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Law of Reflection

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Reflection of Light

The designers of this sign used the fact that light can reflect off of many surfaces, including water, to make the sign legible. It takes a bit of thought, but words can be written in such a way that their reflection is legible, even though the words themselves are not. 

Reflection of Light

The Law of Reflection

When a light ray strikes a reflecting surface, the angle of incidence (measured from the normal line) is equal to the angle of reflection (also measured from the normal line). This is called the Law of Reflection.

If the reflecting surface is a very smooth surface, the reflection will be a  regular reflection, in which the light rays maintain their position relative to each other and objects will be visible and identifiable in the reflected image. If the reflecting surface is rough, the reflection will be a  diffuse reflection, and objects will not be visible or identifiable in the reflection. When you are considering the size of things on the scale of wavelengths of light, even surfaces that appear smooth may be very rough in terms of light waves, and most surfaces produce diffuse reflection.

Left and Right Reversal in a Plane Mirror

The images that appear in a plane (flat) mirror are reversed in some ways and not reversed in other ways.  In the image below, the man's right hand is labeled. The same hand in the mirror image, however, looks like a left hand. While the left and right of the image are reversed, the top and bottom of the image are not. 

Summary

  • The law of reflection states that, when a light ray strikes a reflecting surface, the angle of incidence (measured from the normal line) is equal to the angle of reflection (also measured from the normal line).
  • If the reflecting surface is a very smooth surface, the reflection will be regular, in which the light rays maintain their position relative to each other.
  • If the reflecting surface is rough, the reflection will be diffuse and objects will be distorted in the reflection.
  • Images in a plane mirror are reversed left and right but not reversed top and bottom.

Practice

Use the video on reflection to answer the questions that follow.

http://www.aplusphysics.com/courses/regents/videos/Reflection/Reflection.html

  1. Both the angle of incidence and the angle of reflection are measured from the _________. 
  2. In reflection, the angle of incidence _________  the angle of reflection.

Review

  1. How does regular reflection differ from diffuse reflection?
  2. If a light ray strikes a mirrored surface at an angle of 25° to the surface, what is the angle of incidence?
  3. For problem #2, what will be the angle of reflection?
  4. A dry cement road is a diffuse reflector.  When it rains, the water fills in all the little holes and cracks in the cement road and it becomes a smooth regular reflector.  At night, when you are depending on the light from your headlights to show you the lines on the road, a wet road becomes much darker and it is more difficult to see the lines.  Explain why this occurs.

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