Lesson 3: Shadows

Shadows

You know about shadows, you see them often, even your own but have you ever actually asked the question: 'What exactly is a shadow?'

You know that an opaque object does not allow light to pass through it e.g. a person. So if the sun shines on a person, it's light is unable to pass through the person and a shadow is formed behind him/her. 

Types of shadows

The light source will determine the type of shadow created. A point light source will produce a clearly defined shadow. A larger source of light will produce different types of shadows viz.:

• Umbra
• Penumbra

But what's the difference? This is best answered by way of an image:

Image: Wikipedia (CC BY-SA)

Umbra
This is the 'totally dark' shadow portion. The edges of the umbra are clearly defined. If you are standing in the umbra shadow you will be unable to see the light source.

Penumbra
This shadow type is partially dark and may have slightly blurry edges. If you are standing in the penumbra shadow, you will be able to see part of the light source. 

Watch the YouTube video below which further clarifies the difference between umbra and penumbra shadows.

Science Guru. (2019). Umbra and penumbra light and shadows (Standard YouTube licence)

Activity

Try the following experiment at home:

• The aim of this experiment is to demonstrate how a shadow is formed from a source of light
• You will need the following materials:
• a piece of cardboard with a hole in it
• a source of light e.g. a torch or candle (handle with care)
• an opaque object 
• a screen or piece of paper on which the shadow will be cast

Shine the light through the hole in the cardboard. What do you observe? What type of shadow is formed?

Remove the cardboard and just shine the light in line with the opaque object. do you see a difference in the types of shadows created? Can you name them? What is your conclusion?

Eclipse

If you think back to the course on Sources of Light,  you will recall that both the earth and moon are opaque and non-luminous objects, meaning that light cannot pass through them. Therefore, shadows, called eclipses, can be created. Let's find out more:

Lunar eclipse

A lunar eclipse will occur when the earth comes between the sun and the moon thus forming a shadow on the moon. The shadow can be partial or complete. A total (complete) lunar eclipse occurs when the moon lies in the umbra of the shadow of the earth.

Image: Royalty free (CC)

Solar eclipse

A solar eclipse occurs when the moon comes in between the earth and the sun. The sun is therefore blocked by the moon either totally (total solar eclipse as this image portrays) or partially (partial solar eclipse). Remember: never look directly at the sun!

Image: Wikimedia Commons (CC BY-SA)

Watch the YouTube video below, which although it is seven minutes long, it explains very clearly how a solar and lunar eclipse occur.

Science ABC. (2020). Solar and lunar eclipse explained. (Standard YouTube licence)

Summary

In this short course you learnt about the difference between the propagation and transmission of light:

• Propagation of light is how light is transferred from one point to another
• Transmission of light is how light passes through an object

On the topic of light transmission, the differences between transparent, translucent and opaque objects were identified. Thereafter, you moved onto shadows. When light is directed at an opaque object a shadow is cast behind this object since light is unable to pass through an opaque object. Umbras and penumbras were explored in further detail. Can you remember the difference? We hope so since in the quiz which follows you will be required to answer a question related to this!

Finally, the last topic covered in this course was the topic of solar and lunar eclipses. Can you recall the differences and why they occur? If not, we urge you to review the topic!

In closing, answer the questions in the quiz below. This will test your understanding of the topics covered! We've given you a few clues so good luck!

Attribution

The following resources were consulted in the making of this course:

• Tanzania Institute of Education. (2021). Physics for Secondary Schools Form One (Full copyright)
• ThinkTac. (2021). Light - Rectilinear Propagation (CC BY)
  
• MooMooMath & Science. (2020). Transparent, translucent and opaque (Standard YouTube licence)
  
• Science Guru. (2019). Umbra and penumbra light and shadows (Standard YouTube licence)
  
• Science ABC. (2020). Solar and lunar eclipse explained. (Standard YouTube licence)