Which statement is NOT true about x-ray beams?

Which statement is NOT true about x-ray beams?

• A. Can be focused by a lens
• B. Travel at the speed of light
• C. Cause ionization
• D. Have no mass

Answer: (A)

X-ray beams behave as photons: they have no rest mass and carry energy, and they interact with matter enough to ionize atoms. In vacuum they propagate essentially at the speed of light, and in materials they move so fast that their speed is still very close to c (though slightly reduced by the medium). What makes this question tricky is how they interact with lenses. Conventional lenses rely on refraction, but X-rays have a refractive index very close to 1 and are mainly absorbed rather than refracted by most materials, so focusing with ordinary lenses isn’t feasible. Instead, X-rays are focused using specialized optics like grazing-incidence mirrors or zone plates. So the statement about being focused by a lens is not true, making it the correct choice. The other statements are true: X-rays travel near the speed of light, they cause ionization, and they have no mass.

X-ray beams behave as photons: they have no rest mass and carry energy, and they interact with matter enough to ionize atoms. In vacuum they propagate essentially at the speed of light, and in materials they move so fast that their speed is still very close to c (though slightly reduced by the medium). What makes this question tricky is how they interact with lenses. Conventional lenses rely on refraction, but X-rays have a refractive index very close to 1 and are mainly absorbed rather than refracted by most materials, so focusing with ordinary lenses isn’t feasible. Instead, X-rays are focused using specialized optics like grazing-incidence mirrors or zone plates. So the statement about being focused by a lens is not true, making it the correct choice. The other statements are true: X-rays travel near the speed of light, they cause ionization, and they have no mass.