A) 
The energy of an x-ray photon used for single dental x-rays is
The energy of a photon is related to its wavelength by the equation
where
is the Planck constant
is the speed of light
is the wavelength
Re-arranging the equation for the wavelength, we find
B) 
The energy of an x-ray photon used in microtomography is 2.5 times greater than the energy of the photon used in part A), so its energy is
And so, by using the same formula we used in part A), we can calculate the corresponding wavelength:
Explanation:
Here,
object distance(u)=20cm
image distance(v)=80cm-20cm=60cm
focal length(f)=?
we know ,
(1/f)=(1/v)+(1/u)
1/f = 1/60 + 1/20
1/f = (1+3)/60
60 =4f
f=15cm
Let's take the analogy of the baseball pitcher a step farther. When a baseball is thrown in a straight line, we already said that the ball would fall to Earth because of gravity and atmospheric drag. Let's pretend again that there is no atmosphere, so there is no drag to slow the baseball down. Now, let's assume that the person throwing the ball throws it so fast that as the ball falls towards the Earth, it also travels so far, before falling even a little, that the Earth's surface curves away from the ball's path.
In other words, the baseball falls as it did before, but the ball is moving so fast that the curvature of the Earth becomes a factor and the Earth "falls away" from the ball. So, theoretically, if a pitcher on a 100 foot (30.48 m) high hill threw a ball straight and fast enough,the ball would circle the Earth at exactly 100 feet and hit the pitcher in the back of the head once it circled the globe! The bad news for the person throwing the ball is that the ball will be traveling at the same speed as when they threw it, which is about 8 km/s or several times faster than a rifle bullet. This would be very bad news if it came back and hit the pitcher, but we'll get to that in a minute.
