Week 14 · Lesson 2 Lunar Eclipse: the Moon’s phases, shadow and penumbra, total and partial lunar eclipses, why there is not one every month, a report on solar eclipses · 3 lessons
2
Shadow and penumbra
Science Prep 1 Term 1, Unit 4: pp. 88–93
GoalUse a lamp, a ball and a screen to show how an opaque object makes a shadow with a darker centre and a lighter edge.
New words
opaque · letting no light pass throughshadow · the dark area behind an object that blocks lightpenumbra · the partly lit area around the dark centre of a shadow
Explanation
Light travels in straight lines. An opaque object, such as a ball, blocks the light, so a dark shadow forms behind it. A transparent object lets light pass, so it makes almost no shadow. When the light source is not a tiny point, the shadow has a very dark centre, and around it a lighter edge that gets some light from part of the source. The lighter edge is called the penumbra. In a dark room, a lamp, a ball and a white card show this. Think of the lamp as the Sun, the ball as the Earth and the card as the Moon. Safety: use a small torch or desk lamp with an adult; never look into a bright light, and never use the Sun.
Worked examples
Material
Light passes?
Shadow
wood
no
dark
clear glass
yes
almost none
tissue paper
some
faint
Sort materials. Wood, metal and cardboard are opaque and make dark shadows. Clear glass and clear plastic are transparent and make hardly any. Tissue paper lets some light through, so its shadow is faint.
Move the ball closer to the lamp and the shadow on the card gets larger. Move it closer to the card and the shadow gets smaller and sharper.
In the model
It stands for
lamp
the Sun
ball
the Earth
white card
the Moon
Name the parts of the model. Lamp: the Sun. Ball: the Earth. Card: the Moon. The dark patch on the card is the Earth’s shadow. The lighter ring around it is the penumbra.
Class activity
Lamp, ball and card, with an adult. In a dim room, hold a ball between a small lamp and a white card. Move the ball and see the dark centre and lighter edge of the shadow. Draw what you see and label the shadow and the penumbra. Try a clear plastic cup: what is different?
Practice
1
Why does a ball make a shadow?
It is opaque, so it blocks the light.
2
What is the penumbra?
The lighter, partly lit area around the dark centre of a shadow.
3
Does a clear glass make a dark shadow? Why?
No, because it is transparent and lets light pass.
