Check out example codes for "how to make a solid colour background slightly transparent in css". It will help you in understanding the concepts better.

Code Example 1

img {
  opacity: 0.5;
}

Code Example 2

.opacity30 {
  opacity: 0.3;
  filter: alpha(opacity=30); /* For IE8 and earlier */
}

Code Example 3

.transparent {
    background-color: rgba(255, 255, 255, 0.5);
}
.transparent {
    opacity: 0.5;
}

Code Example 4

img {
  opacity: 0.5;
}

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