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Law of Reflection

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These dancers are practicing in front of a mirror so they can see how they look as they performs. They’re watching their image in the mirror as they dance. What is an image, and how does it get “inside” a mirror? In this article, you’ll find out.  

Reflected Light and Images

Reflection is one of several ways that light can interact with matter. Light reflects off surfaces such as mirrors that do not transmit or absorb light. When light is reflected from a smooth surface, it may form an image. An image is a copy of an object that is formed by reflected (or refracted) light.

Q: Is an image an actual object? If not, what is it?

A: No, an image isn’t an actual object. It is focused rays of light that make a copy of an object, like a picture projected on a screen.

Regular and Diffuse Reflection

If a surface is extremely smooth, as it is in a mirror, then the image formed by reflection is sharp and clear. This is called regular reflection (also called specular reflection). However, if the surface is even slightly rough or bumpy, an image may not form, or if there is an image, it is blurry or fuzzy. This is called diffuse reflection.

Q: Look at the boats and their images in the Figure below . Which one represents regular reflection, and which one represents diffuse reflection?

A: Reflection of the boat on the left is regular reflection. The water is smooth and the image is sharp and clear. Reflection of the boat on the right is diffuse reflection. The water has ripples and the image is blurry and wavy.

Regular and diffuse reflection

In the Figure below , you can see how both types of reflection occur. Waves of light are represented by arrows called rays. Rays that strike the surface are referred to as incident rays, and rays that reflect off the surface are known as reflected rays. In regular reflection, all the rays are reflected in the same direction. This explains why regular reflection forms a clear image. In diffuse reflection, the rays are reflected in many different directions. This is why diffuse reflection forms, at best, a blurry image. You can see animations of both types of reflection at this URL: http://toolboxes.flexiblelearning.net.au/demosites/series5/508/Laboratory/StudyNotes/snReflectionMirrors.htm

Schematic of regular and diffuse reflection

Law of Reflection

One thing is true of both regular and diffuse reflection. The angle at which the reflected rays leave the surface is equal to the angle at which the incident rays strike the surface. This is known as the law of reflection . The law is illustrated in the Figure below and also in this animation: http://www.physicsclassroom.com/mmedia/optics/lr.cfm

Law of reflection

According to the law of reflection, the angle of reflection always equals the angle of incidence. The angles of both reflected and incident light are measured relative to an imaginary line, called normal, that is perpendicular (at right angles) to the reflective surface.

Summary

  • Reflection is one of several ways that light can interact with matter. When light is reflected from a smooth surface, it may form an image. An image is a copy of an object that is formed by reflected (or refracted) light.
  • Regular reflection occurs when light reflects off a very smooth surface and forms a clear image. Diffuse reflection occurs when light reflects off a rough surface and forms a blurry image or no image at all.
  • According to the law of reflection, the angle at which light rays reflect off a surface is equal to the angle at which the incident rays strike the surface.

Vocabulary

  • image : Copy of an object that is formed by reflected or refracted light.
  • law of reflection : Law stating that the angle at which reflected rays of light bounce off a surface is equal to the angle at which the incident rays strike the surface.
  • reflection : Bouncing back of waves from a barrier they cannot pass through.

Practice

At the following URL, review the law of reflection and watch the animation. Then fill in the blanks in the sentence below. http://www.physicsclassroom.com/mmedia/optics/lr.cfm

  1. When a ray of light strikes a plane mirror, the light ray __________ off the mirror.
  2. Reflection involves a change in __________ of a light ray.
  3. The angle of incidence equals the angle between the incident ray and ________.
  4. The angle of __________ equals the angle of incidence.
  5. The normal line is __________ to the mirror.

Review

  1. What is an image?
  2. Identify the object and the image in the Figure below . Which type of reflection formed the image: regular reflection or diffuse reflection? How do you know?

Object and reflection

  1. What is the law of reflection?
  2. Label the angle of incidence and the angle of reflection in the Figure below .

Sun and reflection

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