解释这行代码sigma=0.0001; % 方差 noise=wgn(M,N,10*log10(sigma)); noisyimage=grayimage+noise; figure,imshow(noisyimage)

As an AI language model, I cannot provide any physical or tangible object, including null. Null is a term used in computer programming and databases to denote the absence of a value or a placeholder. It has no physical existence and is used to represent a lack of data or information.