Reflection of light and spherical mirrors

Reflection and the laws of reflection

• Reflection is the bouncing back of light when it falls on a shiny surface such as a mirror.
• The ray that falls on the mirror is the incident ray. The ray that bounces back is the reflected ray. The normal is a line drawn at 90° to the mirror where the ray falls.
• Law 1: the angle of incidence equals the angle of reflection (∠i = ∠r). Both angles are measured from the normal, not from the mirror.
• Law 2: the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.
• These two laws hold for every reflecting surface, flat or curved.
• A plane mirror (a flat mirror) gives an image that is virtual (it cannot be caught on a screen) and erect (upright). The image is the same size as the object, and as far behind the mirror as the object is in front. It is laterally inverted: left and right are swapped.

Words for spherical mirrors

Concave mirror vs convex mirror

Spoon test: the inside of a shiny steel spoon acts as a concave mirror. The back acts as a convex mirror.

How to draw a labelled concave mirror

1. 1
   Draw a straight horizontal line across the page. Label it principal axis.
2. 2
   Draw a shallow curve shaped like a closing bracket ) cutting the line, hollow side facing left. Draw short slanting lines on its right side to show the back of the mirror.
3. 3
   Mark the point where the curve meets the line as P (pole).
4. 4
   Mark C on the line to the left of P. Mark F exactly halfway between P and C.
5. 5
   Draw a double-headed arrow from P to C and label it R. Draw another from P to F and label it f.
6. 6
   Draw two rays parallel to the axis coming from the left. Reflect both so they pass through F. Put an arrowhead on every ray.

   For a convex mirror, draw the curve like an opening bracket ( and put C and F behind it, on the right.

Activity: find the focal length of a concave mirror

You need a concave mirror in a stand, a white card as a screen and a metre scale.

1. 1
   Stand near a window. Point the concave mirror at a distant object, such as a tree or a building far away.

   You may use the Sun instead, but never look at the Sun or at its bright spot. The paper can catch fire.
2. 2
   Hold the white card in front of the mirror, between the mirror and the window.
3. 3
   Move the card slowly forward and back until a sharp, small, inverted image of the far object forms on it.
4. 4
   Measure the distance from the pole of the mirror to the card with the metre scale.
5. 5
   Repeat three times and take the average. This distance is the approximate focal length.

   Rays from a very far object are almost parallel, so they meet at the focus.