A First Course in Physics · Ch. XX, §§ 538–545 · Chapter 20: Formation of Images
63. Concave mirrors: virtual and real images
Goal: Your student can find the focal length of a concave mirror, describe the whole series of images as an object moves back from it, and tell a real image from a virtual one.
You'll need
- The book, open to these pages
- A concave mirror (a shaving or makeup mirror, or the inside of a shiny spoon)
- A white card, a ruler or tape measure, and a small flashlight or candle (with an adult) in a dim room
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Virtual, enlarged images when the object is inside the focus; focal length is half the radius; real, inverted images when the object is outside the focus, shown on a screen or seen in the air; conjugate points; size of image and object; finding the focal length with a distant object and the center of curvature with a candle; the rose-in-a-glass illusion; the full series of images.
The lesson, step by step
Hook question
SayIn a makeup mirror your face looks big and upright. Step far back and it flips upside down. Why?
Read together
Read §§ 538–545. The definition:
This image differs from all which have so far been studied in that it can be thrown upon a screen, i.e. in that light actually emanates from P' as a center. For this reason it is called a real image.
Try it: find the focal length
In a room with one bright window, hold the white card in front of the concave mirror and move it until a small, sharp, upside-down picture of the window (or a tree outside) appears on it (§ 543). The distance from card to mirror is the focal length. (Never use the Sun's image; it can burn.)
Try it: the series of images
Hold a small flashlight or candle very close to the mirror and back away slowly, watching its image: upright and enlarged; then blurry at the focus; then upside down and in front of the mirror (catch it on the card). At twice the focal length the image is the same size and at the same place as the object (§ 544).
Explain the idea
A concave mirror takes curvature away from the waves (the book's way of putting it). Inside the focus, the waves still spread and seem to come from behind the mirror: a virtual image. Outside the focus, the waves are made to converge to a real point in front: a real image (§§ 540–541). The image and object sizes are in the ratio of their distances (§ 542).
Say it back
Your student lists the four kinds of image from § 545 as the candle moves back from the mirror.
Afterwards
Look at the images in both sides of a soup spoon as you move it toward and away from your face.
Tip
The problems on mirrors and lenses come together at the end of the next lesson.