Vertical lines of symmetry

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[School bell rings]

Hello, I'm Mr Firth, and together we're going to learn about lines of symmetry.

Symmetry is when one side of a shape looks exactly like the other—just flipped.

Lots of buildings use symmetry because it looks attractive to the human eye.

Like Buckingham Palace. Even some animals, like butterflies, are symmetrical.

If you can fold a shape in half and it fits into itself perfectly with no overlaps, then you’ve found a line of symmetry.

The line of symmetry can sometimes be called a mirror line.

Because if you put a mirror on the line, you'll see the whole shape in the reflection.

Let’s investigate which shapes have a vertical line of symmetry. Remember, a vertical line is a line that goes down.

Here is a circle.

Let’s try putting our line of symmetry right down the middle.

It’s important to get the line as close to the middle as possible to test the symmetry.

If you’re too close to one edge, it won’t be symmetrical—one side will be bigger than the other.

There’s our line in the middle.

Can you see how both sides of the circle are the same size and look like a mirror image of each other?

If you folded the circle in half, each side would fit perfectly into the other.

Let’s look at a square now.

Do you think it will have a vertical line of symmetry?

Yes, it does!

Both halves of the square now look like identical rectangles.

Let’s have a look at a triangle.

I think this shape will have a line of symmetry. Don’t you?

Once you begin understanding what symmetry is, it becomes a lot easier to start recognising it within shapes.

Doesn’t it? Yes—we were right!

What about this triangle though?

This is a right-angled triangle.

Can you spot the right angle?

This triangle does not have a vertical line of symmetry.

Either side of the mirror line looks different.

This is what one side of the mirror would look like if we placed our mirror on the mirror line.

The other side looks like a triangle—but not the right-angled triangle we were investigating.

Right—quiz time!

Can you spot which of the following shapes will have a vertical line of symmetry?

If you think the shape does, give me a big thumbs up.

If not, show me a thumbs down.

Let’s go!

First shape: the pentagon.

If you drew a line of symmetry vertically down the centre, would both sides look exactly the same but flipped over?

Thumbs up for yes.

Thumbs down for no.

It’s a yes—this pentagon has a vertical line of symmetry.

Next shape: looks like a moon.

Do you think this shape has a vertical line of symmetry?

Remember, the line has to go down, not across.

Thumbs up or thumbs down?

It’s a no—this moon does not have a vertical line of symmetry.

Can you see why?

When we draw the mirror line on, the shape wouldn’t look complete in the reflection.

Last one: a love heart.

Will this heart have a vertical line of symmetry?

What do you think?

Yes—it is symmetrical.

Excellent work!

Lots of shapes are symmetrical.

Look around the room you’re in now—how many objects can you see that are symmetrical?

Or, why not write out the alphabet and find out which letters have a vertical line of symmetry by using a mirror?

For more help, challenges, and games on this subject, visit bbc.co.uk/bitesize.