Reflection of Light Notes

Lesson 1: Reflection of Light

Focus Question:

• Why does light reflected from a mirror make an image while light reflected from a piece of paper does not?

New Vocabulary:

• Specular reflection
• Diffuse reflection
• Plane mirror
• Object
• Image
• Virtual image

Review Vocabulary:

• Normal: The line in a ray diagram that shows the orientation of the barrier or mirror and is drawn at a right angle, or perpendicular, to the barrier or mirror.
• Ray: A line drawn at a right angle to a wavefront; represents the direction of wave travel.

The Law of Reflection:

• The law of reflection states the angle of incidence is equal to the angle of reflection. This applies to both smooth and rough surfaces.
• Specular reflection is caused by a smooth surface, in which parallel light rays are reflected in parallel.
• Diffuse reflection is caused by the scattering of light off a rough surface.

Evaluate the Answer (Example Problem):

• By rotating the mirror $20°$, the angle of incidence and the angle of refraction were both increased by $20°$.
• Problem: A light ray strikes a plane mirror at an angle of $15°$ to the normal. The mirror then rotates $20°$ around the point where the beam strikes it so that the ray’s angle of incidence increases. The axis of rotation is perpendicular to the plane of the incident and to the reflected rays. What is the final angle of reflection of the light ray?
• Response:
  • Sketch and Analyze the Problem:
    • Sketch the situation.
    • List the knowns and unknowns.
      • Known:
        • $\theta_{i, \text{initial}} = 15°$
        • $\Delta \theta_{\text{mirror}} = 20°$
      • Unknown:
        • $\theta_{r, \text{final}} = ?$
  • Solve for the Unknown:
    • Determine the final angle of incidence.
    • Use the law of reflection to determine the final angle of reflection.

Objects and Plane-Mirror Images:

• A plane mirror is a flat, smooth surface from which light is reflected by specular reflection.
• An object is a source of light rays that are to be reflected by a mirrored surface.
• The brain processes reflected light as if it traveled in a straight line, making the light seem to come from a point behind the mirror.
• The combination of the image points produced by reflected light rays forms the image of the object.
• It is a virtual image, which is a type of image formed by diverging light rays. Images of real objects produced by plane mirrors are always virtual images.

Properties of Plane-Mirror Images:

• Looking at yourself in a mirror, you also see that your image is oriented as you are, and it matches your size.
• You can draw rays from the object to the mirror to determine the size of the image. The sight lines of two rays originating from the bottom of an object will converge at the bottom of the image.

Plane-Mirror Image Height:

• Using the law of reflection and congruent-triangle geometry, it can be shown that object height $(h<em>o)$ is equal to the image height $(h</em>i)$.
• $(h<em>o = h</em>i)$

Plane-Mirror Image Position:

• Looking at yourself in a mirror, you can see that your image appears to be the same distance behind the mirror as you are in front of the mirror.
• In a ray diagram, the position of the object with respect to the mirror $(x<em>o)$ has a length equal to the length of the apparent position of the image with respect to the mirror $(x</em>i)$.
• Note that a plane mirror produces an image with the same orientation as the object.

Image Reversal:

• There is a difference between you and the appearance of your image in a mirror.
• Left and right appear to be reversed by a plane mirror, but it actually produces a front-to-back reversal.
• The mirror only reverses the image such that it is facing in the opposite direction as the object.