REFLECTION OF LIGHT FROM SPHERICAL MIRRORS.

Terms Used to Describe Spherical Mirrors

C = Centre of curvature, F = focus, P = pole.

f = focal length.

Reflection of Light from a Concave Mirror
If a ray strikes the pole then the angle of incidence equals the angle of reflection.	A ray through the centre of curvature reflects back along its own path.
Rays parallel to the principal axis reflect through the focus.	Rays through the focus are reflected parallel to the principal axis.

Real Image     (MIX)
A real image is an image formed by the actual intersection of light rays.
Such an image can be located on a screen or by the method of no parallax.

Virtual Image      (MIX)
A virtual image is formed by the apparent intersection of rays.
Such an image can never be formed on a screen. It can be located by the method of no parallax

For a Concave Mirror:     (MIX)
If the object is outside the focus the image is real and is located in front of the mirror.
If the object is inside or at the focus the image is virtual and is located behind the mirror.

Light from a Distant Object      (MIX)
Light from any point on a distant object arrives as a beam of parallel light.

The Natures of the Images in a Concave Mirror
Real Inverted Diminished	Real Inverted Same Size
Real Inverted Magnified	Infinity
Virtual Erect Diminished	Remember: An image in a concave mirror can have any of five natures: RID, RIS, RIM, Infinity, VEM.

CONCAVE MIRROR FORMULAE

where   f  is the focal length,
            u  is the distance between the object and the back of the mirror,
and       v  is the distance between the image and the back of the mirror.

          and     
where   m is the magnification,
            u is the distance between the object and the back of the mirror,
and       v is the distance between the image and the back of the mirror.

Note: if the image is virtual, v is negative.

Note: The fraction button, , makes these questions easy.

TO FIND THE APPROXIMATE FOCAL LENGTH OF A CONCAVE MIRROR

Use the mirror to focus the image of a distant object on a sheet of paper.
The distance between the sheet and the back of the mirror is approximately one focal length.

MANDATORY EXPERIMENT

To find the focal length of a concave mirror.

	Measure u, the distance from the lampbox to the back of the mirror. Measure v, the distance from the back of the mirror to the screen.
	Graph 1/u against 1/v. Draw the best straight line. Take a value for 1/u and a value for 1/v. Use the focal length formula to calculate a value for f.
SOURCES OF ERROR. The image on the screen may not be sharp. The student may have made the error of parallax in taking the measurements of u and v. (The error or parallax occurs when one reads a metre stick at an angle other than ninety degrees.) PRECAUTIONS. Ensure the image is as sharp as possible. Make sure your line of sight is at right angles to the metre stick when reading it.

USES OF A CONCAVE MIRROR

Note: The make-up mirror and the dental mirror make use of the magnifying property.
          When the object is inside the focus, the image is Virtual, Erect and Magnified.

Note: The searchlight makes use of the focal point property.
          Rays through the focus are reflected parallel to the principal axis.

CONVEX MIRRORS.

Notice in the following diagrams how the focus and the centre of curvature are behind the convex mirror.

Reflection of Light from a Convex Mirror
If a ray strikes the pole then the angle of incidence equals the angle of reflection.	A ray heading for the centre of curvature reflects back along its own path.
Rays parallel to the principal axis reflects as if it came from the focus.	Rays heading for the focus are reflected parallel to the principal axis.

Note that unlike a concave mirror, where the image can have any of five natures, RID RIS RIM Infinity VEM, the image in a convex mirror has only one nature - VED.

For a Convex Mirror      (MIX)
The image is always virtual and located behind the mirror.
The image is always diminished. The nearer the object is to the mirror the bigger the image.

CONVEX MIRROR FORMULAE

where   f  is the focal length,
            u  is the distance between the object and the back of the mirror,
and       v  is the distance between the image and the back of the mirror.

REMEMBER: v and f are negative since they are both behind a convex mirror.

          and     
where   m is the magnification,
            u is the distance between the object and the back of the mirror,
and       v is the distance between the image and the back of the mirror.

USES OF A CONVEX MIRROR

Security mirrors in shops

Car door mirror

Note: Convex mirrors have a wide field of view.
          Since the image is diminished, a warning is often placed on the car door mirror.