Topic 1 of 4
Reflection and plane mirrors
A ray diagram shows where light travels. At a mirror, draw a normal before measuring or calculating any angle.
A ray arrow gives the direction of light travel. In the uniform medium shown, rays are straight. The normal is a line at 90° to a surface at the point where the ray meets it; it is a construction line, not a beam of light.
Measure both angles from the normal
- Incident ray
- The ray arriving at the surface. Its angle to the normal is the angle of incidence, i.
- Reflected ray
- The ray leaving the surface on the incident side. Its angle to the normal is the angle of reflection, r.
Measure i and r from the normal
Solid green arrows show light travel. The dashed normal is perpendicular to the reflecting surface.
Worked angle
The given angle is 35° to the mirror
- Find incidence: i = 90° - 35° = 55°.
- Apply reflection: r = i = 55°.
- Check the surface angle: the reflected ray makes 90° - 55° = 35° with the mirror.
Before using a labelled angle, identify the two lines that form it.
Construct and measure a reflected ray
- Draw the mirror's reflecting line and mark the incidence point.
- Draw a normal through that point, perpendicular to the mirror.
- Measure i between the arriving ray and the normal. On the other side of the normal, mark the same angle.
- Draw the reflected ray through that mark and add an arrow pointing away from the mirror.
To investigate the law, place a mirror on its traced reflecting line and direct a narrow ray at it. Mark two well-separated points along each visible ray, remove the apparatus, and join the marks with a ruler. Centre the protractor at the incidence point and use its correct scale to measure from the normal. Compare i and r for several different incident directions.
A broad beam or two marks close together makes the ray direction uncertain. A wrongly drawn normal changes the measured angles systematically; repeating the measurement does not repair that normal. Choose a protractor with suitable degree divisions and record angles to a precision the ray width and markings support.
Why a plane-mirror image appears behind the mirror
Light from one object point reflects from the mirror into the observer's eye. The reflected rays diverge, but their straight backward extensions meet at an apparent point behind the mirror. The eye receives actual reflected light; no light has to travel to that apparent point behind the mirror.
Reflected rays appear to come from behind the mirror
Solid green paths are actual rays. Dashed green paths are backward extensions, with no light travelling behind the mirror.
A separate face view shows the reversed appearance
A plane-mirror image is virtual, upright and the same size as the object. It is as far behind the reflecting plane as the object is in front. It cannot be formed on a screen at its apparent position because actual rays do not converge there.
The reversed appearance of an asymmetric letter is called lateral inversion. The mirror places corresponding image points on the opposite side of its reflecting plane; it does not physically exchange two objects on your left and right.
Image position
An object is 18 cm in front of a fixed mirror
The image is 18 cm behind the reflecting plane, so the object-to-image separation is 18 + 18 = 36 cm.
Move the object 5 cm towards the mirror. Its new distance is 13 cm, and the image is now 13 cm behind. The separation becomes 26 cm. Both distances are measured from the same reflecting plane.
Locate a virtual image experimentally
Trace the reflecting plane and use two sighting directions to locate an image point by backward construction. Alternatively, use a comparison marker behind the mirror with its tip visible alongside the image. Move the marker until a small sideways movement of your eye produces no relative shift between the tip and image.
This no-parallax match means that the marker tip and apparent image point are at the same position along the viewing direction. Measure their distance from the reflecting plane and compare with the object distance. Use a ruler long enough for the separation, read its millimetre divisions carefully, and measure to the reflecting plane rather than a holder or frame.
Virtual does not mean invisible. You can see the image when reflected light enters your eye. The distinction concerns where actual rays meet, not whether the image can be seen.