How light bounces, how a plane mirror forms an image, and why the image looks reversed.
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Light travels in straight lines. When it falls on a polished surface it bounces back: reflection. The angle between the incident ray and the normal is the angle of incidence i; the angle between the reflected ray and the normal is the angle of reflection r. Laws: (1) the angle of incidence equals the angle of reflection, i = r; (2) the incident ray, reflected ray and the normal all lie in one plane.
A ray strikes a mirror with i = 30°. Find r and the angle between incident and reflected rays.
r = 30°. Angle between the rays = i + r = 60°.
A ray falls along the normal. What is the angle of reflection?
0°, because i = 0 so r = 0; the ray returns along its own path.
A ray makes 40° with the mirror surface. Find i.
i is measured from the normal, so i = 90° − 40° = 50°.
A plane mirror forms an image that is virtual (cannot be caught on a screen), erect, 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, which is why AMBULANCE is written reversed on the front of the vehicle.
An object is 3 m from a plane mirror. Find the distance between object and image.
The image is 3 m behind the mirror, so the separation is 3 + 3 = 6 m.
If you walk 1 m towards the mirror, by how much does the gap between you and your image change?
It shortens by 2 m: you move 1 m and the image moves 1 m toward the mirror too.
Why is the word AMBULANCE written laterally inverted?
A driver ahead sees it correctly in the rear-view mirror.
If a mirror is turned through an angle θ, the reflected ray turns through 2θ. Two plane mirrors inclined at angle θ give n = 360° ÷ θ − 1 images when 360 ÷ θ is whole: at 90° there are 3 images, at 60° there are 5, and parallel mirrors give an endless number. Periscopes use two parallel mirrors at 45°, and kaleidoscopes use mirrors at an angle.
A mirror is rotated by 20°. By how much does the reflected ray turn?
2 × 20 = 40°.
How many images are formed by two mirrors at 60°?
360 ÷ 60 − 1 = 5 images.
Name the use of two parallel mirrors at 45° in a tube.
A periscope, to see over obstacles such as in a submarine.
1. State the laws of reflection.
2. What is the angle of reflection if the angle of incidence is 35°?
3. List the properties of the image in a plane mirror.
4. An object is 4 m in front of a plane mirror. How far is the image from the object?
5. What is lateral inversion?
6. A mirror is turned by 15°. By how much does the reflected ray turn?
7. How many images do two mirrors at 90° give?
8. What is a virtual image?
9. Name an instrument that uses plane mirrors.
10. Why does a ray along the normal return along its path?