Everyone walking into Bridget Riley's Tate retros-pective will be confronted, at once, by the largest work she has ever produced. Stretching to a colossal 17.36 metres in width, the drawing is executed in black directly on the white wall opposite the entrance to the show. It is both arresting and inexhaustible.
Riley fills the entire image with a web of circles, some touching and others intersecting. The picture looks at first like a highly simplified abstraction. But a host of possible references to the visible world soon crowd into our minds. The circles suggest planets and suns, jostling for space in an immense cosmic panorama. When sliced by overlapping lines, they also resemble Matissean flowers or broken bursts of light announcing the dramatic advent of day.
This epic Composition With Circles 3 is the newest work on show, proving from the outset that Riley is still working with as much boldness, vigour and inventiveness as ever. Now in her early seventies, she has been a vital force in painting for more than 40 years. But the Tate survey should not be seen as an act of closure, neatly summing up the achievements of a familiar, venerated reputation. The wall drawing demonstrates that she is an artist on the move, still fired by the old urge to ambush and astound.
Not that Riley is motivated merely by a callow desire to shock. The assault on our senses arises from a far more profound source. Just as Wordsworth was driven to recapture the lost intensity of his boyhood response to nature, so Riley aims at recapturing the "sudden, swift and unexpected" pleasures she experienced in her early exploration of land and sea alike. Growing up in Cornwall, she was spellbound by the ever-shifting dynamics of the light. The glittering action of sun on water held her transfixed. Swimming brought her into close, startling proximity to the flash and spatter of reflected brilliance. The young Riley capitulated to its "elusive, unstable, flicking" dazzle, and she later realised that children are far more open than adults to these revelatory encounters. Preoccupied with this sense of loss, she set about forging an art that would enable viewers to regain the freshness and astonishment of their childhood reactions to the natural world.
But Riley had no intention of painting conventional landscapes. Her work in the early 1960s revolutionised our way of seeing. Far from producing tame and predictable images, or following in the path of St Ives painters such as Patrick Heron, she stayed at a remove from her Cornish starting point. The earliest paintings in Riley's show are abstract, and marshalled with crisp, formidable precision. They owe more to the methodical severity of Seurat than to Bonnard, whom Heron revered. All trace of personal brushmarks has been eradicated from her art. Restricting herself to austere black and white, Riley entrusted the execution of her work to assistants. Impersonal in their finish, the works are painted with emulsion on hardboard - as if to stress the absolute toughness of her approach.
Even so, there is nothing remote or unfeeling about the impact of her feisty images, and their freshness remains unimpaired today. Many of their abrupt, one-word titles - Tremor, Shift, Burn, Blaze - match the intensity that Riley conveys. For these are fiercely emotional paintings, and they transmit the essence of her rapt commitment to the act of hard looking. Whether sending hundreds of triangles to dart and swerve across the picture surface, or making an entire work shudder with the volcanic force of the fissures scything through it, Riley strives for maximum engagement.
As well as assailing our eyes with the scintillation of her diagonals, zigzags and dizzying undulations, she makes us feel their prodigious energy surging within our bodies. These paintings involve us so completely that we reel, shiver and sway in front of them. The aptly named Climax, whose surface seems to buckle under the dynamism of the power unleashed here, has a feverish quality. It could overwhelm anyone who scrutinised its corrugations too closely, and in the finest early painting we find ourselves trapped within a mass of eye-bending striations. Called Current, it sends black lines whirling like an electrical discharge. We feel the voltage running through us, but Current also evokes the imperious rhythms of the sea, or magnetic waves rising from the land like a shimmering heat-haze.
All these interpretations are valid, for Riley's art possesses multi-layered references to the visible world. But she always insists on also retaining a clear emphasis on the abstract autonomy of her work. These paintings do not need to be related to observable phenomena at all. They have their own identity, and when Riley introduced colour in the late 1960s she made sure that nothing prevented it from occupying a prime position. Working now largely with acrylic on canvas, she expanded her work to panoramic dimensions. Although the starkness of black and white no longer assails us, the dramatic increase in size means that we often feel engulfed by the images. Riley did not immediately banish the intricacy of her former work: Cataract 3 immerses us in a ceaseless cascade of swelling and dipping lines.
But as her fascination with colour took hold, so she came to stress the simplicity of vertical or horizontal stripes in contrasting colours. By this time, Riley was relying more on the Mediterranean stimulus of the heat, light and sounds in the countryside around her Provencal farmhouse. Painting there for extended periods each year, she responded with particular power to the vibrancy of late morning, when the heat rises and hovers in the air. The ascending bars in her painting of that subject appear to be suspended, and they emanate the luminosity that Riley experienced in the south of France. It is a sensuous canvas, and yet she still organises her responses with bracing rigour.
In a startling 1973 work called Rattle, we are confronted by nine horizontal bands sweeping across a surface of titanic proportions. Within each band, diagonal stripes in hot colours bend our vision in conflicting directions. We feel caught up in a crossfire, organised by an artist who knows precisely how to make our eyes work with invigorating zeal. However much certitude Riley commands, this marvellous exhibition makes clear that she has never slipped into formula. Determined to avoid repeating herself, she spent the rest of the 1970s shunning diagonals and exploring twisted, billowing alternatives. Softer in colour and more lyrical in mood, they seem to offer a protracted paean to the pleasures of summer. But Riley thrives on reacting against her own work. "If you had a pig-headed little plan," she told me recently, "it might prevent wonderful things from occurring." Accordingly, she spent much of the early 1980s reassessing the authority of vertical stripes, assembled in magisterial ranks where colour is able to assert itself with an almost musical resonance. Towards the end of the decade, the diagonals return. Now, however, they lance through the uprights like shafts of light piercing dense clusters of trees. At the same time, Riley's colours become still more scintillating. She allows herself to cast off geometric simplicity in favour of dappled, broken alternatives. We can feel her relaxing and opening herself up to more voluptuous impulses. New Day is the title of a particularly vision-cleansing image and, as the show approaches its close, Riley seems to recharge herself with a new appetite for zestfulness.
The lean, athletic exuberance of Matisse's late cut-outs is emulated in several recent paintings. Invigorated by the spirit of the bacchanal, she has also looked at orgiastic images by Poussin and Rubens. Their wild, lustful figures cannot be detected in Riley's most ambitious new painting, Evoe 3. But its lunging, rearing and leaping rhythms vividly evoke the abandon of a bacchanal. "Evoe!" was the exclamation uttered by the ancient Greeks when they gave themselves up to wine-fuelled revels. It sums up the mood that gives Riley's recent curve paintings their vivacity.
The final room of the Tate show is alive with so much gusto that it seems to be the work of an artist in mid-career. And when we return to the great circular wall drawing on our way out, Riley convinces us that her capacity for renewal is limitless. Refusing to settle for blandness, she promises instead to charge her future work with even more dancing audacity than before.
"Bridget Riley" is at Tate Britain, Millbank, London SW1 (020 7887 8000) until 28 September
Richard Cork is the new NS art critic. His four books on modern art (1970-2000) have just been published by Yale