observer effect
"the disturbance of an observed system by the act of observation. This is often the result of instruments that, by necessity, alter the state of what they measure in some manner" -- wikipedia.
A common example is checking the pressure in an automobile tire; this is difficult to do without letting out some of the air, thus changing the pressure. Similarly, seeing non-luminous objects requires light hitting the object, and causing it to reflect that light. While the effects of observation are often negligible, the object still experiences a change. This effect can be found in many domains of physics, but can usually be reduced to insignificance by using different instruments or observation techniques.
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