In this experiment we will see that there are things in the world that do not have a shadow.
The experiment requires adult supervision.
Equipment and Materials
Lamp or flashlight
Lighter
Matches
Candle
A room that can be darkened
The Experiment
The course of the experiment can be seen in the video
Shadow as the Absence of Light
When we see shadows on the wall or on the ground, what we see are areas where the light intensity is lower than the surroundings.
That is why the shadow appears black or dark because the absence of light is actually darkness.
The lack of light occurs because some object blocks part of the light coming from a source.
But what happens if the object we want to see its shadow shines by itself
The answer is that it depends on several factors.
One of them is the brightness of the object relative to the main light source.
If the object shines brighter than the main light source, we will not see any shadow at all.
Because as mentioned a shadow is an area without light, and if there is another light that shines very strongly there is no lack of light.
We can say playfully that the light hides the shadow.
More accurately the light illuminates the shadow. Sometimes we can even see instead of a shadow a strong illuminated figure coming from the light source
A lamp that is off casts a shadow right.
When the lamp is on left no shadows are seen, and it is even possible to see a line of light on the wall from the filament the lit part of the lamp
It is clear that if I have an object that itself is a strong light source I will not be able to see its shadow
But if the object shines weaker than the main light source in this case shadows can be seen for example when the lamp is far from the wall where it is supposed to cast the shadow so it does not shine strongly on the wall
Shadow as Blocking of Light
A shadow on the wall is as mentioned an area where the light is less compared to the surroundings. What causes the shadow is an object that blocks part of the light, meaning it casts the shadow.
If we go back to the lamp in the example it contains a thin filament made of tungsten, which when electric current passes through it heats up a lot and shines.
But tungsten itself even when it is hot is an opaque material that blocks light meaning it can in principle cast a shadow although it shines. Indeed under certain conditions, when the lamp is far enough from the wall and the light source behind it is strong, it is possible to see the shadow of the lit filament
That is despite light being the opposite of shadow there are cases where it is possible to see even the shadow of a light source if it is weak and opaque
Shadow of a Flame
And what about the flame demonstrated in the experiment When the flame is close to the wall in any case the first situation occurs a lot of light is the opposite of shadow and shadows are not seen
But the truth is that the flame unlike an electric lamp is really unable to cast a shadow.
It is not really opaque meaning light can pass through it.
So even if it is far and the main light source behind it is strong it is impossible to see its shadow.
What can sometimes be seen is the shadow of hot gases.
Air or other gases at different temperatures transmit light differently and slightly bend or deflect it. This is the reason that sometimes a road on a hot summer day looks wet.
We can see these distortions on the wall in front of the flame.
If a lighter is used it is possible to see temperature differences of the gases a cold area near the center of the lighter the gas coming out of the lighter is cold surrounded by a warm halo
As we saw in the experiment when the fire is turned off and smoke rises the smoke itself casts a noticeable shadow because it contains opaque particles that block light and indeed cast a shadow.
It should be noted that sometimes the flame of a candle or lighter is sooty meaning it contains small particles of soot which can also cast a shadow.
But if the flame is clean like a household gas flame there really is no shadow for the flame