What it shows
Two mirrors set at 45 degrees reflect light twice and bring the view from a higher point to your eye. It shows the laws of reflection.
The science
The law of reflection says that the angle of incidence equals the angle of reflection. A mirror at 45 degrees turns a ray of light by 90 degrees. Two such mirrors, one at the top and one at the bottom, send the light twice around a corner and carry the image to the eye. Submarines use periscopes to see above the water. The image is upright and the same size.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A clean 1 litre milk carton or a long cardboard box
- Two small plane mirrors
- Tape and glue
- Scissors (adult help)
- A protractor
- Black paper
Safety
For this project: An adult must cut the carton and handle the mirrors. Cover the mirror edges with tape.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Cut a window on one side near the top and another on the opposite side near the bottom of the carton.
Mark the 45 degree lines with a protractor inside.
Fix a mirror at each 45 degree line, facing the window.
Check that the mirrors face each other.
Paint the inside black to avoid stray light.
Look through the lower window at an object seen through the upper window.
Try the periscope to see over a wall.
Draw the path of a ray through the periscope with the angles marked.
What you should see
You can see objects above and beyond a barrier without moving your head.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the law of reflection?
Angle of incidence equals angle of reflection.
2Why are the mirrors at 45 degrees?
To turn the light by 90 degrees each time.
3Who uses periscopes?
Submarines and soldiers in trenches.
4Is the image upright?
Yes, and the same size.
Take it further
Make a longer periscope and see if the image becomes dimmer.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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