The Unknown Monet exhibition at the Royal Academy is something of a revelation, writes THERESA THOMPSON.

It seems a bit of an oxymoron to call an exhibition The Unknown Monet. After all, Claude Monet is one of the most popular painters in the world, if not the most, and has been so extensively studied that it is hard to imagine there can be anything else to find out.

But The Unknown Monet, which is on at the Royal Academy until June 10, focuses on an area of his output that up till now has never been much addressed: his pastels and drawings. In so doing it uncovers Monet's hidden talent as a draughtsman and overturns the conventional notion of him as an impulsive, unrehearsed painter working directly on to the canvas. It also introduces us to some other Monets, not just the man whose name is synonymous with impressionist painting.

In 80 works - from black chalk or crayon drawings to beautiful pastels, to sketchbook sheets containing pencil studies relating to his paintings - we meet Monet the caricaturist, Monet the artist obsessed by drawing, Monet the master of pastels, Monet the self-promoter, and Monet the printmaker. We also have the chance to flick' through the pages of his sketchbooks, now digitised for the exhibition.

The exhibition opens with the unexpected caricatures, plus early drawings and landscapes. Monet had taught himself how to capture a likeness by looking at newspapers, and his abilities earned him his first modest commissions as well as helping establish him as an up-and-coming artist.

Take a look at the man taking snuff, a charcoal and chalk sketch from around 1858 when Monet was aged 18; it captures an affectionate, almost furtive exchange of glances between the large-nosed man and the viewer. Like the others in this room, it is signed O. Monet'; Oscar did not use Claude, his second name, until the 1860s.

Monet's passion for drawing began in youth and lasted until his old age (despite him publicly disavowing it: part of his promotion of himself as a spontaneous painter).

Apparently, as a young man, he was always sketching; a family friend's unpublished memoir of 1857 confirms this: "Every scrap of paper, no matter how small, was drawn upon with country scenes, tiny seascapes and fishermen. Every sheet of paper that came into his hands was destined for a drawing."

It is clear Monet's love for landscape painting developed early on too. Half of the sheets in his teenage sketchbooks are of places in and around his home town, the seaport of Le Havre. Competent but conventional, these early landscapes inscribed with dates and locations confirm they were original views made direct from nature rather than copied from drawing manuals.

In the next room are Monet's pastels: the highlight for me. Whether of the eroded cliffs at Etretat or Normandy fields or the Thames, they are striking. Take, for example, three small panoramic views lined up on one wall: made in the 1860s, they show at different times of day the same flat empty strip of land with a huge sky hovering above.

The first, After the Rain, shows a tempestuous dark swirling sky, the second is Twilight, and then comes Nightfall where the falling sun highlights the undersides of clouds and the land is left in darkness. Nothing much alters but land and sky. Scenes that back then would barely warrant a sketch let alone a finished work for most artists allow the radical 25-year-old Monet to play with the idea of catching the changing light - and herald his series' paintings, the grainstacks, poplars and Rouen Cathedral we know so well.

On the whole, his pastels were not studies for paintings but commercial products in their own right aimed at the less wealthy buyer. Monet showed seven of them at the First Impressionist Exhibition in Paris in 1874 alongside his oils and although it is not recorded which they were, it is pleasing to think that some may be among those at this exhibition.

He made many pastels in London over the course of several visits. Drawn to its murky atmosphere and river, he would watch the thick fogs intently, finding the more he observed the more colours he would see within the swirling greyness. He hated what he called "appallingly clear" days, was lost without the smog to paint.

In January 1901, staying at the Savoy overlooking the Thames, with one room to live in, one to paint in, he prepared to work. Except his paints and canvases failed to arrive, held at customs. So he resorted to making "some sketches in pastel" to pass the time. When the paints finally arrived and he could begin his intended work he wrote to his wife saying, "It is thanks to my pastels, made swiftly, that I saw what I had to do."

Pastels allowed Monet to draw in colour. This tactile, smudgy, immediate medium liberated him; let him blur the distinction between line and colour; let him capture the fleeting effects of light. It also made him bolder because mistakes could not be corrected.

Seven of the results of this 1901 visit are on display: two of the Thames downstream to Waterloo Bridge and five upstream to Charing Cross Bridge; there's an oil too for comparison.

They show the river, bridges, and feathery suggestions of barges engulfed in luminous explosions of violet, turquoise and aqua. One particularly vaporous view is of Waterloo Bridge, Morning Fog (1901): no one can ever have seen smog like this before.

The London pictures make great colour statements and a fitting finale to a show that, while uncovering the unknown Monet, still has plenty of resonance for lovers of the known and the exuberant colour, hazy light and soul of his trademark impressionist paintings.