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- How to Take Better Reflection Photos - Photography Life
- 16.1 Reflection
- Mirror image
When people look into a mirror, they see an image of themselves behind the glass. That image results from light rays encountering the shiny surface and bouncing back, or reflecting, providing a "mirror image. If you face north and look straight into a mirror, the east side of your face is still on the east side of the image, and the same is true for the west side. The mirror does not reverse the image left to right; it reverses it front to back.
For example, if you are facing north, your reflection is facing south. The reflection of light rays is one of the major aspects of geometric optics; the other is refraction, or the bending of light rays. The other class is physical optics. Geometric optics treats light as continuous rays as opposed to waves or particles that move through transparent media according to three laws. The first law states that light rays move through similar transparent media in straight lines.
The third law governs how light rays behave when they pass between two different media, such as air and water. For example, when you look at a spoon in a glass of water, the submerged part of the spoon appears to be in a different place than expected. This happens because the light rays change direction when they go from one transparent material air into another water. Reflections from flat surfaces are fairly easy to understand. A reflection appears to be the same distance from the "other side" of the mirror as the viewer's eyes are from the mirror.
Also, when light is reflected from a mirror, it bounces off at the same angle in the opposite direction from which it hit. For example, if the light hits a flat or "plane mirror" at a degree angle from the left, it will bounce off at a degree angle to the right. However, if the surface of the mirror is curved, the angles of reflection are different at different points on the surface. If the mirror is convex, or curved outward, it will reflect a wider area, in which images appear smaller and farther away than those from a flat mirror.
These mirrors are often used for outside rearview mirrors on cars and for keeping large areas under surveillance in stores. If the surface is concave, or curved inward, a group of light rays from a distant source is reflected back toward a single location known as the focal point. This generally produces a magnifying effect, such as that seen in a makeup mirror.
The radius of curvature of a mirror determines its magnification factor and its focal length. In a Newtonian reflecting telescope, light rays from distant objects, which are essentially parallel because they come from so far away , strike the concave main mirror at the same angle.
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The rays are then reflected back up through the telescope tube toward the focal point. However, before they reach the focal point, they strike a secondary, flat mirror that is tilted at a degree angle. The secondary mirror diverts the light out through a hole in the side of the tube. The eyepiece lens then focuses the light. Hint—His hat is tilted to one side. This video explains the creation of virtual images in a mirror. It shows the location and orientation of the images using ray diagrams, and relates the perception to the human eye. Some mirrors are curved instead of flat.
A mirror that curves inward is called a concave mirror , whereas one that curves outward is called a convex mirror. Pick up a well-polished metal spoon and you can see an example of each type of curvature. The side of the spoon that holds the food is a concave mirror; the back of the spoon is a convex mirror.
Observe your image on both sides of the spoon. You can remember the difference between concave and convex by thinking, Concave means caved in. Ray diagrams can be used to find the point where reflected rays converge or appear to converge, or the point from which rays appear to diverge. This is called the focal point , F.
Images formed by a concave mirror vary, depending on which side of the focal point the object is placed. For any object placed on the far side of the focal point with respect to the mirror, the rays converge in front of the mirror to form a real image, which can be projected onto a surface, such as a screen or sheet of paper However, for an object located inside the focal point with respect to the concave mirror, the image is virtual.
For a convex mirror the image is always virtual—that is, it appears to be behind the mirror. The ray diagrams in Figure The information in Figure This video uses ray diagrams to show the special feature of parabolic mirrors that makes them ideal for either projecting light energy in parallel rays, with the source being at the focal point of the parabola, or for collecting at the focal point light energy from a distant source. You should be able to notice everyday applications of curved mirrors. One common example is the use of security mirrors in stores, as shown in Figure Some telescopes also use curved mirrors and no lenses except in the eyepieces both to magnify images and to change the path of light.
This design uses a spherical primary concave mirror and a convex secondary mirror.
How to Name a Line in Geometry
The image is projected onto the focal plane by light passing through the perforated primary mirror. The effective focal length of such a telescope is the focal length of the primary mirror multiplied by the magnification of the secondary mirror. The result is a telescope with a focal length much greater than the length of the telescope itself.
A parabolic concave mirror has the very useful property that all light from a distant source, on reflection by the mirror surface, is directed to the focal point. Likewise, a light source placed at the focal point directs all the light it emits in parallel lines away from the mirror. This case is illustrated by the ray diagram in Figure The light source in a car headlight, for example, is located at the focal point of a parabolic mirror. Parabolic mirrors are also used to collect sunlight and direct it to a focal point, where it is transformed into heat, which in turn can be used to generate electricity.
This application is shown in Figure The signs of these values indicate whether the image is inverted, erect upright , real, or virtual. We now look at the equations that relate these variables and apply them to everyday problems.
This equation can be rearranged several ways. For example, it may be written to solve for focal length. Magnification, m , is the ratio of the size of the image, h i , to the size of the object, h o. The value of m can be calculated in two ways. This relationship can be written to solve for any of the variables involved.
How to Take Better Reflection Photos - Photography Life
For example, the height of the image is given by. We saved the simplest equation for last. The radius of curvature of a curved mirror, R , is simply twice the focal length.
We can learn important information from the algebraic sign of the result of a calculation using the previous equations:. A person standing 6. What is the focal length of the mirror? We know that, for this situation, d o is positive. The negative result is expected for a convex mirror. This indicates the focal point is behind the mirror. Electric room heaters use a concave mirror to reflect infrared IR radiation from hot coils.
Note that IR radiation follows the same law of reflection as visible light. Given that the mirror has a radius of curvature of The coils are the object, and we are asked to find their location—that is, to find the object distance d o. Rearranging to solve for d o , we have. You get the most concentrated thermal energy directly in front of the mirror and 3.
In general, this is not desirable because it could cause burns. Usually, you want the rays to emerge parallel, and this is accomplished by having the filament at the focal point of the mirror.
Note that the filament here is not much farther from the mirror than the focal length, and that the image produced is considerably farther away. What is the focal length of a makeup mirror that produces a magnification of 1. Construct a ray diagram using paper, a pencil and a ruler to confirm your calculation. How does the object distance, d o , compare with the focal length, f, for a concave mirror that produces an image that is real and inverted? Skip to main content.
Sections Learning Objectives. Characteristics of Mirrors. The Application of the Curved Mirror Equations. Check Your Understanding. Learning Objectives Learning Objectives By the end of this section, you will be able to do the following: Explain reflection from mirrors, describe image formation as a consequence of reflection from mirrors, apply ray diagrams to predict and interpret image and object locations, and describe applications of mirrors Perform calculations based on the law of reflection and the equations for curved mirrors Section Key Terms angle of incidence angle of reflection central axis concave mirror convex mirror diffused focal length focal point geometric optics law of reflection law of refraction ray real image specular virtual image.go to site
It can travel through a medium, such as air or glass, and typically travels from one medium to another. It can also reflect from an object, such as a mirror. The angles are measured relative to the line perpendicular to the surface at the point where the ray strikes the surface. The incident and reflected rays, along with the normal, lie in the same plane. Here, many parallel rays are incident, but they are reflected at many different angles because the surface is rough. Which image is real? Watch Physics Virtual Image This video explains the creation of virtual images in a mirror.