Q.Electromagnetic waves used as a diagnostic tool in medicine are (A) X-rays. (B) ultraviolet rays. (C) infrared radiation. (D) ultrasonic waves.

The key idea is that X-rays are the electromagnetic waves used in medical diagnostics because of their high penetrating power and ability to create clear images of bones and internal structures.

• X-rays have wavelengths short enough to pass through soft tissue but are absorbed by denser materials like bone, producing a shadow image on a detector.
• Ultraviolet rays are mostly absorbed by the skin and are not used for internal imaging. …

Let’s start with the core idea. The question is about electromagnetic waves used in medical diagnostics. The key is to distinguish between the different types of electromagnetic radiation and their specific applications in medicine. Not all EM waves are suitable for imaging inside the body — some are absorbed too quickly, some don’t penetrate at all, and others are used for therapy rather than diagnosis.

1. What does “diagnostic tool” mean here?

   In medicine, a diagnostic tool helps visualize internal structures or detect abnormalities without surgery. The most common example is an X-ray image of a broken bone. The wave must be able to pass through the body (penetrate) and then be detected on the other side, with different tissues absorbing different amounts.

2. Consider each option:

   • (A) X-rays: These have very short wavelengths ($10^{-11}$ to $10^{-8}$ m) and high energy. They penetrate soft tissues (skin, muscle) but are absorbed by denser materials like bone. This differential absorption creates a shadow image on a detector — that’s a radiograph. X-rays are the backbone of diagnostic radiology.
   • (B) Ultraviolet rays: UV has shorter wavelengths than visible light ($10^{-8}$ to $10^{-7}$ m) and is mostly absorbed by the skin’s outer layers. It causes sunburn and can damage DNA, but it does not penetrate deep enough to image internal organs. UV is used in sterilization or treating skin conditions, not for internal diagnostics.
   • (C) Infrared radiation: IR has longer wavelengths ($10^{-3}$ to $10^{-6}$ m) and is felt as heat. It penetrates only a few millimeters into the skin. While thermal imaging (thermography) can detect surface temperature patterns, it is not a primary diagnostic tool for internal structures like bones or organs.
   • (D) Ultrasonic waves: These are not electromagnetic waves — they are mechanical sound waves with frequencies above human hearing. Ultrasound is indeed a diagnostic tool (e.g., fetal imaging), but the question specifically asks for electromagnetic waves. So this option is out.

3. Why not UV or IR?

   Both UV and IR are electromagnetic, but their penetration depth in human tissue is too shallow for imaging deep structures. UV is absorbed by the epidermis; IR is absorbed by water in tissues within a few millimeters. Only X-rays have the combination of high energy and low absorption in soft tissue to create useful internal images. …