Back To CoursePhysics 101: Help and Review
17 chapters | 212 lessons
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Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering
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.
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.
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.
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).
If the distance from the object to the mirror is bigger than the focal length, the mirror will produce an upside down real image. And if the distance from the object to the mirror is smaller than the focal length, the mirror will produce an upright virtual image.
To see this for yourself, find a metal spoon and look into it. The front of the spoon is curved inward and so it is a concave mirror. Hold your finger up in front of the spoon. You should see an upside down image of your finger that is slightly in front of the spoon. This is a real, upside down image. Now bring your finger closer to the spoon. When you get really close to the spoon, the image of your finger will suddenly get much larger and be upright. That is because your finger is now inside the focal length and the mirror is producing a virtual, upright image!
The law of reflection says that light will bounce back from a mirror at the same angle at which it hits the mirror. When this happens with a flat mirror, it creates a virtual image that you can only see by looking into the mirror. Because a concave mirror is curved inward, the angle that the light hits the mirror varies and this causes light to focus at a certain point.
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 distance from the mirror to the point where the light focuses is called the focal length of a mirror. Because of this, a concave mirror can produce a real image that appears out in front of the mirror. A concave mirror will produce a real image if the object is farther away from the mirror than the focal length and will produce a virtual image if the object is closer to the mirror than the focal length.
Law of reflection - states that the light that hits a mirror bounces back at the same angle at which it hit the mirror
Virtual image - the type of image created by a flat mirror
Concave mirror - a mirror that is curved inward in the middle
Incident angle - the angle at which the light hits the surface
Real images - images that are projected out in front of the mirror at the place where the light focuses; can only be done with a concave mirror
Focus point - the point at which all the light that shines on the mirror will reflect back through; every mirror has one
Focal length - the distance from the center of the mirror to the focus point
After watching this video lesson about concave mirrors, see if you can effortlessly:
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Back To CoursePhysics 101: Help and Review
17 chapters | 212 lessons