A drawing of a woman strolling towards a bridge. Yet look closely (maybe move away form the screen) and a human face will appear.

If you keep your eyes on the black dots, they appear to form and vanish at the intersections of the horizonal and vertical lines. The effect is diminished if one is very close to the screen, further away or if one views at a 45 degree angle

Focus your eyes on the dot in the centre of the circle and then move your head backwards and forwards. The outer wheels of the circle should begin to rotate in opposite directions.

If you focus on the picture, all the wheels start to spin (in both directions). However, if you then concentrate on a single wheel, that wheel will stop whilst the others keep turning.

An elderly couple share a loving embrace...but is that all? Keep looking and an entirely different image will emerge

Look closely at the black squares. You'll begin to see grey spots appearing at the intersections of the white lines amongst the black squares

Of course, optical illusions are now being put to commercial use by graphic designers.

At first glance, it appears to be a black and white chess board with a green cylinder casting a shadow diagonally across the board. However, the black and white squares are simply different shades of grey. The 'white' squares underneath the shadow (including 'B') are actually exactly the same shade of grey as the 'black' squares outside (including 'A')

The horizontal lines appear to be bent, although they in fact parallel straight horizonal lines

the Bezold Effect is such that the red seems lighter when combined with the white and darker when combined with the black

An illusion that plays on our perception of relative size. The first central circle appears smaller than the circle on the right yet they are identical in size

The overlapping black arcs appear to form a spiral; but, the arcs are simply a series of concentric circles.

The Impossible Cube Illusion is an illusion that draws on our natural desire to view the whole rather than individual elements of a drawing or picture. Each part of the drawing is ambiguous, where two lines cross it does not show which is behind and which is in front.

The long black lines are in fact parallel to each other. The illusion is created by the shorter lines being at an angle to the longer lines, this creates the impression that one end of the longer lines is nearer to us

In this image, the two shapes are in fact identical in size

One sees an inverted white equilateral triangle in front of the three black disks. However, the triangle is not drawn and the illusory triangle looks brighter than the background.

At first viewing, one assumes that the blue line is a continuation of the black line. However, on closer viewing it is actually the red line.

Another example of an impossible object. Three circular prongs at one end become two rectangular prongs at the other

When a grey rectangle is mainly surrounded by black it should look lighter. In this case, the grey rectangles are exactly the same shade of grey.

The brain's reaction to the colour contrasts and position of the shapes is such that this static image appears to be moving