Curved Mirrors

Terms

    1) Principal Axis: The horizontal line that goes through the very centre of the mirror

    2) Pole (P): The point where the principal axis meets the mirror

    3) Centre of Curvature (C): If the mirror was a giant glass ball, this would be the centre of this ball

    4) Focal Point (F): The point halfway between P and C. This is where parallel rays “focus” or meet

    5) Focal Length (f): The distance from the mirror (Pole) to the Focal Point (F)

Concave Mirrors (converging)

Think of a cave. These mirrors curve inward. Their job is to bring light rays together (converge)

The 3 Rules for Ray Diagrams

To find where an image is, you draw an object (usually an arrow) on the principal axis and draw these three lines from the tip of an arrow.

    1) The Parallel Ray: Draw a line from the object to the mirror, parallel to the principal axis. It reflects down through the focal point

    2) The Focal Ray: Draw a line from the object through F to the mirror. It reflects parallel to the principal axis

    3) The Center Ray: Draw a line from the object through C. It hits the mirror and reflects directly back on itself.

Put the image wherever those rays cross!!

What happens to the image? (concave)

The nature of the object changes depending on where the object is.

    1) The object is very far away (Beyond C): The image is upside down (inverted), smaller (diminished), and real (you could see it on a screen)

    2) Object is at C: The image is upside down, same size, and real

    3) Object is between C and F: The image is upside down, magnified, and real

    4) Object is INSIDE the focal point (between F and the mirror): *The rays on the outside will never cross - they spread apart.

  • You have to trace the reflected rays backward with dotted lines behind the mirror.

  • The result: The image is upright, magnified, and virtual (this is how a makeup/shaving mirror works)

Convex Mirrors (diverging)

These bulge outward (like the back of a spoon). Their job is to spread light rays apart (diverge)

The big thing for convex mirrors

The image is ALWAYS the same, no matter where you stand

  • Virtual (Behind the mirror)

  • Upright (Right-side up)

  • Diminished (Smaller)

Why use them? Because they make things smaller, they give you a wider field of vision. That’s why they are used as security mirrors in shops or side-mirrors on cars (“Objects in mirror are closer than they appear)

Summary Checklist

  • Real Image: Light rays actually cross. Can be projected on paper. Usually upside down

  • Virtual Image: Light rays only appear to cross behind the mirror. Can’t be projected. Usually right side up.

  • Drawing: Use a ruler! Always include arrows on rays.