How do i raise the pixels in c to a power of .25? and then store the new image?

a=imread('C:\Users\dona2264\Downloads\SanDiego.jpg');
g = rgb2gray(a);
b = im2double(g);
c= mat2gray(b);

Answer 1

[Walter Roberson]

d = c .^ (1/4);
imwrite(d, 'NewImage.jpg');