Explanation:
When two objects are moving in opposite directions, their relative velocity is the sum of their individual velocities. In this case, the electron is moving with a speed of 0.9c in one direction, and the photon is moving in the opposite direction.

Formula:
The formula for calculating relative velocity is as follows:
Relative velocity = Velocity of object 1 - Velocity of object 2

Calculation:
Given that the electron is moving with a speed of 0.9c and the photon is moving in the opposite direction. We can assume the speed of light as c.

So, the velocity of the photon = -c (as it is moving in the opposite direction)
The velocity of the electron = 0.9c

Now, substituting these values in the formula, we get:
Relative velocity = Velocity of electron - Velocity of photon
Relative velocity = 0.9c - (-c)
Relative velocity = 0.9c + c
Relative velocity = 1.9c

Therefore, the relative velocity of the electron with respect to the photon is 1.9c. This means that the electron is moving with a speed of 1.9 times that of the photon in the opposite direction.

Conclusion:
In this question, we have calculated the relative velocity of an electron with respect to a moving photon. The relative velocity is the sum of the velocities of the two objects. By using the formula and the given values, we have calculated the relative velocity of the electron with respect to the photon, which is 1.9c.