Physics 10 Module 3
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Module 3 – Plane Mirrors
Module Learning Objectives
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To recall and understand the difference between real and virtual images
To recall the characteristics of a mirror image
To be able to produce accurate calculations, measurements and diagrams for
mirror reflections
Introduction
In this module you will learn more about ray diagrams which are used to predict the position and
characteristics of images in mirrors (and also lenses and other optical devices).
If you draw the ray diagram for a plane (flat) mirror you will find the characteristics of the image are:
 The image is the same size as the object
 The image is the same distance behind the mirror as the object is in front of the mirror
 The image is laterally inverted (left-right reversed)
 The image is virtual (not real)
All of these characteristics we (sort of) understand from our everyday use of mirrors.
The characteristic which most students find difficult is the idea that the image is virtual (NOT real). It
is important that you understand this. What does this mean? In the everyday use of mirrors we
know that the image we see in the mirror is not really there – it is in a sense an optical illusion –
there is no laterally inverted person that looks like me standing behind the mirror – that is what we
mean by a virtual image.
In Physics we can define a virtual image as being :
an image which cannot be formed on a screen
A virtual image is formed when the rays of light appear to be spreading out from a point.
In the diagram the rays from the mirror to the image don’t really exist – our brain just believes that
is where the light is coming from because light travels in straight lines. We draw the virtual rays as
dotted lines.
Physics 10 Module 3
Lesson Download
A real image is the opposite of a virtual image and is defined as: an image that can be formed on a
screen.
Now have a go at the assignments.
Weblinks
Summary of reflection of waves: http://www.s-cool.co.uk/gcse/physics/properties-of-waves/reviseit/reflection-of-waves
Module 3 – Plane mirrors - Assignments
1. Read pages 8 -10 of textbook A, pages 135-137 of Textbook B
and make notes to help with your revision later in the course.
2. Follow the weblink and read through the summary on reflection of
waves
3. Using a piece of glass (take care), try the experiment outlined on
page 8 of textbook A to find the position of the image in a plane
mirror. Discuss with your parents what you have learned.
4. Complete the sheet “Ray tracing to find the image position”, using
a pencil and ruler. When you have an extended object, as in
number 3 on the second page, you treat each end of the object as
a single point to find the position of the final image. Scan your
completed sheets and send to your tutor for marking.
5. With the sheet “Ray tracing examples”, draw in the position of the
image for each example without tracing any rays, just relying on
what you have found out about the position of the image of an
object in a plane mirror. Scan your completed sheets and send to
your tutor for marking.
6. Look at how a kaleidoscope works, using two mirrors, as explained
on page 9 of textbook A. What happens to the number of images
you are able to see, as you reduce the angle between the mirrors?
7. Answer the questions on page 10 of textbook A.