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Drawing Mirror Diagrams

Construction of the image formed in a plane mirror or a single point object

Transcript

Welcome to cyberphysics video clips.

Today we are going to look at an easy way to construct the diagram that shows how an image is formed in a mirror.

Instead of having to carefully measure angles we are going to cheat – but make it look as if we have carefully measured the angles! You know that the angle of reflection always equals the angle of incidence but what you might not realise is that a student mis-measuring an angle by a couple of degrees can produce a very poor standard optical diagram.

I suggest we cheat – it saves so much time in an exam.

We will start with a simple single point diagram. The commonest examination question on this topic asks you to construct the image of a single point object.

First of all draw the symbol for a plane mirror in the centre of your page. Do not overdo the shading at the back of the mirror – the chevrons look best when the are widely spaced and it saves time to draw fewer of them.

Next add your object point and label it ‘O’.

Take a ray from the point and make it hit the mirror. Remember that for a line to represent a ray in an optical diagram it has to have an arrow on it.

Add a normal at the point where it hits the mirror – a normal is always drawn whenever a ray hits a boundary between two media.

If we were going to construct the reflected ray for this incident ray we would have to carefully measure the angle of incidence and carefully construct a reflected ray so that the angle of reflection equalled the angle of incidence. That is tricky and so we are not going to do that!

Instead we are going to use the fact that the image point will be as far behind the mirror as the object point is in front. Measure the perpendicular distance from the object point to the mirror line and then put the image point an equal distance from it behind the mirror. Label it ‘I’

Now place your ruler so that you can draw a line from the image point through the point where the incident ray hits the mirror.

In front of the mirror line draw a solid line for the reflected ray and behind the mirror draw a dashed construction line that passes through the image point.

Add your arrow to that solid line to make it the reflected ray.

Now repeat this for a second ray from the object. The dotted lines will cross at the image point and the reflected rays will diverge away from the mirror.

When rays diverge as they enter our eyes our brains construct an image at the point the light appears to come from. In this case the diverging reflected rays appear to come from the image point on the opposite side of the mirror line – so the brain would perceive an image point there.

You can now label your diagram. Practice this – you will find that you can construct the diagram in about 30 seconds with practice. This saves a lot of time in an exam..