathers light over a wide area, a convex mirror gives the driver a wider view of the area around the vehicle than a plane mirror would.

Car mirrors are convex mirrors

Summary

  • When a mirror reflects light, it forms an image. An image is a copy of an object formed by reflection (or refraction). A real image is a true image that forms in front of a mirror where reflected light rays actually meet. A virtual image appears to be on the other side of the mirror and doesn’t really exist.
  • Most mirrors are plane mirrors that have a flat reflective surface. A plane mirror forms only virtual, right-side up, and life-sized images.
  • A concave mirror is shaped like the inside of a bowl. The type of image it forms depends on where the object is relative to the focal point. The image may be real, upside down, and reduced in size; or it may be virtual, right-side up, and enlarged.
  • A convex mirror is shaped like the outside of a bowl. It forms only virtual images that are right-side up and reduced in size relative to the object.

Vocabulary

  • concave : Curving inward like the inside of a bowl.
  • convex : Curving outward like the outside of a bowl.
  • image : Copy of an object that is formed by reflected or refracted light.
  • reflection : Bouncing back of waves from a barrier they cannot pass through.

Practice

At the following URL, review how a plane mirror forms an image, and watch the animation. Then answer the multiple choice questions below. http://www.physicsclassroom.com/mmedia/optics/ifpm.cfm

  1. A plane mirror forms an image where the reflected rays
    1. divide.
    2. separate.
    3. begin.
    4. intersect.
  2. The image formed by a plane mirror appears to be
    1. on the same side of the mirror as the object.
    2. on the opposite side of the mirror from the object.
    3. either on the same side or the opposite side, depending on the distance of the object from the mirror.
    4. on both sides of the mirror, regardless of the distance of the object from the mirror.
  3. The distance from the object to a plane mirror equals the distance from the mirror to the
    1. image.
    2. incident ray.
    3. reflected ray.
    4. normal.
  4. Anyone who sees the image formed by a plane mirror is sighting at the same image
    1. size.
    2. line.
    3. distance.
    4. location.

Review

  1. What is an image? How do real and virtual images differ?
  2. Define the focal point of a mirror.
  3. Describe the image formed by a plane mirror.
  4. What type of image is formed by a concave mirror if the object is between the mirror and the focal point?
  5. Mirrors like the one in the Figure below are sometimes placed at street intersections so drivers can see around blind corners. What type of mirror is used for this purpose? What type of image does it form?

Traffic mirror

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Description

Difficulty Level:

Basic

Concept Nodes:

Grades:

7 , 8

Date Created:

Nov 01, 2012

Last Modified:

Aug 15, 2014

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