These poppies were blooming along the Hite Cove trail, probably the most famous wildflower-viewing site in the Yosemite area. Hite Cove peaks in March and April, though if you want to see poppies specifically, plan on March, and likely the first half of March (though it can vary from year to year), before the grass gets tall enough to dilute the effect of all that orange.
Tufted Poppies look much like California's state flower, the California Poppy, which also grows in and near Yosemite. The easiest way to tell them apart is to check just underneath the flower - California Poppies have a little disc there (readily seen here and here) that Tufted Poppies lack. California Poppies also tend to bloom later in the spring.
The little blurs of purple at bottom rear of the photo are Bird's Eye Gilia.
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