Question

Which X-rays have more wavelength, soft X-ray or hard X-ray?

Answer 1

Soft X-rays are those with lower energies and Hard X-rays are those with higher energies. The energy is directly proportional to frequency and is inversely proportional to the wavelength. Thus, higher the energy, higher the frequency and shorter is the wavelength.

Complete answer:
X-rays are a part of the high energy electromagnetic spectrum. X-rays have higher energy than the visible rays and therefore, they have shorter wavelength (since energy is inversely proportional to wavelength). They can pass through many objects including the body. Therefore, they are used to generate images of tissues and bones.

If the electrons are accelerated through a potential difference $V$ and then incident on a suitable target, X-rays of wavelength equal or more than $\dfrac{{hc}}{{eV}}$ are produced. Thus, $\lambda = \dfrac{{hc}}{{eV}}$
where, $h$ is the Planck's constant, $c$ is the speed of light and $e$ is the charge on the electron.

In general, the X-rays of wavelength of the order of $1\mathop {\rm A}\limits^ \circ$ are called hard X-rays. They are named as hard X-rays because they have energies greater than $10{\text{ }}keV$ . As they have high energy, they will have comparatively short wavelengths.

On the other hand, X-rays with wavelength above $4\mathop {\rm A}\limits^ \circ$ are called soft X-rays. They have photon energies below $10{\text{ }}keV$. Therefore, their wavelength will be greater than hard X-rays. Actually, soft X-rays are used to take the pictures of bones and tissues. But, hard X-rays have higher penetrating power. Thus they are used to destroy some tissue in a body, as high energy is required here. Therefore, in that case hard X-rays are used.

Therefore, soft X-rays have more wavelength as compared to hard X-rays.

Note: The X-rays of longer wavelength are soft X-rays and those of shorter wavelength are soft X-rays. X-rays, both hard and soft, are not deflected by any electric or magnetic fields. These X-rays travel with the speed of light that is given by a constant $c$. The hardness of the X-rays depends on the accelerating potential of the electron incident on the target.