This is another classic optical illusion. The name for this is the Ponzo illusion and has a lot to do with our perception of distance and using cues from perspective to judge an item's size.

Enough chitchat for now. Look at the image below and guess which yellow line is longer.

## Step 1: Get an Image With Perspective

OK, all images have perspective, but we want one where many lines are converging to the same vanishing point. In this picture that vanishing point is right above the end of the escalator.

The source image can be found here on Flickr.

## Step 2: Add a Yellow Line

Now, on a new layer, add an yellow line low on the picture.

## Step 3: Copy the Line

Copy this line and place the copy directly above the first one and just a little below the vanishing point.

## Step 4: Try It Out With Other Images

Keep on searching for other images with lines leading to the vanishing point and try it out to see if the effect is stronger.

Source images here and here.

## Step 5: A Simpler View

If you don't fancy pictures of tracks and roads, you can just use black lines instead. See below for this effect.

The effect works because we're tricking our own sense of depth perception. Items that are closer to the horizon are farther away. So naturally if the yellow bar is closer to this it looks bigger even when it's exactly the same length.
Uhhh, the lines all look the same length.
Yeah, same for me. I do several perspective drawings per week, so maybe I'm just not affected. I'd experiment with different patterns on the surface that these lines are on.
its like the one with the paper plates their the same size but look different because their curved!
I agree- in the last image the illusion is pronounced but the bridge picture doesn't really have enough strong linear cues. You might be better off with a higher-contrast image or a photo with more linear perspective (looking down city streets, for instance).
look at the last pic with the black lines i noice the effect good job
when you look at them all over wit the black lines it kinda looks like the length change but when you look up and down the lines one at a time, the illusion doesn't work
yeah.....
They all look the same length to me anyway :/