Light cannot be seen directly. Light is only the medium that makes us see (similar to our own eyes, which we cannot see without aids, but which make us see): We see things because they reflect or absorb light.

In this picture, many people would naively say that there are ‘rays of light’ coming out of the cloud cover. But you know better: what do we really see here?

Some Experiments:

Now you can explain the path of light in a fiber.

Rainbow

When we see a rainbow, this is always only part of a rain ‘circle’, part of which is obscured by the local horizon.

The lower the sun is, the larger the arc, i.e. the more of the 42° circle we see:

Task: Think about where the sun is in these pictures.

With the following experiment, which you can do with a glass ball from a toy or craft store, you will see that the arc is actually a circle.

The rainbow always occurs opposit the source of light: Make an experiment with a little glas ball!

Coloured Shadows

Depending on the source of light, shadows can be colourful. This is typically the case when more than one source of light is involved. 

This image was taken in a planetarium with a chain of LED behind the dome horizon. 

Hint: The finger touches the dome.

You can see the rivet seam, i.e. the dome is obviously made of a very hard material – we don’t see any folds in the material here.

Note our topic: What light sources are obviously installed behind the black panel that suggests the ‘white’ sky?

Colour of the Earth’s atmosphere during a lunar eclipse

  1. the observer is looking at the night side of the Earth, i.e. the dark side that is completely in shadow.
  2. the sun is behind the earth and only grazes through the earth’s atmosphere. As the Earth is three times the size of the Moon, it must measure about three times half a degree, i.e. 1.°5, across the sky from the distance of the Moon. The sun, on the other hand, measures half a degree. The sun can therefore be a maximum of half a degree away from the edge of the earth. The sky therefore glows as brightly as it does at sunrise, when the sun is only half a degree below the horizon.

However, it does not glow red, because the sun itself is only red, not the sky around it. We can illustrate this with a simple laboratory experiment:

Add about 3 drops of milk to a tub of water. This clouds the emulsion. We place a halogen lamp behind the short side of the tub (they have approximately the same spectrum as the sun, i.e. ‘white’ light).

Observation 1: If we look at the side of the tub, the milk solution appears blue like the daytime sky (Rayleigh scattering).

Observation 2: If we look vertically into the lamp (here with slit), the illuminated lamp slit appears reddish, but the emulsion around it appears yellowish, golden (Mie scattering).

However, you can see from this picture that close to the lamp, i.e. about half a degree away from the edge of the light source, the sky (emulsion) should still be almost red, i.e. perhaps about golden yellow.

Flashes/ Saint Elmo’s Fire

CAUTION – Experiment with high voltage!

Lightning is an electrical discharge in the air: We see how an electrical potential discharges from a large negative pole to a charge deficit space, i.e. a charge equalisation takes place.

This can also be simulated in the classroom, you just need a sufficiently large potential that it penetrates the air. During upconversion, sparks are generated at the spark gap (plate capacitors whose air-filled gap is bridged) like a sparkler. In the strong e-field of the Tesla coil (right in the picture), fluorescent tubes begin to glow in a seemingly ‘ghostly’ manner, because they bridge a potential difference from their front to their rear end. This is like applying a voltage with plugs:

a bit dangerous – but very impressing

Here are the details: at the tip of the high Tesla coil, lightning strikes the surrounding air (violet) until it is saturated. Nearby fluorescent tubes begin to flicker or glow: this one is half prepared differently to glow in different colors.

Note: Sometimes there is no tuft discharge at the top of the coil, which could not be changed by airing. Perhaps the humidity was too low? Or was there not enough high voltage at the primary resonance circuit? The cause is still being investigated and a solution may be presented.

Tip: When the ambient air is saturated and the flashes subside, you can stimulate them again a little by smiling. 🙂 At some point, however, you have to stop with the spacey gimmicks. After all, the high voltage is not without danger.

The spark gap attracts attention by spraying kinsters and stars when the transformer transforms the laboratory voltage upwards. With the violet-colored corona discharge – only visible in the darkened room – in the Leiden bottle (exposed for approx. 30 seconds), the color spectacle is perfect.

nota bene: St. Elmo’s Fire (as sung about by John Parr) is the name given to such discharges at the tops of the masts of sailing ships during thunderstorms. They occur there naturally and are a well-known phenomenon among sailors.

tesla trafo in classroom

Solar Eclipse

Solar eclipses have inspired people to perform rituals and calculations since time immemorial. They have therefore played an important role in the development of human science and culture.

Task 1: Observe the partial phase of a solar eclipse in the shade of a tree! Explain why the bright stripes in the shadow look crescent-shaped! What do they normally look like? So what must the rays of light be like here?

(If you are stuck, google the keyword: pinhole camera and compare its functional principle with a camera obscura)

Task 2: With a partner, observe a partial solar eclipse from two different locations. From this we can then use triangulation to determine the ratio of the distances of the sun and moon.

left: under a tree, above: under a shrub during and after a partial solar eclipse

Never look into the Sun directly!

Never look directly at the sun! It can blind you.

General tips for observing the sun

So how can you observe the sun safely? As explained for the partial SoFi, people have found ways to observe partial solar eclipses since ancient times.

It feels cooler and the light is strangely dimmed, even though the sun shines brightly in the sky. Human curiosity leads us to want to see what is happening there.

One variant is projection with a cardboard solar scope: this allows you to observe the sun safely, as you don’t have to look through the telescope at any time. The device is aligned using the shadow (display the image alone in the browser, then it should be large enough for the text to be legible).

Alternatively, you can also make your own pinhole camera:

All you have to do is stick a piece of cardboard in the window that casts a shadow on the floor. Then make a small hole in the cardboard and a small image of the sun will appear on the floor. If there is a dark carpet, simply place a white sheet of paper on the floor so that you can see what is happening better.

Instead of the floor, you can also use a (mobile) office chair.