Light & Optics

Sunset in a Jar

Shine a flashlight through a jar of milky water and watch it look blue from the side and orange from the end, for exactly the reason the sky and the sunset do.

Easy · 45 minutes

Introduction

The sky is not blue because it reflects the ocean, and the sunset is not orange because the sun changes color. They are the same effect seen from two different angles.

You can put that effect in a jar on the kitchen table with a flashlight and a few drops of milk, and look at it from both angles inside the same minute.

The Why

White light is every color at once. When it meets something much smaller than its own wavelength, the short blue waves get knocked sideways far more than the long red ones, roughly sixteen times more, and that sideways-scattered light is what you see through the side of the jar. What is left to come out the far end has had its blue stripped out, so it looks warm and orange. In the sky the scatterers are air molecules, and a sunset is redder than noon because the light is travelling through much more air to reach your eye.

Step-by-Step Instructions

1
Fill the jar with water and turn the lights off. Shine the flashlight in through one end and look at the beam from the side. In clear water you will barely see it, because there is almost nothing in there to scatter the light.
2
Add two or three drops of milk and stir. Look from the side again. The beam should now be visible, and faintly blue.
3
Now look straight down the length of the jar with the flashlight at the far end. Do not look at the flashlight itself, look at the light that has come through the water. It should look warmer than it did going in.
4
Add one more drop and check both views again. Write down what happens to each color as you go.
5
Keep adding milk one drop at a time until the beam from the side turns white instead of blue. Write down how many drops that took.
6
Hold the white paper at the far end and look at the spot the light makes on it. Compare that spot at two drops against the same spot at eight drops.
7
Explain in one sentence why noon and sunset are different colors even though the sun itself has not changed at all.

Drop by Drop

One row per amount of milk. Look from the side and from the end before you add the next drop.

Drops of milk	Color from the side	Color through the end	Notes
Row 1, Drops of milk	Row 1, Color from the side	Row 1, Color through the end	Row 1, Notes
Row 2, Drops of milk	Row 2, Color from the side	Row 2, Color through the end	Row 2, Notes
Row 3, Drops of milk	Row 3, Color from the side	Row 3, Color through the end	Row 3, Notes
Row 4, Drops of milk	Row 4, Color from the side	Row 4, Color through the end	Row 4, Notes
Row 5, Drops of milk	Row 5, Color from the side	Row 5, Color through the end	Row 5, Notes
Row 6, Drops of milk	Row 6, Color from the side	Row 6, Color through the end	Row 6, Notes
Row 7, Drops of milk	Row 7, Color from the side	Row 7, Color through the end	Row 7, Notes
Row 8, Drops of milk	Row 8, Color from the side	Row 8, Color through the end	Row 8, Notes

The row where the side view stops being blue and turns white is the interesting one. Write down what you think changed.

Materials List

A tall clear jar or a straight-sided clear plastic bottle
Water
Milk, only a few drops to begin with
A bright white flashlight, or a phone torch
An eyedropper or a teaspoon
A sheet of white paper to use as a screen
A dark room

Safety First!

Do not look directly into the flashlight, and never use a laser for this project. Use a jar you have permission to use, wipe up spills so nobody slips, and pour the milky water down the drain when you are done.

Challenge Mode

Milk is a cheat, and a useful one. Its fat droplets are far bigger than air molecules, so your jar imitates the sky rather than copying it, which is why your blue comes out pale instead of vivid. Try to get closer: use less milk in more water, or try a pinch of powdered creamer or a single drop of dish soap instead. Which one gives you the bluest side beam for the least cloudiness?