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

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  • 0:01 Mirrors & the Law of…
  • 0:56 Concave Mirrors
  • 2:30 Do Concave Mirrors…
  • 4:32 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

A concave mirror can make images that appear to float in the air! In this lesson, you will learn about what concave mirrors look like and how they work.

Mirrors and the Law of Reflection

Mirrors are something that everyone is familiar with. In fact, you probably have already looked at several mirrors just today! Most mirrors that you use on a daily basis are flat mirrors that produce an image that looks exactly the same size as the object in front of the mirror. When you look at yourself in a flat mirror, you appear to be exactly the same size as you are in real life and you appear to be just as far behind the mirror as you are actually standing in front of the mirror.

This is because of something called the law of reflection. The law of reflection says that the light that hits a mirror bounces back at the same angle at which it hit the mirror. If the mirror is flat, that causes the image to appear life-size. The type of image created by a flat mirror is called a virtual image, because it appears behind the mirror. It can't be seen except by looking into the mirror.

Law of Reflection

Concave Mirrors

A concave mirror is a mirror that is curved inward in the middle. It might help you to remember this if you think that when you look in a concave mirror, it looks like you are looking into a cave. The equation we use when dealing with concave mirrors is called the mirror equation. This equation is used to determine how far away an object is from the mirror (image distance) and how large or small the object is (object size.)

The law of reflection is still true for concave mirrors but because the mirror's surface is curved, the angle at which the light hits the surface, also known as the incident angle, is different depending on which part of the mirror the light hits. This causes concave mirrors to be able to focus light similar to the way that a lens in a camera focuses light and forms an image. Unlike a flat mirror, concave mirrors can form real images that are projected out in front of the mirror at the place where the light focuses.

concave mirror

This creates an image out in front of the mirror that looks like it is floating in the air! You don't have to look into the mirror at all to see this image and that's what makes it a real image. This is really different than the virtual image you see when you look into a flat mirror.

This magic mirror has two concave mirrors placed on top of each other. It creates a real image of the strawberry that appears to be floating on top of the mirror. The strawberry is actually down inside the magic mirror and you cannot see it at all.

magic mirror

Do Concave Mirrors Always Form Real Images?

You just saw that a concave mirror is different from a flat mirror because it can form a real image that is projected out in front of the mirror. Now you may be wondering if a concave mirror can produce a virtual image like a flat mirror as well. It may surprise you to know that the answer is yes! Concave mirrors are the only type of mirror that can produce both real and virtual images.

How do you know which type of image the mirror will make? It depends on two things: where the object is located in front of the mirror and the focal length of the mirror. Every mirror will have a focus point where all the light that shines on the mirror will reflect back through. The distance from the center of the mirror to that point is called the focal length. The focal length only depends on how curved the mirror is and not on how far the object is from the mirror. Mirrors that are more sharply curved will have a small focal length, and those that are flatter will have a larger focal length.

The angle at which light reflects from a concave mirror depends on where on the mirror it hits. The reflected light will all pass through a single point, called the focus point (F). The distance from the center of the mirror to the focus point is known as the focal length (f).

focal length of a concave mirror

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