Lesson 4: Images formed in mirrors

Let's now investigate how images are formed in a plane mirror. Watch the YouTube video below which clearly explains this concept.

Infinity Learn Class. (2019). Properties of images formed by a plane mirror. (Standard YouTube licence)

Video summary

Let's summarise the video into key points:

• Images formed in a plane mirror are virtual and not real since they appear behind the mirror
• The image will always be upright in a plane mirror. (Inverted images can occur but only if the mirror is concave)
• The image will be the same size as the object its reflecting. Plane mirrors do not magnify images
• Images in a plane mirror are laterally inverted, for example if you raise your left hand in front of a plane mirror, the image inverts as appears as if you are raising your right hand i.e. there is left-right reversal
• The image formed is the same distance from the mirror as the object is

Rotating a mirror

What happens to a reflected ray if you rotate a mirror? That's an interesting question!

Let's look at this question with a real example:

• You know that the incident angle and reflected angle are equal on a plane mirror. For this example, we will assume that this angle is 10 degrees
• Rotate the mirror by 15 degrees
• The reflected ray then moves through an angle twice the size of the angle of rotation i.e. the reflected ray will turn through 30 degrees

Right angle mirrors

Refer to the image below as we discuss this topic.

Two mirrors are placed at 90 degrees to each other. Each of the plane mirrors produe a left-right reversal image. These images are referred to as primary images.

But right angle mirrors actually produce three images.

The third image is formed by light rays which reflect off mirror 1 onto and off mirror 2 into your eyes. This third image is known as the secondary image. This secondary image is a result of two reflections and does not show the left-right reversal.

Images in parallel mirrors

• Two mirrors are placed parallel to each other two metres apart
• A person stands in the middle between the two mirrors. There will be one metre between each mirror and the person
• When mirrors are exactly parallel (i.e. there is no angle between them), remember that the image distance equals the object distance

Let's now take this a step further:

• Image 1 can also be considered as an object for mirror 2
• Image 1 will be 3m from mirror 2 and therefore the secondary image will be 3m behind mirror 2

Based on the same principle, there will be a secondary image of image 2 which appears in mirror 1.

These secondary images also act as objects to create a third pair images, and so the process continues. Ultimately, an infinite number of secondary images are created. 

When the mirrors are facing each, these images are not clearly seen since:

• Since there are so many reflections, the images become faint and are not clearly visible
• The human eye cannot comprehend far off images since the angle subtended by these images on the human eye is very small

Differences between an image and a shadow

Reflect on this. Can you identify the differences between an image and a shadow?

Once you have reflected on this and identified what you think are the differences, click on the link below to find out more.

Summary

In this course we explored what reflection is and the difference between incident and reflection rays. We then moved on to specular and diffuse reflections and how they influence the images created when light strikes their surface. 

The reflection of light on plane mirrors, mirrors and 90 degrees to each other and then mirrors parallel to each was thoroughly discussed with the aid of illustrations. 

The reflection of light is a broad topic and when one thinks about how a human actually sees through this process of light reflection, it is an amazing occurrence.

Quiz

Finally, test your understanding on the reflection of light by answering the questions in the short quiz below. A pass mark of 50% is deemed successful - good luck!

Attribution

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

• Tanzania Institute of Education. (2021). Physics for Secondary Schools Form One. (Full copyright)
• MooMooMath and Science. (2021). Reflection of light clearly explained. (Standard YouTube licence)
  
• Infinity Learn Class. (2019). Properties of images formed by a plane mirror. (Standard YouTube licence)
  
• MooMooMath and Science. (2021). Reflection of light clearly explained. (Standard YouTube licence)
  
• Infinity Learn Class. (2019). Properties of images formed by a plane mirror. (Standard YouTube licence)