What it does
A flat (plane) mirror follows one simple rule, the law of reflection: the angle at which light hits it (the angle of incidence, i) equals the angle at which it leaves (the angle of reflection, r). Both are measured from the normal, a line at right angles to the mirror, and the incident ray, the normal and the reflected ray all lie in one flat plane.
This lab has three views:
- Reflection: drag a laser round a mirror and read i and r off the protractor arcs. The laser’s image sits straight behind the mirror, as far behind as the laser is in front, and the reflected ray seems to come from it.
- Two mirrors: set two mirrors at any angle θ, like the inside of a kaleidoscope, and see every image of an F-shaped object. The count follows 360 ÷ θ − 1 when 360 ÷ θ is even; the page also shows the odd and in-between cases by tracing each reflection.
- Periscope: two parallel mirrors in a tube. Tilt them and see that the light only gets through near 45°.
It follows Class 6 to 8 Science on light (reflection, the image in a plane mirror, lateral inversion, the kaleidoscope and periscope) and the start of Class 10 work on reflection of light. Next, curved mirrors and where their images form are in the mirror ray diagram; light bending as it enters glass is in snell's law; and lenses are in the lens ray diagram.
How to use it
- Pick Reflection, Two mirrors or Periscope at the top.
- Reflection: drag the laser dot anywhere above the mirror (left or right of the normal), or set the angle of incidence i from 0° to 80° with the slider or box.
- Two mirrors: set the angle θ between them (30° to 180°, or tap a preset) and pos, where the object sits across the gap (50% is on the middle line). Faded copies are the images.
- Periscope: set the tilt of both mirrors from 30° to 60° and watch the light’s path.
Worked example
The page opens with the laser at i = 35°. The reflected ray leaves at r = 35° on the other side of the normal. Both rays make 90° − 35° = 55° with the mirror surface. The laser is 12.3 cm in front of the mirror on the drawing, and its image is 12.3 cm behind it.
Open Two mirrors: at θ = 60°, 360 ÷ 60 = 6 is even, so you see 6 − 1 = 5 images, arranged round a circle like a kaleidoscope pattern. Try 72°: 360 ÷ 72 = 5 is odd, so you see 5 images with the object off the middle line, but 4 when it sits exactly on it (pos = 50), because two of the images then land in the same place.
In the Periscope, at 45° the top mirror turns the light through 90° straight down the tube, the bottom mirror turns it back level, and you see the object the right way up. At 40° the light goes down 10° off straight and hits the tube wall.
Check yourself
Five fresh questions: the angle of reflection, the angle of incidence from the angle with the surface, images between two mirrors, the distance to your own image, and the angle that gives a number of images. Nothing is saved.
FAQ
Why does my image swap left and right? A plane mirror reverses front and back, which makes a right hand look like a left hand: lateral inversion. Two reflections (as in a periscope, or the even-numbered images in the two-mirror view) undo it.
Why can a kaleidoscope show so many images? Its mirrors are usually at 60°, and the images of images fill the circle. Three mirrors in a triangle repeat the pattern forever.
Is this from the NCERT book? It covers the same topics with fibub’s own pictures and examples. fibub is not part of NCERT.
Where the numbers come from
- The law of reflection, plane mirror images (as far behind as the object is in front, laterally inverted), the number of images between two inclined mirrors and the two-mirror periscope are standard school optics. Wording, pictures and examples are fibub's own.
- Image counts come from tracing the reflections one by one and were checked against the 360/θ − 1 rule (and its odd-number cases) for every whole-degree angle from 20° to 180° where the rule applies.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.