P8.1 – Reflection of Light

Disclaimer: Due to unforeseen difficulties, we have had to take down the images on this notes page. They will be replaced shortly. We apologise for the inconvenience, but hope that the new images will provide you with an even better learning experience.

1. Describe the formation and characteristics of an optical image seen in a plane mirror.

How do we see objects in the mirror? It is due to reflection (of light). Light rays will strike the mirror and reflect off it into our eyes. The optical image formed will be:

• the same size as the object.
• upright
• virtual – a real image is formed on a screen (or some other detector, like your eyes) when all of the rays from a single point on an object strike a single point on a screen. A virtual image is produced when rays of light come into our eyes and appear to come from an object, when in reality, that object is not present at the apparent position of the source. So, due to the direction the light rays come from, our brain makes us think that the object is in one place when in reality, it is in another. The most common example of virtual images are reflections in plane mirrors – look at the diagram below. It looks like the object is at Q’Q, when it’s actually at P’P.
• The image will be behind the plane of the mirror and the object will be in front; the distance between the image and mirror will be equal to the distance between the object and mirror. This is shown in the diagram.

d_o is equal to d_i.

In the diagram, PP’ is the object and QQ’ is the image we see.

2. Perform simple constructions, measurements and calculations based on reflections in plane mirrors

The light ray that hits the mirror from the object is called the incident ray (ray of incidence) and the ray that is reflected off the mirror into our eyes is called the reflected ray (ray of reflection). The normal is the line that runs perpendicular to the mirror between the two rays. The angle of incidence is the angle between the normal and incident ray, and the angle of reflection is the angle between the normal and the reflected ray.

3. Use the law: angle of incidence, i = angle of reflection, r

The law of reflection states that the angle of incidence is always equal to the angle of reflection.

Notes submitted by Lintha and edited by Sarah.

Click here to go to the next topic.

Click here to go back to the previous topic.

Click here go back to the Science menu.