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Why clear sky appears blue or why the rising/setting  sun or moon appears red in colour - both these questions have the same answer. Scattering of the light in the atmosphere of the earth.

This simple instructable can demonstrate just this -  using a FLASHLIGHT

I shall not be going in details of the theory but in short:

The fine constituent particle in the atmosphere of the earth scatters the blue light more than it does the red component of the light.  As the light from the rising/setting sun/moon (in fact it is true for other celestial bodies too) has to travel a longer distance in the atmosphere. In this process most of the blue component of the spectrum is scattered away and what we see is only the red one making the sun or the moon appear red in colour.

For the reasons below I am not going through the details

a) this is my first instructable and that I am submitting for the flashlight contest
b) this one was done some months ago and no sequential photographs were taken
c) it is such an easy one that what ever pictures I am uploading should be good enough

Step 1: Let Us Get Going

All this was done using the material available - lying around

So should I give the part list it goes like this below

1. PVC pipe - inner diameter 8cm, length 32 cm
2. Glass sheet 39x 14 cm one
3. Glass sheet 7x 14 cm 2 no.
4. General purpose epoxy compound  25gm (half a packet was used)
5. Cap portion of a toothpaste tube
6. Black mat finish paint
7. A few drops of Dettol

AND of course

A FLASHLIGHT - I am using a blue LED



Step 2: Back Side of the Apparatus

This is the back side of the apparatus

I cut the PVC pipe into two half along the length and painted one with mat finish black board paint - also sometimes called blackboard paint.

Gave two coats.

Then placed this half section on the 39 x 14 glass strip and glued it with epoxy.

Then glued the other two glass sheets to the side of the tube

This way the apparatus became self standing.

Now drilled a 6mm hole on the pvc pipe and glued the toothpaste cap upper portion.  And one more hole of 2 mm.

After the glue dried - set time given was 1 hr - filled in water into the apparatus.

Found there were a few leaks -  removed the water - and sealed the leaky points

The apparatus is ready now.

Step 3: Front Side of the Apparatus


The front side of the apparatus looked not-so-good so added some pvc sheets - just to give it a face lift.

Now the apparatus was filled in with water and a few drops of Dettol (Antiseptic Liquid Disinfectant )

In past I had experimented with other liquids such as water colour, milk etc but Dettol works best.



Step 4:


Now all one need to do is shine flashlight from one side (in this case from left). 

We see that color changes from left to right from blue to red in the column of the dettol mixed water in the tube

Step 5:


And if one sees from the other end - the flashlight that was distinctly blue/white is now red - just like colour sun close to the horizon

Step 6:


This has been a good tool to explain this atmospheric phenomena.

But one could extend it to show one more property of light passing from denser medium to rarer medium - the total internal reflection.


Step 7:


And lastly the flashlight can be used for showing / studying how two liquids mix -

just like this one -

quite dramatic - nice to watch -


And the last beam of the setting sun is green. Really!!!

About This Instructable

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Bio: I had been interested in DIY activities since my childhood. One of my DIY activity - astronomy and telescope making took me on a path to ... More »
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