What is a Convex Mirror? - Definition, Uses & Equation

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  • 0:04 Types of Mirror Images
  • 1:00 Convex Mirrors
  • 3:32 The Mirror Equation
  • 4:27 Convex Mirror Uses
  • 4:58 Lesson Summary
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Lesson Transcript
Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

Convex mirrors have reflective surfaces that curve outward. In this lesson, learn about the types of images that can be formed by a convex mirror and some of the uses of convex mirrors.

Types of Mirror Images

When you look in a mirror, you see yourself looking back out. Depending on the mirror, you may not look exactly like you do in real life, though. You may appear bigger, smaller, or even upside down! The type of mirror you are looking at plays a critical role in determining exactly what kind of image you will see.

First, the image may be real or virtual. A virtual image is one that you can only see if you look into the mirror. It appears to be inside the mirror. In contrast, a real image is one that is projected out in front of the mirror and can be seen in front of the mirror without looking into the mirror. Only certain types of mirrors can produce real images, but all mirrors can produce virtual images.

Second, the image can be upright or inverted. An inverted image appears upside down. Also, the image can be exactly the same size as the object being reflected, or the image can be bigger or smaller than the real life object.

Convex Mirrors

One type of mirror, known as a convex mirror, has a reflective surface that curves outward.

A convex mirror has a point where all the light that hits it appears to focus. This is called the focal point or sometimes simply the focus. The distance from the front of the mirror to the focal point is the focal length.

How can we tell what kind of image will be formed by a convex mirror? One way is to construct a ray diagram, which is simply a picture that shows the paths of a few important light rays as they hit the surface of the mirror and bounce back.

To make a ray diagram, first, draw the mirror in the middle of the page and then draw a horizontal axis right through the center of the mirror. Carefully measure the focal length, and mark this on the diagram. For a convex mirror, the focus will be behind the mirror. Now, measure the distance to the object and mark this distance in front of the mirror. You don't have to worry about drawing the object accurately. Just draw an upright arrow to represent the object.

Next, make the following three special rays. The first ray should be parallel to the horizontal axis and should go from the top of the object to the surface of the mirror. After it hits the mirror, it will be reflected back along a path lined up with the focal point.

The second ray will also begin at the object and will go towards the mirror along a path that is lined up with the focal point. When it hits the surface of the mirror, it will be reflected back parallel to the horizontal axis.

Finally, the third ray will extend from the top of the object to the center of the mirror. It will reflect back at the same angle that it hits the mirror.

Once you have these three rays, you can use them to find out where the image will be formed and the characteristics of the image (real or virtual, upright or inverted, bigger or smaller).

If the three rays you just drew all meet at a point in front of the mirror, then you have a real image. If they don't meet, you need to continue the three reflected lines back behind the mirror until they do meet. It's a good idea to use dashed lines for these rays because the light does not actually go behind the mirror, it just appears to. If the lines meet behind the mirror, the image is virtual. In this case, we can see that the image is virtual and upright. It's also a lot smaller than the original object.

ray diagram - step 5

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